Metagenomic analysis of a thermophilic bacterial consortium and its use in the bioremediation of a petroleum-contaminated soil

Biodegradation is difficult at high temperatures due to the limited capacity of microorganisms to survive and function outside their optimum temperature range. Here, a thermophilic petroleum-degrading consortium was enriched from compost at a temperature of 55 °C. 16S rDNA and metagenomic techniques were used to analyze the composition of the consortium and the mechanisms of degradation. The consortium degraded 17000 mg total petroleum hydrocarbons (TPHs) L-1 with a degradation efficiency of 81.5% in 14 days. The consortium utilized a range of substrates such as n-hexadecane, n-docosane, naphthalene and pyrene and grew well over a wide range of pH (4-10) and salinity (0-90 g L-1). The hydrocarbon-degrading extremophilic consortium contained, inter alia, (relative abundance >1%) Caldibacillus, Geobacillus, Mycolicibacterium, Bacillus, Chelatococcus, and Aeribacillus spp. Metagenomic analysis was conducted to discover the degradation and environmental tolerance functional genes of the consortium. Two alkane hydroxylase genes, alkB and ladA, were found. A microcosm study shows that the consortium promoted the bioremediation of soil TPHs. The results indicate that the consortium may be a good candidate for the high-temperature bioremediation of petroleum-contaminated soils.

Potential high-risk release sources of thallium and arsenic from surrounding rocks of a typical thallium and arsenic mining area in southwest China

Abundant naturally and anthropogenically exposed surrounding rocks (NESRs and AESRs) in mining areas may pose persistent threats as sources of potentially toxic elements (PTEs), but this has been historically overlooked, especially for thallium (Tl) and arsenic (As). Here, the release risks of Tl and As from both NESRs and AESRs in a typical TlAs sulfide mining area were investigated. In a single leaching process, AESRs released 10.4 % of total Tl (157 μg L-1) and 32.5 % of total As (4089 μg L-1), 2-3 orders of magnitude higher than NESRs. Prolonged multiple leaching tests revealed notable and long-term risks of release of Tl and As from AESRs, associated with oxidation and dissolution of iron/sulfur-bearing minerals. Substantial release of PTEs was linked to the transformation/degradation of the -OH functional group and extensive dissolution of secondary sulfate minerals in AESRs. Ultrafiltration and STEM-EDS indicate that 18.4 % of water-extracted As released from AESRs existed as natural nanoparticles consisting of iron/sulfur-bearing minerals. This study highlights the high risks of Tl and As release from anthropogenically exposed surrounding rocks and the importance of nanoparticles in PTE transport, and provides insights into the control of PTEs in mining areas.

Differences in the activities of six soil enzymes in response to cadmium contamination of paddy soils in high geological background areas

East Yunnan province in southwest China is a region with elevated natural abundance (high geological background levels) of Cd due to high metal (loid) contents in the soils. Enzyme activities are useful indicators of metal (loid) toxicity in contaminated soils and whether Cd inhibits enzyme activities in paddy soils in high geological background areas is of considerable public concern. A pot experiment combined with field investigation was conducted to assess the effects of Cd on six soil enzymes that are essential to the cycling of C, N, and P in soils. Inhibitory effects of Cd fractions on enzyme activities were assessed using ecological dose-response models. The impact of soil properties on the inhibition of sensitive soil enzymes by Cd were assessed using linear and structural equation models. Cadmium was enriched in the paddy soils with 72.2 % of soil samples from high geological background areas exceeding the Chinese threshold values (GB 15618-2018) of Cd. Enzyme responses to Cd contamination varied markedly with a negative response by catalase but a positive response by invertase. Urease, β-glucosidase, and alkaline phosphatase activities were stimulated at low Cd concentrations and inhibited at high concentrations. The average inhibition ratios of β-glucosidase, urease, and catalase in high Cd levels were 19.9, 38.9, and 51.9%, respectively. Ecological dose-response models indicate that catalase and urease were the most Cd-sensitive of the enzymes studied and were suitable indicators of soil quality in high geological background areas. Structural equation modeling (SEM) indicates that soil properties influenced sensitive enzymes through various pathways, indicating that soil properties were factors determining Cd inhibition of enzyme activities. This suggests that Cd concentrations and soil physicochemical properties under a range of environmental conditions should be considered in addressing soil Cd pollution.

Cadmium toxicity to and accumulation in a soil collembolan (Folsomia candida): major factors and prediction using a back-propagation neural network model

Accurate prediction of cadmium (Cd) ecotoxicity to and accumulation in soil biota is important in soil health. However, very limited information on Cd ecotoxicity on naturally contaminated soils. Herein, we investigated soil Cd ecotoxicity using Folsomia candida, a standard single-species test animal, in 28 naturally Cd-contaminated soils, and the back-propagation neural network (BPNN) model was used to predict Cd ecotoxicity to and accumulation in F. candida. Soil total Cd and pH were the primary soil properties affecting Cd toxicity. However, soil pH was the main factor when the total Cd concentration was < 3 mg kg-1. Interestingly, correlation analysis and the K-spiked test confirmed nutrient potassium (K) was essential for Cd accumulation, highlighting the significance of studying K in Cd accumulation. The BPNN model showed greater prediction accuracy of collembolan survival rate (R2 = 0.797), reproduction inhibitory rate (R2 = 0.827), body Cd concentration (R2 = 0.961), and Cd bioaccumulation factor (R2 = 0.964) than multiple linear regression models. Then the developed BPNN model was used to predict Cd ecological risks in 57 soils in southern China. Compared to multiple linear regression models, the BPNN models can better identify high-risk regions. This study highlights the potential of BPNN as a novel and rapid tool for the evaluation and monitoring of Cd ecotoxicity in naturally contaminated soils.

