Effects of warming on soil organic carbon pools mediated by mycorrhizae and hyphae on the Eastern Tibetan Plateau, China

Mycorrhizae and their hyphae play critical roles in soil organic carbon (SOC) accumulation. However, their individual contributions to SOC components and stability under climate warming conditions remain unclear. This study investigated the effects of warming on the SOC pools of Picea asperata (an ectomycorrhizal plant) and Fargesia nitida (an arbuscular mycorrhizal plant) mycorrhizae/hyphae on the eastern Tibetan Plateau. The results indicated that mycorrhizae made greater contributions to SOC accumulation than hyphae did by increasing labile organic carbon (LOC) components, such as particle organic carbon (POC), easily oxidizable organic carbon, and microbial biomass carbon, especially under warming conditions. Plant species also had different effects on SOC composition, resulting in higher mineral-associated organic carbon (MAOC) contents in F. nitida plots than in P. asperata plots; consequently, the former favored SOC stability more than the latter, with a lower POC/MAOC. Partial least-squares path modelling further indicated that mycorrhizae/hyphae indirectly affected LOC pools, mainly by changing soil pH and enzyme activities. Warming had no significant effect on SOC content but did change SOC composition by reducing LOC through affecting soil pH and iron oxides and ultimately increasing SOC stability in the presence of mycorrhizae for both plants. Therefore, the mycorrhizae of both plants are major contributors to the variation of SOC components and stability under warming conditions.

Mineral protection mediates soil carbon temperature sensitivity of nine old-growth temperate forests across the latitude transect

Temperature sensitivity (Q10) of soil microbial respiration serves as a crucial indicator for assessing the response of soil organic carbon (SOC) to global warming. However, the biogeographic variation in Q10 remains inconsistent. In this study, we examined Q10 and its potential drivers in nine old-growth mixed broad-leaved Korean pine (Pinus koraiensis Sieb. et Zucc.) forests (the climax community of Asian temperate mixed forest) under a wide range of climatic conditions. We found that stand characteristics (arbuscular mycorrhizal tree basal area to ectomycorrhizal tree basal area ratio and root to shoot ratio) contributed to soil C sequestration by facilitating the accumulation of soil recalcitrant C components. Contrary to the C quality-temperature hypothesis, Q10 was not correlated with C quality (soil C to nitrogen ratio and recalcitrant C to labile C ratio). Soil mineral protection parameters (Fe/Al oxides) had negative effect on Q10 because they inhibited microbial activities by decreasing substrate accessibility. Additionally, soils with high microbial biomass C and microbial biomass C to soil organic C ratio had high Q10. Overall, understanding the complex relationships among Q10, mineral protection, and microbial attributes on a spatial scale is essential for accurately predicting soil C cycling in forest ecosystems.

The functional division of arbuscular mycorrhizal fungi and earthworm to efficient cooperation on phytoremediation in molybdenum (Mo) contaminated soils

Single phytoremediation has limited capacity to restore soil contaminated with extreme Mo due to its low metal accumulation. Soil organisms can help compensate for this deficiency in Mo-contaminated soils. However, there is limited information available on the integrated roles of different types of soil organisms, particularly the collaboration between soil microorganisms and soil animals, in phytoremediation. The objective of this study is to investigate the effects of a combination of arbuscular mycorrhizal fungi (AMF) and earthworms on the remediation of Mo-contaminated soils by alfalfa (Medicago sativa L.). The results indicated that in the soil-alfalfa system, earthworms effectively drive soil Mo activation, while AMF significantly improve the contribution of the translocation factor to total Mo removal (TMR) in alfalfas (p < 0.05). Meanwhile, compared to individual treatments, the combination of AMF and earthworm enhanced the expression of alfalfa root specific Mo transporter - MOT1 family genes to increase alfalfa uptake Mo (p < 0.05). This alleviated the competition between P/S nutrients and Mo on non-specific Mo transporters-P/S transporters (p < 0.05). Additionally, the proportion of organelle-bound Mo in the root was reduced to decrease Mo toxicity, while the cell wall-bound Mo proportion in the shoot was increased to securely accumulate Mo. The contributions of inoculants to alfalfa TMR followed the order (maximum increases): AMF + E combination (274.68 %) > alone treatments (130 %). Overall, the "functional division and cooperation" between earthworm and AMF are of great importance to the creation of efficient multi-biological systems in phytoremediation.

Afforestation enhances glomalin-related soil protein content but decreases its contribution to soil organic carbon in a subtropical karst area

Afforestation on degraded croplands has been proposed as an effective measure to promote ecosystem functions including soil organic carbon (SOC) sequestration. Glomalin-related soil protein (GRSP) plays a crucial role in promoting the accumulation and stability of SOC. Nevertheless, mechanisms underlying the effects of afforestation on GRSP accumulation have not been well elucidated. In the present study, 14 pairs of maize fields and plantation forests were selected using a paired-site approach in a karst region of southwest China. By measuring soil GRSP and a variety of soil biotic and abiotic variables, the pattern of and controls on GRSP accumulation in response to afforestation were explored. The average content of total GRSP (T-GRSP) and its contribution to SOC in the maize field were 5.22 ± 0.29 mg g-1 and 42.33 ± 2.25%, and those in the plantation forest were 6.59 ± 0.32 mg g-1 and 25.77 ± 1.17%, respectively. T-GRSP content was increased by 26.4% on average, but its contribution to SOC was decreased by 39.1% following afforestation. T-GRSP content decreased as soil depth increased regardless of afforestation or not. Afforestation increased T-GRSP indirectly via its positive effects on arbuscular mycorrhizal fungi biomass, which was stimulated by afforestation through elevating fine root biomass or increasing the availability of labile C and N. The suppressed contribution of T-GRSP to SOC following afforestation was due to the relatively higher increase in other SOC components than T-GRSP and the significant increase of soil C:N ratio. Our study reveals the mechanisms underlying the effects of afforestation on T-GRSP accumulation, and is conducive to improving the mechanistic understanding of microbial control on SOC sequestration following afforestation.

Removal of sulfamethoxazole and Cu, Cd compound pollution by arbuscular mycorrhizal fungi enhanced vertical flow constructed wetlands

The combined pollution of antibiotics and heavy metals (HMs) has a serious impact on the water ecological environment. Previous researches mainly focused on the removal of antibiotics or HMs as single pollutants, with limited investigation into the treatment efficiencies and underlying mechanisms associated with their co-occurring pollution. In this study, 16 micro vertical flow constructed wetlands (MVFCWs) were constructed to treat composite wastewater consisting of sulfamethoxazole (SMX), copper (Cu) and cadmium (Cd), involving two different inoculation treatments (arbuscular mycorrhizal fungi (AMF) inoculated and uninoculated) and eight kinds of pollutant exposure (Control Check (CK), SMX, Cu, Cd, SMX + Cu, SMX + Cd, Cu + Cd, SMX + Cu + Cd). The findings of this study demonstrated that the inoculation of AMF in MVFCWs resulted in removal efficiencies of SMX, Cu, and Cd ranging from 18.70% to 80.52%, 75.18% to 96.61%, and 40.50% to 89.23%, respectively. Cu and CuCd promoted the degradation of SMX in the early stage and inhibited the degradation of SMX in the later stage. Cd did not demonstrate a comparable promotive impact on SMX degradation, and its addition hindered Cu removal. However, comparatively, the presence of Cu exerted a more pronounced inhibitory effect on Cd removal. Furthermore, the addition of Cu augmented the abundances of Proteobacteria, Bacteroidetes (at the phylum level) and Rhodobacter, Lacunisphaera and Flavobacterium (at the genus level), and Cu exposure showed a substantially stronger influence on the microbial community than that of Cd and SMX. AMF might confer protection to plants against HMs and antibiotics by enriching Nakamurella and Lacunisphaera. These findings proved that AMF-C. indica MVFCW was a promising system, and the inoculation of AMF effectively enhanced the simultaneous removal of compound pollution.

Nitrogen fertilization and arbuscular mycorrhizal fungi do not mitigate the adverse effects of soil contamination with polypropylene microfibers on maize growth

Soil contamination with microplastics may adversely affect soil properties and functions and consequently crop productivity. In this study, we wanted to verify whether the adverse effects of microplastics in the soil on maize plants (Zea mays L.) are due to a reduction in nitrogen (N) availability and a reduced capacity to establish symbiotic relationships with arbuscular mycorrhizal (AM) fungi. To do this, we performed a pot experiment in which a clayey soil was exposed to two environmentally relevant concentrations of polypropylene (PP; one of the most used plastic materials) microfibers (0.4% and 0.8% w/w) with or without the addition of N fertilizer and with or without inoculation with AM fungi. The experiment began after the soil had been incubated at 23 °C for 5 months. Soil contamination with PP considerably reduced maize root and shoot biomass, leaf area, N uptake, and N content in tissue. The adverse effects increased with the concentration of PP in the soil. Adding N to the soil did not alleviate the detrimental effects of PP on plant growth, which suggests that other factors besides N availability played a major role. Similarly, although the presence of PP did not inhibit root colonization by AM fungi (no differences were observed for this trait between the uncontaminated and PP-contaminated soils), the addition of the fungal inoculum to the soil failed to mitigate the negative impact of PP on maize growth. Quite the opposite: mycorrhization further reduced maize root biomass accumulation. Undoubtedly, much research remains to be done to shed light on the mechanisms involved in determining plant behavior in microplastic-contaminated soils, which are most likely complex. This research is a priority given the magnitude of this contamination and its potential implications for human and environmental health.