Potential mobilization of water-dispersible colloidal thallium and arsenic in contaminated soils and sediments in mining areas of southwest China

Water-dispersible colloids (WDCs) are vital for trace element migration, but there is limited information about the abundance, size distribution and elemental composition of WDC-bound thallium (Tl) and arsenic (As) in mining-contaminated soils and sediments solutions. Here, we investigated the potential mobilization of WDC-bound Tl and As in soils and sediments in a typical Tl/As-contaminated area. Ultrafiltration results revealed on average > 60% of Tl and As in soil solution (< 220 nm) coexisted in colloidal form whereas Tl and As in sediment solution primarily existed in the truly dissolved state (< 10 kDa) due to increased acidity. Using AF4-UV-ICP-MS and STEM-EDS, we identified Fe-bearing WDCs in association with aluminosilicate minerals and organic matter were main carriers of Tl and As. SAED further verified jarosite nanoparticles were important components of soil WDC, directly participating in the migration of Tl and As. Notably, high pollution levels and solution pH promoted the release of Tl/As-containing WDCs. This study provides quantitative and visual insights into the distribution of Tl and As in WDC, highlighting the important roles of Fe-bearing WDC, soil solution pH and pollution level in the potential mobilization of Tl and As in contaminated soils and sediments.

Water-dispersible colloids facilitate the release of potentially toxic elements from contaminated soil under simulated long-term acid rain

The release behaviors of potentially toxic elements (PTEs) associated with water-dispersible colloids (WDCs) in contaminated soils are of considerable public concern. However, little information is available on the size distribution and elemental composition of WDCs and their effects on the release of PTEs in contaminated soils under long-term acid rain. Here, a quantitative accelerated aging leaching test was conducted to evaluate the long-term release risks of PTEs from four contaminated agricultural soil types exposed to acid rain. Asymmetric flow field-flow fractionation (AF4), scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) and ultrafiltration were used to clarify the size distribution and elemental composition of WDCs containing PTEs. Solution dynamics of successive leaching indicate high release potential for As, Cd, and Pb depending on soil properties under long-term (∼65 years) acid rain. Both ultrafiltration and AF4 analysis show that As in leachate was mainly in the "truly dissolved" fraction, while Pb, Cu, Cd and Fe were predominantly in the colloidal fraction and their percentages increased with increasing extraction time by acid rain. AF4-UV-ICP-MS and STEM-EDS reveal that nanoparticles at 1-7 nm most likely composed of organic matter (OM)-Fe/Al(/Si) oxides composite were the main carriers of Pb, Cu, As and Cd. Lead was also verified in Fe-oxide colloids at 34-450 nm in the first extracts but disappeared in the tenth extracts. This indicates that WDC-bearing PTEs become smaller as leaching proceeds. The study indicates the quantitative description and size-resolved understanding of WDC- and nanoparticle-bound PTEs in leachates of contaminated soils subjected to long-term acid rain.

New insights into the key role of node I in thallium accumulation in seed of coix (Coix lacryma-jobi L.)

The mechanisms underlying the distribution of many toxic metal(loid)s in shoots and metal(loid) transport to grains have been well documented in the quest for food safety but there remains a lack of knowledge on thallium (Tl) accumulation in food crops. Here, field investigations combined with a glasshouse pot experiment were conducted to investigate the characteristics of Tl distribution and accumulation in coix, a major food crop in south Guizhou province, China, and the role of node I in restricting Tl transport to the seed. Fourteen percent of coix seed samples collected from the Lanmuchang Tl-As-Hg mine contained higher Tl concentrations than the recommended limit for foods and feedstuffs in Germany (0.5 mg kg-1), with the highest exceedance rate of the metal(loid)s determined, when grown in soils surrounding the mine with a very high Tl concentration of 0.07-89.5 mg kg-1 and a general low pH of 4.19-6.48. Thallium concentrations were higher in coix nodes than in internodes, followed by roots and grains. The Tl translocation factors from node I to grains were 0.01-0.21 and were the lowest of any translocation factors between different tissues. Node I is therefore the key tissue restricting Tl transport to coix grains. Thallium was localized mainly in the diffuse vascular bundles (DVBs) in node I. The co-localization of Tl and sulfur in the DVBs and Tl contamination-induced phytochelatin (PC) accumulation indicate that Tl storage in the DVBs involving complexation with PCs in node I is an important process in Tl accumulation in coix grains. Moreover, the area of DVBs in node I increased with increasing soil Tl pollution level, providing more channels for Tl transport to the panicles and grains and thereby acting as a key factor restricting Tl transport to the grains. These results provide new insights into the key role of node I in Tl accumulation in coix grains and indicate key points to minimize Tl accumulation in grains for food safety.