Isolation and identification of arbuscular mycorrhizal fungi from an abandoned uranium mine and their role in soil-to-plant transfer of radionuclides and metals

Arbuscular mycorrhizal fungi were recovered from soil samples from the naturally radioactive soil at the long-abandoned South Terras uranium mine in Cornwall, UK. Species of Rhizophagus, Claroideoglomus, Paraglomus, Septoglomus, and Ambispora were recovered, and pot cultures from all except Ambispora were established. Cultures were identified to species level using morphological observation and rRNA gene sequencing combined with phylogenetic analysis. These cultures were used in pot experiments designed with a compartmentalised system to assess the contribution of fungal hyphae to the accumulation of essential elements, such as copper and zinc, and non-essential elements, such as lead, arsenic, thorium, and uranium into root and shoot tissues of Plantago lanceolata. The results indicated that none of the treatments had any positive or negative impact on shoot and root biomass. However, Rhizophagus irregularis treatments showed higher accumulation of copper and zinc in shoots, while R. irregularis and Septoglomus constrictum enhanced arsenic accumulation in roots. Moreover, R. irregularis increased uranium concentration in roots and shoots of the P. lanceolata plant. This study provides useful insight into fungal-plant interactions that determine metal and radionuclide transfer from soil into the biosphere at contaminated sites such as mine workings.

Global distributions of foliar nitrogen and phosphorus resorption in forest ecosystems

Nutrient resorption is an important mechanism for nutrient conservation and can maintain ecosystem stoichiometry. Here, we examined the global-scale variation of nitrogen resorption efficiency (NRE) and phosphorus resorption efficiency (PRE) by analyzing observations from 218 published papers. We used Pagel's λ to test the phylogenetic limitation on NRE and PRE and applied the random forest model to assess biotic and abiotic drivers, which included climate, soil, species characteristics, and topographical factors, and predicted the global NRE and PRE distributions. We found that NRE and PRE had oppositing trends among climatic zones, plant functional groups, and foliar nitrogen (N) to phosphorus (P) ratios. Nutrient resorption was higher in ectomycorrhizal trees than in arbuscular mycorrhizal trees. Moreover, foliar NRE and PRE were not linked to phylogeny. On average, the random forest overall explained 38 % (21 %-55 %) variation in NRE and 36 % (16 %-55 %) variation in PRE. Both NRE and PRE varied greatly with climate and soil organic carbon (SOC). The spatial variation of NRE and PRE was coupled to N-limitation and P-limitation, respectively. Our evaluation of the factors that influenced NRE and PRE and their global distributions, and our novel approach for evaluating plant utilization of nutrients, advances our understanding of the relative stability of ecosystem randomness in forest ecosystems and the global forest nutrient cycle.

Global magnitude of rhizosphere effects on soil microbial communities and carbon cycling in natural terrestrial ecosystems

The rhizosphere is one of the most dynamic interfaces on the Earth. Understanding the magnitudes of rhizosphere effects (RE, difference in bio-physicochemical properties between rhizosphere and bulk soils) on soil microbial communities and their moderators is important for studying on below-ground carbon (C) cycling. A comprehensive meta-analysis was conducted to quantify the REs on soil microbial biomass, community structure, respiration, and C-degrading enzymes. We found that REs on soil C and nutrients, total microbial biomass, the abundance of specific microbial groups, fungi to bacteria ratio, respiration, and C-degrading enzymes were positive, but the magnitudes were varied with biomes, plant functional types, and mycorrhizal types. REs on microbial biomass, respiration, and C-degrading enzymes increased with the increase of mean annual temperature and mean annual precipitation, but decreased with the increase of soil clay, C, nitrogen (N), and phosphorus (P) contents. The REs on microbial biomass and respiration also increased as the REs on soil C:N:P increased. Compared with bulk soil, per unit rhizosphere soil C supported more microbial biomass, per unit of which respired more C, leading to faster C decomposition in rhizosphere. Our findings indicate that the increase in microbial biomass, co-metabolism induced by labile and energy-rich organic C of root exudates, and overflow respiration induced by stoichiometric imbalance together contribute to the enhanced C decomposition in rhizosphere. The global pattern of REs on soil microbial communities is critical to revealing the plant-microbe-soil interactions in terrestrial ecosystems.

Mycorrhizal Infection Can Ameliorate Abiotic Factors in Urban Soils

Once abandoned, urban and post-industrial lands can undergo a re-greening, the natural regeneration and succession that leads to surprisingly healthy plant communities, but this process is dependent upon microbial activity and the health of the parent soil. This study aimed to evaluate the effects of arbuscular mycorrhizal fungi (AMF) in facilitating plant production in post-industrial soils. In so doing, we helped to resolve the mechanism through which AMF ameliorate environmental stress in terrestrial plants. An experiment was established in which rye grass (Lolium perenne) was grown in two heavy metal-contaminated soils from an urban brownfield in New Jersey, USA, and one non-contaminated control soil. One set of the treatments received an AMF inoculum (four species in a commercial mix: Glomus intraradices, G. mosseae, G. etunicatum and G. aggregatum) and the other did not. Upon harvest, dried plant biomass, root/shoot ratio, AMF colonization, and extracellular soil phosphatase activity, a proxy for soil microbial functioning, were all measured. Plant biomass increased across all treatments inoculated with AMF, with a significantly higher average shoot and root mass compared to non-inoculated treatments. AMF colonization of the roots in contaminated soil was significantly higher than colonization in control soil, and the root/shoot ratio of plants in contaminated soils was also higher when colonized by AMF. Mycorrhizal infection may help plants to overcome the production limits of post-industrial soils as is seen here with increased infection and growth. The application of this mechanistic understanding to remediation and restoration strategies will improve soil health and plant production in urban environments.

Dual RNA and 16S ribosomal DNA sequencing reveal arbuscular mycorrhizal fungi-mediated mitigation of selenate stress in Zea mays L. and reshaping of soil microbiota

Excessively high concentrations of selenium (Se) in soil are toxic to crop plants, and inoculation with arbuscular mycorrhizal fungi (AMF) can reverse Se stress in maize (Zea mays L.). To investigate the underlying mechanisms, maize seedlings were treated with sodium selenate (5 mg Se[VI] kg^-1) and/or AMF (Funneliformis mosseae and Claroideoglomus etunicatum). Dual RNA sequencing in mycorrhiza and 16 S ribosomal DNA sequencing in soil were performed. The results showed that Se(VI) application alone decreased plant dry weight, but increased plant Se concentration, total Se content (mainly selenocysteine), and root superoxide content. Inoculation with either F. mosseae or C. etunicatum increased plant dry weight, decreased Se accumulation and selenocysteine proportion, enhanced root peroxidase activity, and alleviated oxidative stress in Se(VI)-treated plants. Inoculation also downregulated the expression of genes encoding Se transporters, assimilation enzymes, and cysteine-rich receptor-like kinases in Se(VI)-stressed plants, similar to plant-pathogen interaction and glutathione metabolism related genes. Conversely, genes encoding selenium-binding proteins and those related to phenylpropanoid biosynthesis were upregulated in inoculated plants under Se(VI) stress. Compared with Se(VI)-free plants, Se tolerance index, symbiotic feedback percentage on plant dry weight, and root colonization rate were all increased in inoculated plants under Se(VI) stress, corresponding to upregulated expression of 'key genes' in symbiosis. AMF inoculation increased bacterial diversity, decreased the relative abundances of selenobacteria related to plant Se absorption (e.g., Proteobacteria and Firmicutes), and improved bacterial network complexity in Se(VI)-stressed soils. We suggest that stress-mediated enhancement of mycorrhizal symbiosis contributed to plant Se(VI) tolerance, whereas AMF-mediated reshaping of soil bacterial community structure prevented excessive Se accumulation in maize.

Altering microbial community for improving soil properties and agricultural sustainability during a 10-year maize-green manure intercropping in Northwest China

Maize is a crop that is cultivated worldwide. The Hexi Oasis is one of the most important areas for high-yield maize seed production in China. Green manure, a plant fertilizer, has great potential for increasing crop yield and agricultural sustainability. However, the role of microorganisms in soil health and the microbiological mechanism of green manure in improving soil fertility and crop production in the Hexi Oasis area remain unknown. The effects of maize-green manure intercropping on the soil microbial community structure and diversity and the mechanism of soil improvement were investigated in a 10-year field experiment. The study revealed that microbial phylotypes were grouped into four major ecological clusters. Module #2 is a soil core ecological cluster enriched with many plant growth-promoting rhizobacteria and arbuscular mycorrhizal fungi. The application of green manure led to significantly increased soil pH, nutrient contents, and enzyme activities, and significantly reduced the relative abundance of potential plant pathogens compared with monocropping, which should ensure high and stable maize yield under long-term continuous cropping. It also increased the economic benefits by 56.39% compared with monocropping, owing to the additional products produced by the green manure. These improvements were associated with changes in the microbial community structure and activity, consistent with the structural equation model results. Therefore, soil microorganisms are the key drivers of the potential benefits of maize-green manure on agricultural sustainability.

Effects of urbanization intensity on glomalin-related soil protein in Nanchang, China: Influencing factors and implications for greenspace soil improvement

Glomalin-related soil protein (GRSP) is a stable and persistent glycoprotein secreted by arbuscular mycorrhizal fungi that plays an important role in sequestering soil organic carbon (SOC) and improving soil quality. Rapid urbanization disturbs and degrades the soil quality in the greenspace. However, few studies have investigated the effects of urbanization on GRSP and its influencing factors. This study selected impervious surface area as a measure of urbanization intensity. A total of 184 soil samples were collected from the 0-20 cm soil layer in the greenspace of Nanchang, China (505 km²). The GRSP content, soil properties, urban forest characteristics, and land-use configuration were determined. The total GRSP (TG) and easily extractable GRSP (EEG) contents were 2.38 and 0.57 mg g^-1, respectively. TG and EEG decreased by 16.22% and 19.35%, respectively, from low to heavy urbanized areas. Moreover, SOC decreased from 39.9 to 1.4 mg g^-1, while EEG/SOC and TG/SOC increased by approximately 17% and 34%, respectively, indicating the significant contribution of GRSP to the SOC pool. Pearson and redundancy analysis showed that GRSP was positively correlated with SOC, phosphorus, nitrogen, vegetation richness, and tree height, but negatively correlated with pH, bulk density, and impervious area. The partial least squares path model demonstrated that urbanization affected soil properties, forest characteristics, and land use factors, resulting in GRSP changes. This study clarifies the key factors of urbanization that affect GRSP and provides insight for urban greenspace soil improvement from the new perspective of enhancing the GRSP content.

Suppression of arbuscular mycorrhizal fungi increased lead uptake in maize leaves and loss via surface runoff and interflow from polluted farmland

Arbuscular mycorrhizal fungi (AMF) are ubiquitous in farmland. But the knowledge on AMF impact on lead (Pb) migration in farmland is limited. A field experiment was conducted in the rainy season (May-October) for two years in a Pb-polluted farmland. Benomyl was used to specifically suppress the native AMF growth in the farmland. The effect of benomyl-induced AMF suppression on the Pb uptake in maize, and Pb loss via surface runoff and interflows (20 cm and 40 cm depth) from the farmland was investigated. The benomyl significantly inhibited the AMF growth, resulting in decreases in the colonization rate, spore number, and contents of total and easily extractable glomalin-related soil protein (GRSP); and promoted the Pb migration into maize shoots and mainly enriched in leaves. The particulate Pb accounted for 83.2%-90.6% of Pb loss via surface runoff, while the proportion of particulate Pb loss via interflow was decreased and the proportion of dissolved Pb loss increased with the increase of soil depth. The AMF suppression led to a decrease in dissolved Pb concentration and loss, but an increase in particulate Pb concentration and loss, and enhanced the total Pb loss via surface runoff and interflows. Moreover, significant or very significant negative correlations were observed between the AMF colonization rate in roots with the Pb uptake in leaves, and the content of easily extractable GRSP with the particulate Pb loss. These results indicated the native AMF contributed to immobilizing Pb in soil and inhibited its migration to crops and the surrounding environment.

Arbuscular mycorrhizal fungi reduce potassium, cadmium and ammonium losses but increases nitrate loss under high intensity leaching events

BACKGROUND

Nutrients and heavy metals can be lost from soils via leaching, and arbuscular mycorrhizal fungi (AMF) can influence these events. Soil column experiments were carried out to examine whether leaching intensity and AMF can alter nutrient and Cd uptake in white clover plants and the extent of their losses through leaching.

RESULTS

The presence of AMF significantly increased shoot and total biomass, as well as increased N, P, Cu and Zn uptake independent of water amount applied; while root P and Cu uptakes were promoted by AMF at any water amount treatments. Higher water amounts led to reductions in total N, K and Zn uptake for AMF-colonized plants in comparison to moderate water amount treatments. In the absence of AMF, white clover at low water amount treatment exhibited maximal root Cd uptake. At high water amount treatments, the presence of AMF significantly decreased leachate volumes and the amount of leached NH₄⁺, K and Cd while AMF significantly increased the amounts of leached NO₃^-.

CONCLUSIONS

Overall we found that AMF-colonized white clover plants reduced NH₄⁺, K and Cd loss from soils but increased the risk of NO₃^- loss under high intensity leaching conditions.

Effects of solid waste-based soil conditioner and arbuscular mycorrhizal fungi on crop productivity and heavy metal distribution in foxtail millet (Setaria italica)

Shanxi is a large coal-producing province, and it also produces a lot of solid waste. Solid waste can leach heavy metals, which can harm soil and affect food security at the beginning of the food chain. To investigate the impacts of solid waste-based soil conditioner (SWSC) and arbuscular mycorrhizal fungi (AMF) on millet safety and crop production, a field experiment with foxtail millet (Setaria italica) was conducted in Tunliu. The results of this study demonstrate that SWSC + AMF, SWSC and AMF can increase millet yield by 28.0%, 27.1% and 19.5%, respectively, compared with CK. This is mainly due to increased mycorrhizal infection. Besides, the pollution index (P_i) and the Nemerow-integrated pollution index (P_N) of the soil with SWSC and AMF were both below 0.7, indicating safe pollution levels. The application of AMF and SWSC inhibits plants from absorbing heavy metals from the soil and reduces the TF_root/soil of the millet. SWSC + AMF application inhibited the transfer of heavy metals from the roots to the upper part of the ground and reduced the TF_shoot/root of the millet. The TF_grain/soil of the millet was below 1. The HQ and HI of the millet grains did not exceed 1, indicating the absence of a potential health risk. Therefore, SWSC combined with AMF is applicable for millet production in Tunliu, and the combined treatment can decrease heavy metal phytoavailability and post-harvest transfer risks. This work provides a way to utilize solid waste while also improving millet yields in dry farming. Based on the review, we suggested future researches to better understand the mechanisms of SWSC + AMF long-term application to promote awareness on its role over time through alterations in its surface chemistry, soil microbial community and environmental implications.

A new mushroom hyperaccumulator: Cadmium and arsenic in the ectomycorrhizal basidiomycete Thelephora penicillata

Fruit-bodies of six Thelephora species (Fungi, Basidiomycota, Thelephoraceae) were analyzed for their trace element concentrations. In Thelephora penicillata, extremely high concentrations of Cd and As were found, followed by highly elevated concentrations of Cu and Zn. The highest accumulation ability was found for Cd with a mean concentration of 1.17 ± 0.37 g kg^-1 (dry mass) in fruit-bodies collected from 20 unpolluted sites; the mean As concentration was 0.878 ± 0.242 g kg^-1. Furthermore, striking accumulation of Se (923 ± 28 mg kg^-1) was found in one sample of T. vialis and elevated concentrations of S were detected in T. palmata (19.6 ± 5.9 g kg^-1). The analyzed Thelephora species were sequenced and, based on the Maximum Likelihood phylogenetic analysis (ITS rDNA) of the genus, possible other Thelephora (hyper)accumulators were predicted on the basis of their phylogenetic relationship with the discovered (hyper)accumulators. The striking ability of T. penicillata to accumulate simultaneously Cd, As, Cu, and Zn has no parallel in the Fungal Kingdom and raises the question of a biological importance of metal(loid) hyperaccumulation in mushrooms.

Combined application of marble waste and beneficial microorganisms: toward a cost-effective approach for restoration of heavy metals contaminated sites

Heavy metal (HM) pollution and the need to preserve the environment have gathered increasing scientific attention. The immobilization of HMs into less-soluble, less mobile, and less toxic forms in addition to the improvement of Medicago sativa L. growth and HMs accumulation were evaluated after the application of marble waste (MW) and/or beneficial PGP rhizobacteria and mycorrhizae to the mining soil compost. A greenhouse assay was conducted to elucidate the influence of both amendment and beneficial microorganisms. The application of marble waste to the soil-compost resulted in decreasing the bioavailability of metals (Cu, Zn, Pb, and Cd), thus ameliorating the installation of the vegetal cover for 6 months of culture. Cultivation of M. sativa under 5% MW-amended soil for 6 months increased the shoot dry weight by almost twofold, while the inoculation with rhizobacteria-mycorrhizae combined with the application of 15% MW resulted in an improvement of 3.5-fold in case of shoot dry weight. In addition, the application of marble waste amendment or their combination with metallo-resistant bacteria resulted in decreasing HM accumulation leading to HM content below the threshold recommended for animal grazing. Thus, the application of amendments and beneficial microorganisms appeared to guarantee the safe cultivation of alfalfa for 6 months of culture. The dual combination amendments and beneficial microorganisms showed the good potential to restore HM polluted soils and could stand as a novel approach for restoration of HM-contaminated soils.

Heavy metal domestication enhances beneficial effects of arbuscular mycorrhizal fungi on lead (Pb) phytoremediation efficiency of Bidens parviflora through improving plant growth and root Pb accumulation

Native arbuscular mycorrhizal fungi (AMF) generally provide more effective assistance for phytoremediation to remove heavy metal (HM) from polluted soils than non-native AMF. Nevertheless, it is a time-consuming work to isolate, identify, and propagate AMF inoculum for practical application. This study aims to explore an alternative method to improve the phytoremediation efficiency of Bidens parviflora using domesticated AMF under HM stress condition for a certain period of time. Our results showed that Funneliformis mosseae inoculation alleviated oxidative damage to plant membranes by enhancing activities of superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase. Furthermore, mycorrhizal plants had higher chlorophyll concentration, photosynthesis efficiency, and root Pb content to protect the aerial parts from damage. These protective mechanisms were found to be more efficient in domesticated AMF inoculation compared with non-domesticated AMF inoculation. Overall, this study suggests that F. mosseae domesticated for 12 months could greatly enhance plant root Pb accumulation and plant growth mainly through strengthening antioxidant defenses as well as the photosynthesis efficiency under Pb stress conditions. Plants inoculated with pre-domesticated AMF provided a promising new strategy to enhance phytoremediation of Pb-contaminated soils.

Development of an organomineral fertiliser formulation that improves tomato growth and sustains arbuscular mycorrhizal colonisation

Achieving sustainable agricultural development requires the efficient use of nutrient resources for crop production. Recovering nutrients from animal manures may play a key role in achieving this. Animal manures typically have low nutrient concentrations, and in ratios that are often not ideal for balanced crop nutrition. Here, combinations of organic and inorganic phosphorus (P) were formulated as granular products (organomineral fertilisers) with granule size suitable for transport and spreading. The fertilisers were produced by granulating powdered chicken litter with MAP and urea powders making the following formulations: 0:4, 1:3, 2:2, 3:1, 4:0. The kinetics of NH₄⁺-N and P release from the fertilisers, and the effects on tomato growth and nutrition, as well as arbuscular mycorrhizal formation in roots following fertiliser application, were determined. Cumulative NH₄⁺-N release ceased within 12 h, and was lower in the formulations with higher proportions of chicken litter. The cumulative P released reached approximately 80% of total P in all formulations, and the time to obtain maximum P dissolution was 19 days in the formulation that contained only chicken litter. The organomineral fertilisers increased tomato shoot growth by 15-28% compared to the chicken litter only, MAP only and MAP/urea formulations. Reasonable levels of mycorrhizal colonisation of tomato roots was achieved with the organomineral fertilisers. The results demonstrated that optimum plant growth does not depend solely on immediately available P, and that timing of nutrient supply to match plant demand is important. The combination of chicken litter with MAP sustained nutrient supply and improved plant growth. Taken together, organomineral fertiliser formulations are potential alternatives to inorganic P fertilisers that can improve crop growth and nutrition, while provide a sustainable use for animal production wastes.