Cadmium and zinc isotope compositions indicate metal sources and retention mechanisms in different soil particle size fractions

Soil particle size may significantly affect metal distribution and stable isotopic behavior. Here, two soils were separated into four particle size fractions, namely fine sand, silt, fine silt, and colloidal particles and used to determine cadmium (Cd) and zinc (Zn) concentrations and isotope compositions. Concentrations of Cd and Zn were generally enriched in the finer particles and positively correlated with the iron (Fe) and manganese (Mn) oxide contents. However, Cd concentration in the fine sand was higher than in the silt fraction due to the higher soil organic matter contents in the former particle fraction. The maximum δ114/110Cd value was found in the colloidal particles (-0.02 and 0.01‰) of both soils while the minimum was in the silt particles (-0.12 and 0.06‰). Incorporation into the mineral lattice of Fe and Mn oxides is suggested to explain the slight enrichment of heavy Cd isotopes in the colloidal fraction. The similar δ66Zn values of the four particle fractions (0.20-0.29‰ with a mean of 0.25‰) indicate similar Zn sources in different particle sizes. Metal isotopic fingerprint of different soil particle size fractions provides further insight into the underlying metal retention mechanisms within soil micro-zones and helps in tracing metal sources and biogeochemical processes.

Prediction of cadmium and zinc phytoextraction by the hyperaccumulator Sedum plumbizincicola using a dynamic geochemical mechanical combination model

Phytoextraction with hyperaccumulators is an environmentally friendly and cost-effective technique for soil remediation but remediation time is largely dependent on experience due to variations in soil properties which restrict the application of this technique. Here, a novel dynamic multi-surface model (MSM) framework is proposed to predict the efficiency and duration of cadmium (Cd) and zinc (Zn) phytoextraction using the hyperaccumulator Sedum plumbizincicola. First, the application of MSM to S. plumbizincicola was investigated using 95 naturally contaminated soils. Using the 'default' settings and considering the pH and DOC content in the rhizosphere, the dissolved Cd/Zn predicted by MSMs showed strong correlations with metal uptake by shoots (R2 = 0.825/0.802 for Cd/Zn, n = 95) and outperformed chemical extraction methods. Then the MSMs were further integrated with time and Cd and Zn interactions to form dynamic-MSM combined (D-MSM-C) models to evaluate and predict phytoextraction efficiency and duration based on a six-season continuous pot experiment. The D-MSM-C models well predicted metal contents remaining in soils after each season with mean absolute percentage error (MAPE) = 20.4 % (Cd) and 2.46 % (Zn) (n = 66). This model is a powerful tool for assessing and predicting phytoremediation efficiency and duration and is applicable across diverse soil properties and multiple metal-contamination scenarios.

Root-associated bacterial microbiome shaped by root selective effects benefits phytostabilization by Athyrium wardii (Hook.)

The root-associated microbiome assembly substantially promotes (hyper)accumulator plant growth and metal accumulation and is influenced by multiple factors, especially host species and environmental stress. Athyrium wardii (Hook.) is a phytostabilizer that grows in lead (Pb)-zinc (Zn) mine tailings and shows high root Pb accumulation. However, there remains little information on the assembly of the root-associated microbiome of A. wardii and its role in phytostabilization. A field study investigated the structural and functional variation in the root-associated bacterial microbiome of Athyrium wardii (Hook.) exposed to different levels of contamination in Pb-Zn mine tailings. The root compartment dominated the variation in the root-associated bacterial microbiome but the levels of contaminants showed less impact. Bacterial co-occurrence was enhanced in the rhizosphere soil and rhizoplane but tended to be much simpler in the endosphere in terms of network complexity and connectivity. This indicates that the microbial community assembly of A. wardii was non-random and shaped by root selective effects. Proteobacteria, Chloroflexi, Actinobacteria, Cyanobacteria, and Acidobacteriota were generally the dominant bacterial phyla. The genera Crossiella and Bradyrhizobium were enriched in the rhizosphere and cyanobacterial genera were enriched in the endosphere, demonstrating substantial advantages to plant survival and adaptation in the harsh mine environment. Functional categories involved in amino acid and carbohydrate metabolism were abundant in the rhizosphere soil, thus contributing to metal solubility and bioavailability in the rhizosphere. Membrane transporters, especially ATP-binding cassette transporters, were enriched in the endosphere, indicating a potential role in metal tolerance and transportation in A. wardii. The study shows substantial variation in the structure and function of microbiomes colonizing different compartments, with the rhizosphere and endophytic microbiota potentially involved in plant metal tolerance and accumulation during phytostabilization.

Climate and mineral accretion as drivers of mineral-associated and particulate organic matter accumulation in tidal wetland soils

Tidal wetlands sequester vast amounts of organic carbon (OC) and enhance soil accretion. The conservation and restoration of these ecosystems is becoming increasingly geared toward "blue" carbon sequestration while obtaining additional benefits, such as buffering sea-level rise and enhancing biodiversity. However, the assessments of blue carbon sequestration focus primarily on bulk SOC inventories and often neglect OC fractions and their drivers; this limits our understanding of the mechanisms controlling OC storage and opportunities to enhance blue carbon sinks. Here, we determined mineral-associated and particulate organic matter (MAOM and POM, respectively) in 99 surface soils and 40 soil cores collected from Chinese mangrove and saltmarsh habitats across a broad range of climates and accretion rates and showed how previously unrecognized mechanisms of climate and mineral accretion regulated MAOM and POM accumulation in tidal wetlands. MAOM concentrations (8.0 ± 5.7 g C kg-1 ) (±standard deviation) were significantly higher than POM concentrations (4.2 ± 5.7 g C kg-1 ) across the different soil depths and habitats. MAOM contributed over 51.6 ± 24.9% and 78.9 ± 19.0% to OC in mangrove and saltmarsh soils, respectively; both exhibited lower autochthonous contributions but higher contributions from terrestrial or marine sources than POM, which was derived primarily from autochthonous sources. Increased input of plant-derived organic matter along the increased temperature and precipitation gradients significantly enriched the POM concentrations. In contrast, the MAOM concentrations depended on climate, which controlled the mineral reactivity and mineral-OC interactions, and on regional sedimentary processes that could redistribute the reactive minerals. Mineral accretion diluted the POM concentrations and potentially enhanced the MAOM concentrations depending on mineral composition and whether the mineral accretion benefited plant productivity. Therefore, management strategies should comprehensively consider regional climate while regulating sediment supply and mineral abundance with engineering solutions to tap the OC sink potential of tidal wetlands.