Cadmium transfer between maize and soybean plants via common mycorrhizal networks

More than 80% terrestrial plants establish mutualistic symbiosis with soil-borne arbuscular mycorrhizal fungi (AMF). These fungi not only significantly improve plant nutrient acquisition and stress resistance, but also mitigate heavy metal phytotoxicity, Furthermore, the extraradical mycorrhizal mycelia can form common mycorrhizal networks (CMNs) that link roots of multiple plants in a community. Here we show that the networks mediate migration of heavy metal cadmium (Cd) from maize (Zea mays L.) to soybean (Glycine max (Linn.) Merr.) plants. CMNs between maize and soybean plants were established after inoculation of maize plants with AMF Funneliformis mosseae. Application of CdCl2 in maize plants led to 64.4% increase in the shoots and 48.2% increase in the roots in Cd content in CMNs-connected soybean plants compared to the control without Cd treatment in maize. Meanwhile, although the CMNs-connected soybean plants did not directly receive Cd supply, they upregulated transcriptional levels of Cd transport-related genes HATPase and RSTK 2.13- and 5.96-fold, respectively, induced activities of POD by 44.8% in the leaves, and increased MDA by 146.2% in the roots. Furthermore, Cd addition inhibited maize growth but mycorrhizal colonization improved plant performance in presence of Cd stress. This finding demonstrates that mycorrhizal networks mediate the transfer of Cd between plants of different species, suggesting a potential to use CMNs as a conduit to transfer toxic heavy metals from main food crops to heavy metal hyperaccumulators via intercropping.

Transfer of La, Ce, Sm and Yb to alfalfa and ryegrass from spiked soil and the role of Funneliformis mosseae

Rare earth elements (REEs) are widely used in high-tech industries, and REE waste emissions have become a concern for ecosystems, food quality and human beings. Arbuscular mycorrhizal fungi (AMF) have repeatedly been reported to alleviate plant stress in metal-contaminated soils. To date, little information is available concerning the role of AMF in REE-contaminated soils. We recently showed that there was no transfer of Sm to alfalfa by Funneliformis mosseae, but only a single REE was examined, while light and heavy REEs are present in contaminated soils. To understand the role of AMF on the transfer of REEs to plants, we carried out an experiment using alfalfa (Medicago sativa) and ryegrass (Lolium perenne) in compartmented pots with separate bottom compartments that only were accessible by F. mosseae fungal hyphae. The bottom compartments contained a mixture of four REEs at equal concentrations (La, Ce, Sm and Yb). The concentration of REEs in plants was higher in roots than in shoots with higher REE soil-root than root-shoot transfer factors. Moreover, significantly higher light-REEs La and Ce were transferred to ryegrass shoots than Sm and the heavy-REE Yb, but this was not observed for alfalfa. Alfalfa dry weight was significantly increased by F. mosseae inoculation, but not ryegrass dry weight. For both plant species, there was significantly higher P uptake by the mycorrhizal plants than the nonmycorrhizal plants, but there was no significant transfer of La, Ce, Sm or Yb to alfalfa and ryegrass roots or shoots due to F. mosseae inoculation.

Arbuscular mycorrhizal fungi mitigate soil nitrogen and phosphorus losses: A meta-analysis

Nutrient loss from terrestrial ecosystems via leaching and gaseous emissions is increasingly threatening global environmental and human health. Although arbuscular mycorrhizal fungi (AMF) have been shown to regulate soil N and P losses, a comprehensive quantitative overview of their influences on the losses of these soil nutrients across global scales is currently lacking. This study used a meta-analysis of 322 observations from 36 studies to assess the effect of AMF inoculum on 11 variables related to the loss of soil N and P. We found that the presence of AMF significantly reduced soil N and P losses, with the most pronounced reduction occurring in soil NO₃^--N (-32%), followed by total P (-21%), available P (-16%) and N₂O (-10%). However, the mitigation effects of AMF on soil N and P loss were dependent on the identity of AMF inoculum, plant type and soil biotic and abiotic factors. Generally, the mitigation effects of AMF increased with increasing AMF root colonization rate, microbial diversity of inoculants, soil organic carbon (SOC) content and experimental duration as well as with decreasing soil sand contents and soil N and P availability. Overall, this meta-analysis highlights the importance of AMF inoculation in mitigating N and P nutrient loss and environmental pollution for terrestrial ecosystem sustainability.

Structure and diversity of fungal communities in long-term copper-contaminated agricultural soil

Copper (Cu) contamination threatens the stability of soil ecosystems. As important moderators of biochemical processes and soil remediation, the fungal community in contaminated soils has attracted much research interest. In this study, soil fungal diversity and community composition under long-term Cu contamination were investigated based on high-throughput sequencing. The co-occurrence networks were also constructed to display the co-occurrence patterns of the soil fungal community. The results showed that the richness and Chao1 index both significantly increased at 50 mg kg^-1 Cu and then significantly decreased at 1600 and 3200 mg kg^-1 Cu. Soil fungal diversity was significantly and positively correlated with plant dry weight. Specific tolerant taxa under different Cu contamination gradients were illustrated by linear discriminant analysis effect size (LEfSe). Soil Cu concentration and shoot dry weight were the strongest driving factors influencing fungal composition. The relative abundance of arbuscular mycorrhizal fungi increased first and then declined along with elevating Cu concentrations via FUNGuild analysis. The interactions among fungi were enhanced under light and moderate Cu contamination but weakened under heavy Cu contamination by random matrix theory (RMT)-based molecular ecological network analysis. Penicillium, identified as a keystone taxon in Cu-contaminated soils, had the function of removing heavy metals and detoxification, which might be vital to trigger the resistance of the fungal community to Cu contamination. The results may facilitate the identification of Cu pollution indicators and the development of in situ bioremediation technology for contaminated cultivated fields.

Effect of arbuscular mycorrhizal fungi in roots on antioxidant enzyme activity in leaves of Robinia pseudoacacia L. seedlings under elevated CO2 and Cd exposure

Arbuscular mycorrhizal fungi (AMF) are easily influenced by increasing atmospheric CO₂ concentration and heavy metals including cadmium (Cd), which can regulate antioxidant enzyme in host plants. Although the effect of AMF under individual conditions such as elevated CO₂ (ECO₂) and Cd on antioxidant enzyme in host plants has been reported widely, the effect of AMF under ECO₂ + Cd receives little attention. In this study, a pot experiment was conducted to study the effect of AMF community in roots on superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) activities in leaves of 135-d Robinia pseudoacacia L. seedlings under ECO₂ + Cd. The activities of SOD and CAT increased and POD activity and the richness and diversity of AMF community decreased under ECO₂ + Cd relative to Cd alone. The richness and diversity of AMF were negatively related to Cd content in roots and leaves. The richness and OTUs of AMF community positively and AMF gene abundance negatively affected POD activity under the combined treatments. Superoxide dismutase and POD activities were negatively and positively related to Archaeospora and Scutellospora, respectively, under ECO₂ + Cd. Cadmium in roots and leaves was negatively and significantly related to Glomus, Scutellospora, and Claroideoglomus abundance under ECO₂ + Cd. Overall, AMF diversity and Archaeospora and Scutellospora in roots significantly influenced SOD, POD, and CAT activities. The response of AM symbiosis to ECO₂ might regulate antioxidant capacity in host plants upon Cd exposure. Glomus, Scutellospora, and Claroideoglomus might be applied to phytoremediation of Cd-contaminated soils.

Metabolic alterations elicited by Cd and Zn toxicity in Zea mays with the association of Claroideoglomus claroideum

The concentrations of cadmium (Cd) and zinc (Zn) in arable lands exceed the maximum permissible levels due to the excessive use of phosphorus fertilizers and fungicides by farmers. The increasing issues related to the application of agrochemicals have lead to the demand for the implementation of sustainable agricultural approaches. Association of arbuscular mycorrhizae with crop plants is an appropriate strategy due to the potential of these microorganisms to augment the metals tolerance of plants through the immobilization of Cd and Zn in an eco-friendly manner. In the present study, 45 d old Zea mays (var. CoHM6) plants inoculated with AM fungi (Claroideoglomus claroideum) were exposed to 1.95 g Zn Kg^-1 soil and 0.45 g Cd Kg^-1 soil. The major objective of this study was to determine the metabolic alterations in the leaves and roots of mycorrhizal and non-mycorrhizal plants exposed to CdCl₂ and ZnSO₄. Both non AM and AM plants exhibited alterations in the quantity of primary and secondary metabolites on exposure to Zn and Cd toxicity. Moreover, Zn and Cd-induced accumulation of γ-sitosterol reduced the quantity of neophytadiene (a well-known terpenoid) and aided the production of 3-β-acetoxystigmasta-4,6,22-triene in maize leaves. Mycorrhization and heavy metal toxicity induced significant metabolic changes in the roots by producing 4,22-stigmastadiene-3-one, eicosane, 9,19-cyclolanost-24-en-3-ol, pentacosane, oxalic acid, heptadecyl hexyl ester, l-norvaline, and n-(2-methoxyethoxycarbonyl). In addition, the metal-induced variations in leaf and root lignin composition were characterized with the aid of the FTIR technique. Mycorrhization improved the tolerance of maize plants to Cd and Zn toxicity by stabilizing these metal ions in the soil and/or limiting their uptake into the plants, thus ensuring normal metabolic functions of their roots and shoots.

Radiocaesium in Tricholoma spp. from the Northern Hemisphere in 1971-2016

A considerable amount of data has been published on the accumulation of radiocaesium (¹³⁴Cs and particularly, ¹³⁷Cs) in wild fungi since the first anthropogenically influenced releases into the environment due to nuclear weapon testing, usage and subsequently from major accidents at nuclear power plants in Chernobyl (1986) and Fukushima (2011). Wild fungi are particularly susceptible to accumulation of radiocaesium and contamination persists for decades after pollution events. Macromycetes (fruiting bodies, popularly called mushrooms) of the edible fungal species are an important part of the human and forest animal food-webs in many global locations. This review discusses published occurrences of ¹³⁴Cs and ¹³⁷Cs in twenty four species of Tricholoma mushrooms sourced from the Northern Hemisphere over the last five decades, but also includes some recent data from Italy and Poland. Tricholoma are an ectomycorrhizal species and the interval for contamination to permeate to lower soils layers which host their mycelial networks, results in a delayed manifestation of radioactivity. Available data from Poland, over similar periods, may suggest species selective differences in accumulation, with some fruiting bodies, e.g. T. portentosum, showing lower activity levels relative to others, e.g. T. equestre. Species like T. album, T. sulphurescens and T. terreum also show higher accumulation of radiocaesium, but reported observations are few. The uneven spatial distribution of the data combined with a limited number of observations make it difficult to decipher any temporal contamination patterns from the observations in Polish regions. When data from other European sites is included, a similar variability of ¹³⁷Cs activity is apparent but the more recent Ukrainian data appears to show relatively lower activities. ⁴⁰K activity in mushrooms which is associated with essential potassium, remains relatively constant. Further monitoring of ¹³⁷Cs activity in wild mushrooms would help to consolidate these observations.

Ozone does not diminish the beneficial effects of arbuscular mycorrhizas on Medicago sativa L. in a low phosphorus soil

Enriched surface ozone (O₃) can impose harmful effects on plants. Conversely, arbuscular mycorrhizal (AM) symbiosis can enhance plant tolerance to various environmental stresses and facilitate plant growth. The interaction of AM fungi and O₃ on plant performance, however, seldom has been investigated. In this study, alfalfa (Medicago sativa L.) was used as a test plant to study the effects of O₃ and AM symbiosis on plant physiology and growth under two O₃ levels (ambient air and elevated O₃ with 60 nmol·mol^-1 O₃ enrichment) and three AM inoculation treatments (inoculation with exogenous or indigenous AM fungi and non-inoculation control). The results showed that elevated O₃ decreased plant net photosynthetic rate and biomass, and increased malondialdehyde concentration, while AM inoculation (with both exogenous and indigenous AM fungi) could promote plant nutrient acquisition and growth irrespective of O₃ levels. The positive effects of AM symbiosis on plant nutrient acquisition and antioxidant enzyme (superoxide dismutase and peroxidase) activities were most likely offset by increased stomatal conductance and O₃ intake. As a result, AM inoculation and O₃ generally showed no significant interactions on plant performance: although elevated O₃ did not diminish the beneficial effects of AM symbiosis on alfalfa plants, AM symbiosis also did not alleviate the harmful effects of O₃ on plants.

Citric acid and AMF inoculation combination-assisted phytoextraction of vanadium (V) by Medicago sativa in V mining contaminated soil

The use of citric acid (CA) chelator to facilitate metal bioavailability is a promising approach for the phytoextraction of heavy metal contaminants. However, the role of the CA chelator associated with arbuscular mycorrhizal fungi (AMF) inoculation on phytoextraction of vanadium (V) has not been studied. Therefore, in this study, a greenhouse pot experiment was conducted to evaluate the combined effect of CA chelator and AMF inoculation on growth performance and V phytoextraction of plants in V-contaminated soil. The experiment was performed via CA (at 0, 5, and 10 mM kg^-1 soil levels) application alone or in combination with AMF inoculation by Medicago sativa Linn. (M. sativa). Plant biomass, root mycorrhizal colonization, P and V accumulation, antioxidant enzyme activity in plants, and soil chemical speciation of V were evaluated. Results depicted (1) a marked decline in plant biomass and root mycorrhizal colonization in 5- and 10-mM CA treatments which were accompanied by a significant increased V accumulation in plant tissues. The effects could be attributed to the enhanced acid-soluble V fraction transferring from the reducible fraction. (2) The presence of CA significantly enhanced P acquisition while the P/V concentration ratio in plant shoots and roots decreased, owing to the increased V translocation from soil to plant. (3) In both CA-treated soil, AMF-plant symbiosis significantly improved dry weight (31.4-73.3%) and P content (37.3-122.5%) in shoots and roots of M. sativa. The combined treatments also showed markedly contribution in reduction of malondialdehyde (MDA) content (12.8-16.2%) and higher antioxidants (SOD, POD, and CAT) activities in the leaves. This suggests their combination could promote growth performance and stimulate antioxidant response to alleviate V stress induced by CA chelator. (4) Taken together, 10 mM kg^-1 CA application and AMF inoculation combination exhibited a higher amount of extracted V both in plant shoots and roots. Thus, citric acid-AMF-plant symbiosis provides a novel remediation strategy for in situ V phytoextraction by M. sativa in V-contaminated soil.

Mycorrhiza and Iron Tailings Synergistically Enhance Maize Resistance to Arsenic on Medium Arsenic-Polluted Soils Through Increasing Phosphorus and Iron Uptake

Agricultural arsenic (As, CAS. No. 7440-38-2) over the issue of pollution has been related to people's livelihood, security and moderate use of As contaminated soil is an important aspect of contaminated soil remediation. In this potted plant experiment, synergistic effects of arbuscular mycorrhizal fungi (AMF) Funneliformis mosseae and iron (Fe, CAS. No. 7439-89-6) oxides on plant growth and phosphorus (P, CAS. No. 7723-14-0), As and Fe uptake by maize (Zea mays L.) were studied on simulating medium As-polluted soils in greenhouse. Different amounts (0, 5, 10, 20, 40 g kg^- 1) of iron tailings (IT) were added. The results showed that IT20 and IT40 addition significantly increased mycorrhizal infection rate, plant biomass, root length and P, Fe uptake under FM treatment; IT40 addition decreased As concentration in roots. In addition, FM inoculation increased biomass, root length and P uptake by shoots, but decreased Fe and As concentration in shoots. Therefore, the combined FM inoculation and IT40 addition promoted maize growth and decreased As concentration in shoots by decreasing As absorption efficiency, increasing P and Fe uptake and P/As ratio.

Impacts of biochar-based fertilization on soil arbuscular mycorrhizal fungal community structure in a karst mountainous area

The application of biochar-based fertilizer can improve soil properties in part by stimulating microbial activity and growth. Karst ecosystems, which make up large areas of Southwest China, are prone to degradation. Understanding the response of arbuscular mycorrhizal fungal (AMF) community structure to biochar-based fertilizer application is of great significance to karst soil restoration. A field experiment was conducted in a typical karst soil (calcareous sandy loam) in Southwest China. A high-throughput sequencing approach was used to investigate the effect of biochar-based fertilization on AMF community structure in the karst soil. With the control (CK), compost with NPK fertilizer (MF), biochar (B), a lower amount of biochar with compost and NPK fertilizer (B1MF), biochar with compost and NPK fertilizer (BMF), and a higher amount of biochar with compost and NPK fertilizer (B4MF), the field trials were set up for 24 months. Soil amendments increased soil nutrient content and AMF diversity. The composition and structure of the AMF community varied among the treatments. AMF community composition was significantly impacted by soil chemical properties such as TC (total carbon), TN (total nitrogen), TP (total phosphorus), and AP (available phosphorus). Furthermore, network analysis showed that biochar-based fertilization increased the scale and complexity of the microbial co-occurrence network. Biochar-based fertilization enabled more keystone species (such as order Diversisporales and Glomerales) in the soil AMF network to participate in soil carbon resource management and soil nutrient cycling, indicating that biochar-based fertilizer is beneficial for the restoration of degraded karst soils.

Unraveling the effects of arbuscular mycorrhizal fungi on cadmium uptake and detoxification mechanisms in perennial ryegrass (Lolium perenne)

Cadmium (Cd) is a major environmental pollutant and one of the most toxic metals in the environment. Arbuscular mycorrhizal fungi (AMF) assisted phytoremediation can be used to remove Cd from polluted soils but the role of AMF, which mediate in Cd accumulation and tolerance, remains poorly understood. Here we inoculated Lolium perenne with two different AMF species (Glomus etunicatum and Glomus mosseae). Mycorrhizal L. perenne and non-mycorrhizal controls were exposed to Cd stress and we tested the effects of AMF mycorrhization on Cd uptake and subsequent tolerance, as well as the underlying mechanisms. Mycorrhizal infection increased root Cd²⁺ uptake and we observed that net Cd²⁺ influx was coupled with net Ca²⁺ influx. The inactivation of Ca²⁺ transporter channels decreased Cd²⁺ uptake in non-inoculated roots to a greater extent than in inoculated roots, indicating that AMF activates additional ion transport channels. In consequence, inoculated plants exhibited higher Cd accumulation in both roots and shoots than non-inoculated controls. However, AMF-inoculated plants showed higher chlorophyll concentrations, photosynthesis, and growth under Cd, indicating lower Cd toxicity in AMF-inoculated plants, despite the increase in Cd uptake. We observed that AMF-inoculated favored the isolation of Cd within cell walls and vacuoles, and had higher concentrations of superoxide dismutase activity and glutathione concentration in roots than non-inoculated plants, consequently experiencing less stress upon Cd exposure. Our results highlight the potential and mechanism of AMF for enhancing phytoremediation of L. perenne in heavy metal contaminated environments.