Phytoextraction of highly cadmium-polluted agricultural soil by Sedum plumbizincicola: An eight-hectare field study

Phytoextraction with Sedum plumbizincicola is an in-situ, environmentally friendly and highly efficient remediation technique for slightly Cd-polluted soils but it remains a challenge to remediate highly Cd-polluted soils under field conditions. Here, an 8-ha field experiment was conducted to evaluate the feasibility of repeated phytoextraction by S. plumbizincicola of a highly Cd-polluted acid agricultural soil (pH 5.61, [Cd] 2.58 mg kg-1) in Yunnan province, southwest China. Mean shoot dry biomass production, Cd concentration and Cd uptake were 1.95 t ha-1, 170 mg kg-1 and 339 g ha-1 at the first harvest, and 0.91 t ha-1, 172 mg kg-1 and 142 g ha-1 at the second harvest. After two seasons of phytoextraction, soil total and CaCl2-extractable Cd concentrations decreased from 2.58 ± 0.69 to 1.53 ± 0.43 mg kg-1 and 0.22 ± 0.12 to 0.14 ± 0.07 mg kg-1, respectively. Stepwise multiple linear regression analysis shows that the shoot Cd concentration and uptake of S. plumbizincicola were positively related to soil CaCl2-extractable Cd concentrations, especially in the first crop. A negative relationship indicates that soil organic matter content played an important role in soil Cd availability and shoot Cd concentration in the first crop. In addition, the rhizosphere effect on soil CaCl2-extractable Cd concentration was negatively correlated with soil pH in the first crop. The accuracy of the calculation of soil Cd phytoextraction efficiency at field scale depends on all of the following factors being considered: shoot Cd uptake, cropping pattern, standardized sampling points, and the leaching and surface runoff of Cd. Phytoextraction with S. plumbizincicola is a feasible technique for efficient Cd removal from highly polluted soils and wide variation in soil properties can influence phytoextraction efficiency at the field scale.

Metagenomics reveals mechanism of pyrene degradation by an enriched bacterial consortium from a coking site contaminated with PAHs

A bacterial consortium, termed WPB, was obtained from polycyclic aromatic hydrocarbons (PAHs) contaminated soil from a coking site. The consortium effectively degraded 100 mg L-1 pyrene by 94.8 % within 12 days. WPB was also able to degrade phenanthrene (98.3 %) and benzo[a]pyrene (24.6 %) in 12 days, while the individual isolates showed no PAHs degrading ability. Paracoccus sp. dominated the bacterial consortium (65.0-86.2 %) throughout the degradation process. Metagenomic sequencing reveals the proportion of sequences with xenobiotics biodegradation and metabolism increased throughout the degradation process indicating the great potential of WPB to degrade pollutants. The annotation of genes by metagenomic analysis help reconstruct the degradation pathways ("phthalate pathway" and "naphthalene degradation") and reveal how different bacteria contribute to the degradation process. Mycobacterium gilvum was found to carry nidAB genes that catalyze the first step of high-molecular-weight (HMW) PAHs in the degradation process despite Mycobacterium gilvum accounting for only 0.005-0.06 %. In addition, genomes of Paracoccus denitrificans and some other genera affiliated with Devosia, Pusillimonas caeni and Eoetvoesia caeni were successfully recovered and were found to carry genes responsible for the degradation of the intermediates of pyrene. These results enable further understanding of the metabolic patterns of pyrene-degrading consortia and provide direction for further cultivation and discovery of key players in complex microbial consortia.

Optimizing cropping systems to close the gap between economic profitability and environmental health

Supporting food security while maintaining ecosystem sustainability is one of the most important global challenges for humanity. Optimization of cropping systems is expected to promote the ecosystem services of agroecosystems. Yet, how and why cropping system influences the trade-offs between economic profitability and multiple ecosystem services remain poorly understood. We investigate the influence of six cropping systems on trade-offs between economic profitability and multiple ecosystem services after considering 36 agricultural ecosystem properties using field experiment data from 2020 to 2022. We show that designing cropping system is a critical tool to closing the gap between ecosystem sustainability and commercial profitability. Cropping system with three harvests within 2 yr had higher performance in overall ecosystem multiple services through enhancement of supporting, regulating, and economic performance without compromising provisioning compared with four other systems. These systems diminished the trade-off among multiple services, resulting in a 'win-win' situation for economics and multiple services. By contrast, the monoculture and double cropping systems lead to a strong trade-off between pairwise services including ecosystem health and profitability. Our work illustrates the substantial potential of rotation systems with three harvests within 2 yr in enforcing ecosystem services and closing the trade-offs among multiple agricultural ecosystem services.

Dynamics of trace element enrichment in blue carbon ecosystems in relation to anthropogenic activities

Blue carbon ecosystems (BCEs), located at the land-sea interface, provide critical ecological services including the buffering of anthropogenic pollutants. Understanding the interactions between trace element (TE) loads in BCEs and socioeconomic development is imperative to informing management plans to address pollution issues. However, the identification of anthropogenic TE pollution in BCEs remains uncertain due to the complex geochemical and depositional processes and asynchronous socioeconomic development along continental coastlines. Here, priority-controlled TE (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) concentrations in the mangrove, saltmarsh and seagrass soils and plant tissues along the coastline of China were investigated while taking bare flat and upland soils as corresponding references. We demonstrate that blue carbon (BC) soils accumulated markedly higher concentrations of anthropogenic TEs than the reference soils, mainly due to the effective trapping of fine-grained particles and higher binding capacities. We identify the time course of TE changes over the last 100 years which shows increasing anthropogenic TE accumulation resulting from military activities (1930-1950) and the growth of industrial and agricultural activities (1950-1980), then reaching a maximum after national economic reform (1980-2000). Since the 2000s, decreases in TE discharges driven by socioeconomic reform and strengthened environmental regulations have led to a widespread reversal of anthropogenic TE concentrations in BC soils. Based on the current TE flux we estimate that BCEs can filter over 27.3-100 % of the TEs emitted in industrial wastewaters from Chinese coastal provinces annually. However, the uptake of these TEs by plants can be substantially reduced through various mechanisms offered by edaphic properties such as organic carbon, clay, and sulfur contents. Therefore, enhancing TE filtering while preventing TEs from entering food webs through the conservation and restoration of BCEs will greatly aid in achieving the sustainable development goal of the coastal zone under intensified anthropogenic activities.

Impact of lignin constituents on the bacterial community and polycyclic aromatic hydrocarbon co-metabolism in an agricultural soil

Lignin is a complex biopolymer comprising phenolic monomers with different degrees of methoxylation and may potentially enhance the degradation of soil pollutants such as polycyclic aromatic hydrocarbons (PAHs) through co-metabolism. However, the contribution of lignin constituents, including phenolic and methoxy subunits, to PAH biodegradation remains unclear. Here, p-hydroxybenzoate (pHBA), vanillate and methanol were selected to simulate phenolic units and methoxy groups of lignin. Soil microcosms receiving these compounds were established to evaluate their regulation on the bacterial community and PAH co-metabolism. There were different effects of different components on the biodegradation of a four-ring PAH, benzo(a)anthracene (BaA), as characterized using an isotopic tracer. Only vanillate significantly stimulated BaA mineralization to CO2, with pHBA and methanol leading to no appreciable change in the allocation of BaA in soil compartments. The lignin constituents had differential impacts on the soil bacterial community, with substantial enrichment of methylotrophs occurring in methanol-supplemented microcosms. Both vanillate and pHBA selected several aromatic degraders. Vanillate caused additional enrichment of methylotrophs, suggesting structure-dependent stimulation of bacterial functional guilds by lignin monomers. Compared with its constituents, lignin produced more extensive responses in terms of bacterial diversity and composition and the fate of BaA. However, it was difficult to link BaA co-metabolism to any specific bacterial taxa in the presence of lignin or its subunits. The results indicate that the co-metabolism effects of lignin may not be directly associated with phenolic or methoxy metabolism but with its regulation of the soil microbiome.

Does phytoextraction with Sedum plumbizincicola increase cadmium leaching from polluted agricultural soil?

Sedum plumbizincicola is a cadmium (Cd) and zinc hyperaccumulator that can activate Cd by rhizosphere acidification. However, there is little understanding of the Cd leaching risk from polluted soil during phytoextraction process. Here, pot and column experiments were conducted to monitor soil Cd leaching characteristics under different rainfall simulation conditions during S. plumbizincicola phytoextraction. Soil Cd leaching increased significantly with increasing simulated rainfall intensity. Compared with normal rainfall (NR), weak rainfall (WR) resulted in a 34.3% decrease in Cd uptake by S. plumbizincicola and also led to a 68.7% decline in Cd leaching. In contrast, Cd leaching under heavy rainfall (HR) was 2.12 times that of NR in the presence of S. plumbizincicola. After two successive growing periods, phytoextraction resulted in a 53.5-66.4% decline in the amount of soil Cd leached compared with controls in which S. plumbizincicola was absent. Even compared with maize cropping as a control, S. plumbizincicola did not instigate a significant increase in Cd leaching. The contribution of Cd leaching loss to the decline in soil total Cd concentration was negligible after phytoextraction in the pot experiment. Overall, the results contribute to our understanding of soil Cd leaching risk by phytoextraction with S. plumbizincicola.