Enrichment of nosZ-type denitrifiers by arbuscular mycorrhizal fungi mitigates N2 O emissions from soybean stubbles

Hotspots of N2 O emissions are generated from legume residues during decomposition. Arbuscular mycorrhizal fungi (AMF) from co-cultivated intercropped plants may proliferate into the microsites and interact with soil microbes to reduce N2 O emissions. Yet, the mechanisms by which or how mycorrhizal hyphae affect nitrifiers and denitrifiers in the legume residues remain ambiguous. Here, a split-microcosm experiment was conducted to assess hyphae of Rhizophagus aggregatus from neighbouring maize on overall N2 O emissions from stubbles of nodulated or non-nodulated soybean. Soil microbes from fields intercropped with maize/soybean amended with fertilizer nitrogen (SS-N1) or unamended (SS-N0) were added to the soybean chamber only. AMF hyphae consistently reduced N2 O emissions by 20.8%-61.5%. Generally, AMF hyphae promoted the abundance of N2 O-consuming (nosZ-type) denitrifiers and altered their community composition. The effects were partly associated with increasing MBC and DOC. By contrast, AMF reduced the abundance of nirK-type denitrifiers in the nodulated SS-N0 treatment only and that of AOB in the non-nodulated SS-N1 treatment. Taken together, our results show that AMF reduced N2 O emissions from soybean stubbles, mainly through the promotion of N2 O-consuming denitrifiers. This holds promise for mitigating N2 O emissions by manipulating the efficacious AMF and their associated microbes in cereal/legume intercropping systems.

Combined effect of putrescine and mycorrhizal fungi in phytoremediation of Lallemantia iberica in Pb-contaminated soils

As soil contamination with heavy metals is increasing and polyamines have roles in the growth of mycorrhiza and plants, it is important to study phytoremediation, growth, tolerance, and mycorrhization in Lallemantia iberica as a multi-purpose plant, by the application of putrescine along with mycorrhiza in Pb-contaminated soils. For this purpose, the study was performed in a factorial arrangement with Pb (0, 300, 600, and 900 mg Pb/kg soil), mycorrhiza (non-inoculation, Funneliformis mosseae (Fm), and Rhizophagus intraradices (Ri)), and putrescine (0, 0.5, and 1 mM) in a greenhouse. Results showed that antioxidant activities, plant Pb, and mycorrhizal features enhanced, while transfer factor (TF), biomass, and tolerance decreased under Pb levels. Mycorrhiza improved growth, greenness, defense, and tolerance and reduced TF, Pb, and H₂O₂ content under Pb stress. Putrescine (0.5 mM) increased catalase activity, biomass, and colonization and reduced Pb content and TF under Pb levels. Combination of 0.5 mM putrescine with Fm increased shoot biomass (13%), peroxidase (17.2%), root P (7.5%), shoot tolerance (14.4%), colonization (5.1%), and hyphal width (5.5%) and decreased malondialdehyde (20.5%) and shoot Pb content (28.1%). Putrescine (1 mM) had negative effects on all traits in combination with Ri but not with Fm. Combination of putrescine and Fm showed more efficiency in decreasing Pb content in L. iberica and was effective in phytostabilization. It is generally concluded that 0.5 mM putrescine was the beneficial concentration in combination with mycorrhiza, Pb stress, and single use to improve plant performance, and Fm was a useful species for improving the growth and tolerance of L. iberica under Pb levels.

Demonstrational gardens with EDTA-washed soil. Part II: Soil quality assessment using biological indicators

In this study, we evaluated the impact of washing of Pb, Zn and Cd contaminated soil using EDTA-based technology (ReSoil®) on soil biological properties by measuring some of the most commonly used/sensitive biological indicators of soil perturbation. We estimated the temporal dynamics of the soil respiration, the activities of soil enzymes (dehydrogenase, β-glucosidase, urease, acid and alkaline phosphatase), and the effect of the remediation process on arbuscular mycorrhizal (AM) fungi in original (Orig), remediated (Rem) and remediated vitalized (Rem+V) soils during a more than one-year garden experiment. ReSoil® technology initially affected the activity level of soil microbial respiration and all enzyme activities except urease and reduced AM fungal potential in the soil. However, after one year of vegetable cultivation and standard gardening practices, soil microbial respiration, acid and alkaline phosphatase in the Rem and Rem+V reached similar activities as in the Orig. Only the activities of dehydrogenase and β-glucosidase remained lower in the remediated soil compared to the Orig. The frequency of arbuscular mycorrhiza in the root system, arbuscular density in the colonized root fragment, and the intensity of mycorrhizal colonization in the colonized root fragments in the remediated treatments increased with time; at the end of the experiment, no consistent differences in these parameters of mycorrhizal colonization were found among the treatments. Our results suggest a restored biological functioning of the remediated soil after one year of vegetable cultivation. In general, no differences were found between the Rem and Rem+V treatments, indicating that simple common garden practices are sufficient to restore soil functioning after remediation.

Plant response to mycorrhizal inoculation and amendments on a contaminated soil

Understanding the combined effects of soil amendments and inoculation of mycorrhizal fungi on the response of different plant species during the phytostabilization process of trace elements contaminated soils is a challenge. This task is more difficult but more realistic when studied under field conditions. We assess the combined effects of two amendment doses and mycorrhizal inoculation on the response of saplings of two tree species planted in a contaminated field. The amendments were a mix of sugar beet lime and biosolid compost. The inoculation treatments were made with a commercial inoculum of arbuscular mycorrhizal fungi for wild olive and ectomycorrhizal fungi for stone pine. Results showed a weak or null effect of the mycorrhizal inoculation on plant growth, survival and trace element accumulation. There was a significant increase on P nutrition for stone pine, growing on non-amended conditions. Soil amendments were very effective reducing trace elements availability and their accumulation in both plant species, especially in roots. However, the effects on plant biomass were species-dependent and contrasted; low-dose amendments increased the biomass of wild olive by 33.3%, but reduced by 28% that of pine. The high doses of amendments (60 T ha^-1) produced some negative effects on plant growth and nutrition, probably related to the increase of soil salinity. Both plant species, stone pine and wild olive, have been proved to be adequate for phytostabilization of contaminated soils under Mediterranean climate, due to their drought tolerance and the low transfer of trace elements from root to shoot, thus reducing toxicity for the food web. To implement microbial-assisted phytoremediation approaches, a better understanding of the diversity and ecology of plant-associated microorganisms is needed. The use of indigenous fungi, locally adapted and tolerant to contamination, would be more suitable for phytostabilization purposes.

Differential responses of 23 maize cultivar seedlings to an arbuscular mycorrhizal fungus when grown in a metal-polluted soil

Modern breeding efforts have been accelerating crop improvement and yielding numerous cultivars with distinct genetic traits; however, interactions between different cultivars and their root-associated arbuscular mycorrhizal fungi (AMF) are not clear. Herein, we selected the 22 most common commercial maize (Zea mays) varieties in China and an inbred line (B73) to study the differential responses of these 23 cultivars to mycorrhizal inoculation when grown in an arable soil polluted by multiple metals (Pb, Zn, and Cd). We found that the different cultivars exhibited significant variations in plant metal accumulation, ranging from strong metal exclusion (ZYY9) to strong metal accumulation (B73). Mycorrhizal colonization substantially altered metal uptake and repartitioning, while bioaugmenting the inherent characteristics of metal accumulation; for example, the AMF enhanced leaf accumulation of the metal-accumulator B73, and markedly reduced the root uptake of the metal-excluder ZYY9. However, such AMF-induced alterations were also substantially dependent on plant organs (roots and shoots) and metal species. We found that the extent of the AMF-induced leaf alterations was substantially greater than that of the root alterations. Similarly, the number of instances where the AMF significantly altered the Zn and Cd accumulation was far higher than the number of instances where Pb accumulation was significantly altered by AMF. In addition, the presence of AMF appeared to trigger the maize antioxidant systems, which may have alleviated the toxicity of excessive Cd, increased the leaf chlorophyll content, augmented the net photosynthetic rate, and promoted the growth of 17.39% of the maize cultivars. Our results suggest that a future crop breeding challenge is to produce cultivars for safe production or phytoremediation, thereby optimizing the combinations of crop cultivars and their root-associated AMF in slightly to moderately metal-polluted arable soils.

Nitrogen isotopes in the soil-to-tree continuum - Tree rings express the soil biogeochemistry of boreal forests exposed to moderate airborne emissions

Anthropogenic N emissions represent a potential threat for forest ecosystems, and environmental indicators that provide insight into the changing forest N cycle are needed. Tree ring N isotopic ratios (δ¹⁵N) appear as a contentious choice for this role as the exact mechanisms behind tree-ring δ¹⁵N changes seldom benefit from a scrutiny of the soil-to-tree N continuum. This study integrates the results from the analysis of soil chemistry, soil microbiome genomics, and δ¹⁵N values of soil N compounds, roots, ectomycorrhizal (EcM) fungi and recent tree rings of thirteen white spruce trees sampled in five stands, from two regions exposed to moderate anthropogenic N emissions (3.9 to 8.1 kg/ha/y) with distinctive δ¹⁵N signals. Our results reveal that airborne anthropogenic N with distinct δ¹⁵N signals may directly modify the NO₃^- δ¹⁵N values in surface soils, but not the ones of NH₄⁺, the preferred N form of the studied trees. Hence, the tree-ring δ¹⁵N values reflect specific soil N conditions and assimilation modes by trees. Along with a wide tree-ring δ¹⁵N range, we report differences in: soil nutrient content and N transformation rates; δ¹⁵N values of NH₄⁺, total dissolved N (TDN) and EcM mantle enveloping the root tips; and bacterial and fungal community structures. We combine EcM mantle and root δ¹⁵N values with fungal identification to infer that hydrophobic EcM fungi transfer N from the dissolved organic N (DON) pool to roots under acidic conditions, and hydrophilic EcM fungi transfer various N forms to roots, which also assimilate N directly under less acidic conditions. Despite the complexities of soil biogeochemical properties and processes identified in the studied sites, in the end, the tree-ring δ¹⁵N averages inversely correlate with soil pH and anthropogenic N inputs, confirming white spruce tree-ring δ¹⁵N values as a suitable indicator for environmental research on forest N cycling.

Effect of cadmium on sorghum root colonization by glomeral fungi and its impact on total and easily extractable glomalin production

Glomalin is a specific glycoprotein compound synthesized by glomeral fungi and its amount varies according to soil conditions. Due to the symbiosis of these fungi with a multitude of plants, a remarkable amount of glomalin is produced inside the hypha wall of these fungi. Furthermore, while increasing the symbiosis stability, glomalin plays a critical role in reducing the availability of potentially toxic elements (PTEs) through their fixation. In this regard, soil contamination with PTEs such as cadmium (Cd) affects the glomalin production of mycorrhiza fungi. In order to investigate the effect of different levels of Cd on glomalin production of three species of arbuscular mycorrhiza fungi in the presence of sorghum, a greenhouse experiment was conducted in a completely randomized design with factorial arrangement and three repetitions. Factors include four levels of Cd (0, 5, 10, 20 milligrams per kilogram of soil) and second factor included these three types of mycorrhiza: Funneliformis mosseae (FM), Rhizophagus intraradices (RI), and Claroideoglomus etunicatum (CE). The results of this study showed that when increasing soil Cd concentration, shoot dry weight, root colonization percentage, total and easily extractable glomalin decreased while leaf proline concentration, shoot, and root Cd concentration increased. The presence of mycorrhiza in comparison with the control increased the dry weight of shoots and plant height. The results showed that mycorrhizal treatments increased total and easily extractable glomalin compared to the absence of mycorrhiza. Also, the highest amount of glomalin production was observed in two species of FM and CE. Complexation of Cd by total glomalin and easily extractable glomalin was higher in two species of FM and CE rather than RI. The results showed that the use of three species of mycorrhizae reduced the adverse effects of high levels of Cd. Also, the concentration of leaf proline, soluble sugars, shoot, and root Cd concentration was higher in presence of mycorrhizal fungal species than in plant not inoculated with mycorrhizal fungal species. This increase was significant in both FM and CE species. Finally, a notable positive correlation was also observed between glomalin measured by Bradford method and percentage of root colonization.

Arbuscular mycorrhizal fungi potentially regulate N2O emissions from agricultural soils via altered expression of denitrification genes

Agricultural soils are an important source of nitrous oxide (N₂O), a potent greenhouse gas involved in the destruction of the protective ozone layer that contributes to global warming. During N₂O production, soil microorganisms play important driving and regulating roles. A few recent studies have revealed the potential effects of arbuscular mycorrhizal fungi (AMF), a widely distributed soil fungi, on controlling N₂O emissions. However, how AMF regulate N₂O production from soils remains poorly understood. To address the knowledge gap, we manipulated two independent soil environments, which were either allowed (AM) or prevented (NM) access by AMF hyphae in a microcosm experiment (n = 5). Soil physicochemical properties, N₂O flux, the diversity of bacterial communities, and the abundance of key genes responsible for N₂O production were assessed in both treatments over three months. Results showed that the presence of AMF significantly decreased N₂O emissions from agricultural soils in the 1st month, and the abundance of key genes responsible for denitrification (nirK and nosZ) significantly decreased in AM treatments, indicating that the regulation of N₂O emissions is transmitted by AMF-induced changes in the denitrification process. A structural equation model further revealed that AMF indirectly influenced N₂O emissions by altering the abundance of N metabolism-related genes, rather than by altering soil chemical properties or the diversity of bacterial communities. Thus, we proposed a possible mechanism by which AMF can regulate denitrification activities and therefore N₂O emissions from agricultural soils.

Improvement of alfalfa resistance against Cd stress through rhizobia and arbuscular mycorrhiza fungi co-inoculation in Cd-contaminated soil

Rhizobia and arbuscular mycorrhiza fungi (AMF) are important symbiotic microbes that are advantageous to plants growing in metal-contaminated soil. However, it remains unclear how inoculated microbes affect rhizosphere microbial communities or whether subsequent changes in rhizosphere microbiomes contribute to improving plant resistance under metal stress. This study investigated the effects of rhizobia and AMF inoculation on alfalfa resistance to Cd stress. The response of rhizosphere microbial communities to inoculation and its role in increasing alfalfa' ability to cope with stress were further analyzed using high-throughput sequencing of 16S and ITS rRNA genes. Results showed that single rhizobia or AMF inoculation significantly improved alfalfa resistance to Cd stress, while their co-inoculation resulted in the greatest overall improvement. Improved resistance was reflected by the significant mitigation of Cd-induced lipid peroxidation and reactive oxygen species (ROS) stress caused by increases in antioxidant enzyme activities along with co-inoculation. Furthermore, co-inoculation significantly altered the rhizosphere microbial community structure by decreasing fungal community diversity and increasing bacterial community diversity. Results of partial least squares path modeling (PLS-PM) and variation partitioning analysis (VPA) showed that the rhizosphere bacterial community predominated over the fungal community with respected to improvements in resistance to Cd stress under the co-inoculation treatments. This improvement was specifically seen in the enrichment of certain key bacterial taxa (including Proteobacteria, Actinobacteria, Acidobacteria, and Chloroflexi) induced by the rhizobia and AMF co-inoculation, enhancing alfalfa' ability to uptake rhizosphere nutrients and reduce its release of photosynthetically-derived carbon (C) into soil. Our findings revealed that the co-inoculation of multiple symbiotic microbes can assist plants to effectively cope with Cd stress, providing a greater understanding of rhizosphere bacterial taxa in the microbe-induced phytomanagement.

Effects of elevated CO2 on arbuscular mycorrhizal fungi associated with Robinia pseudoacacia L. grown in cadmium-contaminated soils

As symbionts capable of reciprocal rewards, arbuscular mycorrhizal fungi (AMF) can alleviate heavy metal toxicity to host plants and are easily influenced by elevated CO₂ (ECO₂). Although the individual effects of ECO₂ and cadmium (Cd) on AMF have been widely reported, the response of AMF to ECO₂ + Cd receives little attention. We evaluated the combined effects of ECO₂ and Cd on AMF in the rhizosphere soil and roots of Robinia pseudoacacia L. seedlings. Under ECO₂ + Cd relative to Cd, AMF gene copies and richness in rhizosphere soils increased (p < 0.05) and the diversity reduced (p < 0.05) at 4.5 mg Cd kg^-1 dry soil; whereas root AMF abundance at 4.5 mg Cd kg^-1 dry soil and the diversity and richness reduced (p < 0.05). Elevated CO₂ caused obvious differences in the dominant genera abundance between rhizosphere soils and roots upon Cd exposure. Responses of C, water-soluble organic nitrogen (WSON), pH, and diethylene triamine penta-acetic acid (DTPA)-Cd in rhizosphere soils and root N to ECO₂ shaped dominant genera in Cd-polluted rhizosphere soils. Levels of DTPA-Cd, WSON, C and pH in rhizosphere soils and C/N ratio, N, and Cd in roots to ECO₂ affected (p < 0.05) dominant genera in roots under Cd exposure. AMF richness and diversity were lower in roots than in rhizosphere soils. Elevated CO₂ altered AMF communities in rhizosphere soils and roots of R. pseudoacacia seedlings exposed to Cd. AMF associated with R. pseudoacacia may be useful/interesting to be used for improving the phytoremediation of Cd under ECO₂.

Partial and full mycoheterotrophy in green and albino phenotypes of the slipper orchid Cypripedium debile

Most green orchids form mycorrhizal associations with rhizoctonia fungi, a polyphyletic group including Serendipitaceae, Ceratobasidiaceae, and Tulasnellaceae. Although accumulating evidence indicated that partial mycoheterotrophy occurs in such so-called rhizoctonia-associated orchids, it remains unclear how much nutrition rhizoctonia-associated orchids obtain via mycoheterotrophic relationships. We investigated the physiological ecology of green and albino individuals of a rhizoctonia-associated orchid Cypripedium debile, by using molecular barcoding of the mycobionts and stable isotope (¹³C and ¹⁵ N) analysis. Molecular barcoding of the mycobionts indicated that the green and albino individuals harbored Tulasnella spp., which formed a clade with the previously reported C. debile mycobionts. In addition, stable isotope analysis showed that both phenotypes were significantly enriched in ¹³C but not in ¹⁵ N. Therefore, green and albino individuals were recognized as partial and full mycoheterotrophs, respectively. The green variants were estimated to obtain 42.5 ± 8.2% of their C from fungal sources, using the ¹³C enrichment factor of albino individuals as a mycoheterotrophic endpoint. The proportion of fungal-derived C in green C. debile was higher than that reported in other rhizoctonia-associated orchids. The high fungal dependence may facilitate the emergence of albino mutants. Our study provides the first evidence of partial mycoheterotrophy in the subfamily Cypripedioideae. Partial mycoheterotrophy may be more general than previously recognized in the family Orchidaceae.

Physiological and biochemical responses of soybean plants inoculated with Arbuscular mycorrhizal fungi and Bradyrhizobium under drought stress

BACKGROUND

The present study aims to study the effects of biofertilizers potential of Arbuscular Mycorrhizal Fungi (AMF) and Bradyrhizobium japonicum (B. japonicum) strains on yield and growth of drought stressed soybean (Giza 111) plants at early pod stage (50 days from sowing, R3) and seed development stage (90 days from sowing, R5).

RESULTS

Highest plant biomass, leaf chlorophyll content, nodulation, and grain yield were observed in the unstressed plants as compared with water stressed-plants at R3 and R5 stages. At soil rhizosphere level, AMF and B. japonicum treatments improved bacterial counts and the activities of the enzymes (dehydrogenase and phosphatase) under well-watered and drought stress conditions. Irrespective of the drought effects, AMF and B. japonicum treatments improved the growth and yield of soybean under both drought (restrained irrigation) and adequately-watered conditions as compared with untreated plants. The current study revealed that AMF and B. japonicum improved catalase (CAT) and peroxidase (POD) in the seeds, and a reverse trend was observed in case of malonaldehyde (MDA) and proline under drought stress. The relative expression of the CAT and POD genes was up-regulated by the application of biofertilizers treatments under drought stress condition. Interestingly a reverse trend was observed in the case of the relative expression of the genes involved in the proline metabolism such as P5CS, P5CR, PDH, and P5CDH under the same conditions. The present study suggests that biofertilizers diminished the inhibitory effect of drought stress on cell development and resulted in a shorter time for DNA accumulation and the cycle of cell division. There were notable changes in the activities of enzymes involved in the secondary metabolism and expression levels of GmSPS1, GmSuSy, and GmC-INV in the plants treated with biofertilizers and exposed to the drought stress at both R3 and R5 stages. These changes in the activities of secondary metabolism and their transcriptional levels caused by biofertilizers may contribute to increasing soybean tolerance to drought stress.