Depth and contaminant-shaped bacterial community structure and assembly at an aged chlorinated aliphatic hydrocarbon-contaminated site

Chlorinated aliphatic hydrocarbons (CAHs) are potentially toxic substances that have been detected in various contaminated environments. Biological elimination is the main technique of detoxifying CAHs in the contaminated sites, but the soil bacterial community at CAH-contaminated sites have been little investigated. Here, high-throughput sequencing analysis of soil samples from different depths (to 6 m depth) at an aged CAH-contaminated site has been conducted to investigate the community composition, function, and assembly of soil bacteria. The alpha diversity of the bacterial community significantly increased with increasing depth and bacterial community also became more convergent with increasing depth. Organohalide-respiring bacteria (OHRB) is considered keystone taxa to reduce the environmental stress of CAHs by reductive dechlorinate CAHs into nontoxic products, increases the alpha diversity of bacterial community and improves the stability of bacterial co-occurrence network. The high concentration of CAHs in deep soil and the stable anaerobic environment make deterministic processes dominate bacterial community assembly, while the topsoil is dominated by dispersal limitation. In general, CAHs at contaminated sites have a great impact on bacterial community, but the CAHs metabolic community acclimated in deep soil can reduce the environmental stress of CAHs, which provides foundation for the monitored natural attenuation technology in CAHs-contaminated sites.

Endogenous biohydrogen from a rhizobium-legume association drives microbial biodegradation of polychlorinated biphenyl in contaminated soil

Endogenous hydrogen (H₂) is produced through rhizobium-legume associations in terrestrial ecosystems worldwide through dinitrogen fixation. In turn, this gas may alter rhizosphere microbial community structure and modulate biogeochemical cycles. However, very little is understood about the role that this H₂ leaking to the rhizosphere plays in shaping the persistent organic pollutants degrading microbes in contaminated soils. Here, we combined DNA-stable isotope probing (DNA-SIP) with metagenomics to explore how endogenous H₂ from the symbiotic rhizobium-alfalfa association drives the microbial biodegradation of tetrachlorobiphenyl PCB 77 in a contaminated soil. The results showed that PCB77 biodegradation efficiency increased significantly in soils treated with endogenous H₂. Based on metagenomes of ¹³C-enriched DNA fractions, endogenous H₂ selected bacteria harboring PCB degradation genes. Functional gene annotation allowed the reconstruction of several complete pathways for PCB catabolism, with different taxa conducting successive metabolic steps of PCB metabolism. The enrichment through endogenous H₂ of hydrogenotrophic Pseudomonas and Magnetospirillum encoding biphenyl oxidation genes drove PCB biodegradation. This study proves that endogenous H₂ is a significant energy source for active PCB-degrading communities and suggests that elevated H₂ can influence the microbial ecology and biogeochemistry of the legume rhizosphere.

Surfactant-enhanced bioremediation of petroleum-contaminated soil and microbial community response: A field study

Surfactant-enhanced bioremediation (SEBR) is frequently employed to clean up soil polluted with petroleum hydrocarbons, but few studies have focused on how surfactants affect microbial communities and different fractions of petroleum hydrocarbons, particularly in the field. Here, the surfactants sodium dodecyl benzene sulfonate (SDBS), alpha olefin sulfonate (AOS), Triton X-100 (TX-100), Tween80, and rhamnolipid were combined with the oil-degrading bacterium Pseudomonas sp. SB to remediate oil-contaminated soil in the laboratory. AOS gave the highest removal efficiency (65.1%) of total petroleum hydrocarbons (TPHs). Therefore, AOS was used in a field experiment with Pseudomonas sp. SB and the removal efficiency of TPHs and long-chain hydrocarbons C21-C40 reached 57.4 and 53.0%, respectively, significantly higher than the other treatments. During bioremediation the addition of Pseudomonas sp. SB significantly stimulated the growth of bacterial genera such as Alcanivorax, Luteimonas, Parvibaculum, Stenotrophomonas, and Pseudomonas and AOS further stimulated the growth of Sphingobacterium, Pseudomonas and Alcanivorax. This study validates the feasibility of surfactant-enhanced bioremediation in the field and partly reveals the mechanism of surfactant-enhanced bioremediation from the perspective of changes in different fractions of petroleum and microbial community dynamics.

Zinc Supply Affects Cadmium Uptake and Translocation in the Hyperaccumulator Sedum Plumbizincicola as Evidenced by Isotope Fractionation

This study employs stable isotope analysis to investigate the mechanisms of cadmium (Cd) and zinc (Zn) interaction in the metal hyperaccumulating plant species Sedum plumbizincicola. To this end, the Cd and Zn isotope compositions of root, stem, leaf, and xylem sap samples were determined during metal uptake and translocation at different Cd and Zn concentrations. The enrichment of light isotopes of both elements in plants during uptake was less pronounced at low metal supply levels, likely reflecting the switch from a low-affinity to a high-affinity transport system at lower levels of external metal supply. The lower δ114/110Cd values of xylem sap when treated with a metabolic inhibitor decreasing the active Cd uptake further supports the preference of heavier Cd isotopes during high-affinity transport. The Δ66Znplant-initial solution or Δ66Znplant-final solution values were similar at different Cd concentrations, indicating negligible interaction of Cd in the Zn uptake process. However, decreasing Zn supply levels significantly increased the enrichment of light Cd isotopes in plants (Δ114/110Cd = -0.08‰) in low-Cd treatments but reduced the enrichment of light Cd isotopes in plants (Δ114/110Cd = 0.08‰) under high Cd conditions. A systematic enrichment of heavy Cd and light Zn isotopes was found in root-to-shoot translocation of the metals. The Cd concentrations of the growth solutions thereby had no significant impact on Zn isotope fractionation during root-to-shoot translocation. However, the Δ114/110Cdtranslocation values hint at possible competition between Cd and Zn for transporters during root-to-shoot transfer and this may impact the transport pathway of Cd. The stable isotope data demonstrate that the interactions between the two metals influenced the uptake and transport mechanisms of Cd in S. plumbizincicola but had little effect on those of Zn.

Contamination levels of and potential risks from metal(loid)s in soil-crop systems in high geological background areas

Soil metal(loid)s in high geological background areas occur mainly in the residual form with low bioavailability, and whether these potentially toxic elements (PTEs) in agricultural soils are harmful to human health is of considerable public concern. A paired survey using both soil and crop samples was conducted using 437 contaminated sites in east Yunnan province, southwest China. The concentration, distribution, and source of PTEs (arsenic (As), copper (Cu), zinc (Zn), lead (Pb), cadmium (Cd), and chromium (Cr)) in agricultural soils, and the contamination levels of and potential health risks from PTEs from different pollution sources were evaluated. Soil Pb, Cu, Zn, Cd, Cr, and As concentrations were higher than the Chinese screening values (GB 15618-2018) of 10.98, 36.16, 24.71, 86.96, 14.19, and 6.64 %, respectively, and Cd greatly exceeded the screening values. Spatial distribution maps indicate that areas with high concentrations of Pb, Cu, Cd, and As were located mainly in mining areas. However, the Zn and Cr concentrations were relatively homogeneous and more dependent on natural processes. The source identification of PTEs shows that Zn and Cr in soils were controlled mainly by the geological background, Pb and As were closely related to anthropogenic activities, and Cu and Cd were related to both sources. Different pollution sources affected crop PTE contents, with average concentrations of Zn, Cd, Cr, and As in high geological background areas significantly lower than in anthropogenic activity areas (p < 0.001), while Cu and As did not differ significantly. Although soil PTEs in high geological background areas represent a relatively high potential risk, they had little impact on crop quality. The hazard indices of different crop products for adults and children followed the sequence: cereals > leafy vegetables > rootstalk vegetables > fruit vegetables. Rootstalk and fruit vegetables are recommended to be grown in the study areas because they are safe for human consumption.

Potential mitigation of environmental impacts of intensive plum production in southeast China with maintenance of high yields: Evaluation using life cycle assessment

INTRODUCTION

Intensive plum production usually involves high yields but also high environmental costs due to excessive fertilizer inputs. Quantitative analysis of the environmental effects of plum production is thereby required in the development of optimum strategies to promote sustainable fruit production.

METHODS

We collected survey questionnaires from 254 plum production farms in Zhao'an county, Fujian province, southeast China to assess the environmental impacts by life cycle assessment (LCA) methodology. The farms were categorized into four groups based on yield and environmental impacts, i.e., LL (low yield and low environmental impact), LH (low yield but high environmental impact), HL (high yield but low environmental impact), and HH (high yield and high environmental impact).

RESULTS

The environmental impacts, i.e., average energy depletion, global warming, acidification, and eutrophication potential in plum production were 18.17 GJ ha^-1, 3.63 t CO₂ eq ha^-1, 42.18 kg SO₂ eq ha^-1, and 25.06 kg PO₄ eq ha^-1, respectively. Only 19.7% of farmers were in the HL group, with 13.3% in the HH group, 39.0% in LL, and 28.0% LH. Plum yields of the HL group were 109-114% higher than the mean value of all 254 farms. Additionally, the HL group had a lower environmental impact per unit area compared to the overall mean value, with a reduction ranging from 31.9% to 36.7%. Furthermore, on a per tonne of plum production basis, the energy depletion, global warming potential, acidification potential, and eutrophication potential of HL farms were lower by 75.4%, 75.0%, 75.6%, and 75.8%, respectively. Overall, the total environmental impact index of LL, LH, HL, and HH groups were 0.26, 0.42, 0.06, and 0.21, respectively.

DISCUSSION

Excessive fertilizer N application was the main source of the environmental impacts, the potential to reduce fertilizer N rate can be achieved without compromising plum yield by studying the HH group. The results provide an important foundation for enhancing the management of plum production, in order to promote 'green' agricultural development by reducing environmental impacts.

Microbial community assembly of the hyperaccumulator plant Sedum plumbizincicola in two contrasting soil types with three levels of cadmium contamination

Microbial communities are closely related to plant performance and numerous studies have shown their involvement with the growth and development of host plants, resistance to pathogen invasion and adaptation to environmental stress. Here we described in detail the ecological process of the microbial community assembly in hyperaccumulator plant Sedum plumbizincicola. We divided the microbiota into four ecological compartments (bulk soil, rhizosphere, root endosphere and aboveground endosphere). The results showed that host selection strongly controlled the aggregation of microbial community. So that microbes occupied different niches from the bulk soil to the aboveground endosphere, and bacterial diversity and network complexity decreased gradually. Soil types were the second influencing factor, especially for the microbial community in the root endosphere. The SourceTracker analysis further confirmed the vertical migration of microbes from bulk soil to aboveground endosphere. In addition, under the condition of heavy metal pollution, the microbial community of S. plumbizincicola tended to form a microbial pool dominated by Proteobacteria and Actinobacteria. Ellin6067, Sphingomonas, Ralstonia, SC-I-84_uncultured bacterium, Burkholderiaceae_Undibacterium and Pedosphaeraceae_uncultured bacterium etc. were identified as the vital biomarker taxa. Among these genera, the relative abundance of last three was significantly positively correlated with the activation and transfer of cadmium, and they mainly enriched in paddy soil. This study provides evidence for the mechanism by which the microbial community assembly occurs and experience for regulating the microbial community and increasing the accumulation efficiency of potentially toxic metals in S. plumbizincicola.