CONCLUSIONS

The results of this study suggest that application of biofertilizers to soybean plants is a promising approach to alleviate drought stress effects on growth performance of soybean plants. The integrated application of biofertilizers may help to obtain improved resilience of the agro ecosystems to adverse impacts of climate change and help to improve soil fertility and plant growth under drought stress.

A review on metal-based nanoparticles and their toxicity to beneficial soil bacteria and fungi

The unregulated deposition of metal-based nanoparticles in terrestrial ecosystems particularly in agricultural systems has alarmingly threatened the sustainability of the environment and diversity of beneficial microbial populations such as soil bacteria and fungi. This occurs due to the poor treatment of biosolids during wastewater treatment and their application in agricultural fields to enhance the fertility of soils. Continuous deposition, low biodegradability, and longer persistence of metal nanoparticles in soils adversely impact the population of soil beneficial bacteria and fungi. The current literature suggests the toxic outcome of nanoparticle-fungi and nanoparticle-bacteria interactions based on various toxicity endpoints. Therefore, due to the extreme importance of beneficial soil bacteria and fungi for soil fertility and plant growth, this review summarizes the production, application, release of metal nanoparticles in the soil system and their impact on various soil microbes specifically plant growth-promoting rhizobacteria, cellular toxicity and impact of nanoparticles on bioactive molecule production by microbes, destructive nanoparticle impact on unicellular, mycorrhizal, and cellulose/lignin degrading fungi. This review also highlights the molecular alterations in fungi and bacteria-induced by nanoparticles and suggests a plausible toxicity mechanism. This review advances the understanding of the nano-toxicity aspect as a common outcome of nanoparticles and fungi/bacteria interactions.

Arbuscular mycorrhizal fungus-induced decrease in nitrogen concentration in pore water and nitrogen leaching loss from red soil under simulated heavy rainfall

Arbuscular mycorrhizal fungus (AMF) is generally colonized in plant roots and influences the migration of mineral elements such as nitrogen (N) in soils. However, its effect on N leaching loss in red soils is limited. In the present study, red soils were collected from wasteland, farmland, and slopeland in the Yunnan Plateau. Maize, as a host plant, was cultured in a dual-compartment cultivation system. There were mycorrhizal and hyphal compartments for the AMF inoculation treatment and root and soil compartments for the non-inoculation treatment. The N concentration and uptake in maize, N concentration in pore water within two depth (0-20 and 20-40 cm), and N leaching losses from soil under simulated heavy rainfall (40 and 80 mm/h) were analyzed. Results showed that AMF inoculation significantly enhanced the biomass and N uptake in maize. Compared with the root and soil compartments, the N concentrations in pore water and their leaching losses from the mycorrhizal and hyphal compartments were decreased by 48-77% and 51-74%, respectively. Moreover, significant or extremely significantly positive correlations were observed between the N concentrations in pore water with the N leaching losses from soil. The three-way ANOVA showed that AMF highly significantly decreased N concentrations in pore water and their leaching losses from wasteland, farmland, and slopeland; rainfall intensity had strong influences on the N concentration in pore water from farmland and N leaching losses from wasteland and farmland, whereas the maize root's effect was insignificant. The study indicated that the AMF-induced decreases in the N leaching loss from red soils were caused by increased N uptake by maize and decreased N concentrations in pore water. These results have implications for reducing nutrient leaching loss through the management of beneficial microorganisms in soils.

Suppression of arbuscular mycorrhizal fungi decreases the temporal stability of community productivity under elevated temperature and nitrogen addition in a temperate meadow

Global change alters how terrestrial ecosystems function and makes them less stable over time. Global change can also suppress the development and effectiveness of arbuscular mycorrhizal fungi (AMF). This is concerning, as AMF have been shown to alleviate the negative influence of global changes on plant growth and maintain species coexistence. However, how AMF and global change interact and influence community temporal stability remains poorly understood. Here, we conducted a 4-year field experiment and used structural equation modeling (SEM) to explore the influence of elevated temperature, nitrogen (N) addition and AMF suppression on community temporal stability (quantified as the ratio of the mean community productivity to its standard deviation) in a temperate meadow in northern China. We found that elevated temperature and AMF suppression independently decreased the community temporal stability but that N addition had no impact. Community temporal stability was mainly driven by elevated temperature, N addition and AMF suppression that modulated the dominant species stability; to a lesser extent by the elevated temperature and AMF suppression that modulated AMF richness associated with community asynchrony; and finally by the N addition and AMF suppression that modulated mycorrhizal colonization. In addition, although N addition, AMF suppression and elevated temperature plus AMF suppression reduced plant species richness, there was no evidence that changes in community temporal stability were linked to changes in plant richness. SEM further showed that elevated temperature, N addition and AMF suppression regulated community temporal stability by influencing both the temporal mean and variation in community productivity. Our results suggest that global environmental changes may have appreciable consequences for the stability of temperate meadows while also highlighting the role of belowground AMF status in the responses of plant community temporal stability to global change.

Factors that influence the absorption of uranium by indigenous plants on the spoil tip of an abandoned mine in western Spain

The purpose of this work was to study the factors affecting the absorption of U by plants growing on the spoil tip of an abandoned mine in western Spain. The plant species were selected based on how palatable they were to livestock and were sampled for four consecutive years during which, we also recorded rainfall data. The factors related to the plants studied were the leaf size and the percentage and characteristics of the arbuscular mycorrhizae (AM) fungi present in their roots. Our results showed a correlation between the annual rainfall and the U concentration in the plants. The percentage of mycorrhization and AM vesicles is a predominant factor in the uptake of U by plants. Spergularia rubra (L.) J.Presl & C.Presl, which is resistant to mycorrhization, contained higher U concentrations relative to the plants that grew with AM mycorrhization. The absorption curves of the different plants studied indicated that these plants were tolerant to ²³⁸U from 875 Bq kg^-1 (70 mg kg^-1), with a hormesis effect below that concentration. The annual U removal was 0.068%, suggesting that AM are responsible for limiting the incorporation of U into the food chain, favouring its retention in the soil and preventing its dispersion.

Combined effects of arbuscular mycorrhizae fungus and composted pig manure on the growth of ryegrass and uptake of Cd and Zn in the soil from an e-waste recycling site

Little attention has been paid to the combined effects of arbuscular mycorrhizae (AM) fungus and composted manure on heavy metal bioavailability and its uptake by plants grown in heavy metal-contaminated soils from electronic-waste (e-waste) recycling sites. A greenhouse pot experiment was conducted to investigate the effects of AM fungus, composted pig manure (CM) and AM fungus + CM (ACM) on the growth of ryegrass and uptake of Cd and Zn in the soil collected from an e-waste recycling site. The calcium chloride (CaCl₂) and Tessier sequential extraction procedure were adopted to evaluate the bioavailability and chemical speciation of Cd and Zn in the soil. Results showed that the application of CM and ACM significantly increased the pH but decreased the CaCl₂-extractable Cd and Zn concentrations in the rhizosphere and bulk soils. ACM treatment significantly shifted Cd from exchangeable fraction to other more stable fractions, and transformed the exchangeable Zn fraction to the carbonate-bound and reducible iron and manganese-bound fractions. Furthermore, the application of ACM can enhance the growth of plant shoots, and decrease the uptake of Cd and Zn in the ryegrass plants. This work suggests that AM fungus in combination with CM amendment may be a potential method for not only remediation of soil Cd and Zn pollution, but also reduction of Cd and Zn uptake by ryegrass grown in the soil from e-waste recycling sites.

Earthworm and arbuscular mycorrhiza interactions: Strategies to motivate antioxidant responses and improve soil functionality

Earthworms and arbuscular mycorrhizal fungi (AMF) act synergistically in the rhizosphere and may increase host plant tolerance to Cd. However, mechanisms by which earthworm-AMF-plant partnerships counteract Cd phytotoxicity are unknown. Thus, we evaluated individual and interactive effects of these soil organisms on photosynthesis, antioxidant capacity, and essential nutrient uptake by Solanum nigrum, as well as on soil quality following Cd exposure (0-120 mg kg^-1). Decreases in biomass and photosynthetic activity, as well as nutrient imbalances were observed in Cd-stressed plants; however, the addition of AMF and earthworms reversed these effects. Cd exposure increased superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities, whereas inoculation with Rhizophagus intraradices decreased those. Soil enzymatic activity decreased by 15-60% with increasing Cd concentrations. However, Cd-mediated toxicity was partially reversed by soil organisms. Earthworms and AMF ameliorated soil quality based on soil enzyme activity. At 120 mg kg^-1 Cd, the urease, catalase, and acid phosphatase activities were 1.6-, 1.4-, and 1.2-fold higher, respectively, in soils co-incubated with earthworms and AMF than in uninoculated soil. Cd inhibited shoot Fe and Ca phytoaccumulation, whereas AMF and earthworms normalized the status of essential elements in plants. Cd detoxification by earthworm-AMF-S. nigrum symbiosis was manifested by increases in plant biomass accumulation (22-117%), chlorophyll content (17-63%), antioxidant levels (SOD 10-18%, POD 9-25%, total polyphenols 17-22%, flavonoids 15-29%, and glutathione 7-61%). It also ameliorated the photosynthetic capacity, and macro- and micronutrient statuses of plants; markedly reduced the levels of malondialdehyde (20-27%), superoxide anion (29-36%), and hydrogen peroxide (19-30%); and upregulated the transcription level of FeSOD. Thus, the combined action of earthworms and AMF feasibly enhances metal tolerance of hyperaccumulating plants and improves the quality of polluted soil.