Mycorrhiza-mediated recruitment of complete denitrifying Pseudomonas reduces N2O emissions from soil

BACKGROUND

Arbuscular mycorrhizal fungi (AMF) are key soil organisms and their extensive hyphae create a unique hyphosphere associated with microbes actively involved in N cycling. However, the underlying mechanisms how AMF and hyphae-associated microbes may cooperate to influence N₂O emissions from "hot spot" residue patches remain unclear. Here we explored the key microbes in the hyphosphere involved in N₂O production and consumption using amplicon and shotgun metagenomic sequencing. Chemotaxis, growth and N₂O emissions of isolated N₂O-reducing bacteria in response to hyphal exudates were tested using in vitro cultures and inoculation experiments.

RESULTS

AMF hyphae reduced denitrification-derived N₂O emission (max. 63%) in C- and N-rich residue patches. AMF consistently enhanced the abundance and expression of clade I nosZ gene, and inconsistently increased that of nirS and nirK genes. The reduction of N₂O emissions in the hyphosphere was linked to N₂O-reducing Pseudomonas specifically enriched by AMF, concurring with the increase in the relative abundance of the key genes involved in bacterial citrate cycle. Phenotypic characterization of the isolated complete denitrifying P. fluorescens strain JL1 (possessing clade I nosZ) indicated that the decline of net N₂O emission was a result of upregulated nosZ expression in P. fluorescens following hyphal exudation (e.g. carboxylates). These findings were further validated by re-inoculating sterilized residue patches with P. fluorescens and by an 11-year-long field experiment showing significant positive correlation between hyphal length density with the abundance of clade I nosZ gene.

CONCLUSIONS

The cooperation between AMF and the N₂O-reducing Pseudomonas residing on hyphae significantly reduce N₂O emissions in the microsites. Carboxylates exuded by hyphae act as attractants in recruiting P. fluorescens and also as stimulants triggering nosZ gene expression. Our discovery indicates that reinforcing synergies between AMF and hyphosphere microbiome may provide unexplored opportunities to stimulate N₂O consumption in nutrient-enriched microsites, and consequently reduce N₂O emissions from soils. This knowledge opens novel avenues to exploit cross-kingdom microbial interactions for sustainable agriculture and for climate change mitigation. Video Abstract.

Maize/peanut intercropping has greater synergistic effects and home-field advantages than maize/soybean on straw decomposition

INTRODUCTION

The decomposition of plant litter mass is responsible for substantial carbon fluxes and remains a key process regulating nutrient cycling in natural and managed ecosystems. Litter decomposition has been addressed in agricultural monoculture systems, but not in intercropping systems, which produce species-diverse litter mass mixtures. The aim here is to quantify how straw type, the soil environment and their combined effects may influence straw decomposition in widely practiced maize/legume intercropping systems.

METHODS

Three decomposition experiments were conducted over 341 days within a long-term intercropping field experiment which included two nitrogen (N) addition levels (i.e. no-N and N-addition) and five cropping systems (maize, soybean and peanut monocultures and maize/soybean and maize/peanut intercropping). Experiment I was used to quantify litter quality effects on decomposition; five types of straw (maize, soybean, peanut, maize-soybean and maize-peanut) from two N treatments decomposed in the same maize plot. Experiment II addressed soil environment effects on root decomposition; soybean straw decomposed in different plots (five cropping systems and two N levels). Experiment III addressed 'home' decomposition effects whereby litter mass (straw) was remained to decompose in the plot of origin. The contribution of litter and soil effects to the home-field advantages was compared between experiment III ('home' plot) and I-II ('away' plot).

RESULTS AND DISCUSSIONS

Straw type affected litter mass loss in the same soil environment (experiment I) and the mass loss values of maize, soybean, peanut, maize-soybean, and maize-peanut straw were 59, 77, 87, 76, and 78%, respectively. Straw type also affected decomposition in the 'home' plot environment (experiment III), with mass loss values of maize, soybean, peanut, maize-soybean and maize-peanut straw of 66, 74, 80, 72, and 76%, respectively. Cropping system did not affect the mass loss of soybean straw (experiment II). Nitrogen-addition significantly increased straw mass loss in experiment III. Decomposition of maize-peanut straw mixtures was enhanced more by 'home-field advantage' effects than that of maize-soybean straw mixtures. There was a synergistic mixing effect of maize-peanut and maize-soybean straw mixture decomposition in both 'home' (experiment III) and 'away' plots (experiment I). Maize-peanut showed greater synergistic effects than maize-soybean in straw mixture decomposition in their 'home' plot (experiment III). These findings are discussed in terms of their important implications for the management of species-diverse straw in food-production intercropping systems.

Removal of nanoplastics from aqueous solution by aggregation using reusable magnetic biochar modified with cetyltrimethylammonium bromide

Nanoplastics (NPs) pollution has become an emerging threat to the aquatic environment and its organisms. The removal of NPs from contaminated water is a global challenge. In this study, an efficient and reusable composite was prepared from cetyltrimethylammonium bromide (CTAB) modified magnetic biochar. The performances of CTAB modified magnetic biochar (CMB) to remove polystyrene (PS) and carboxylate-modified polystyrene (CPS) nanoparticles from water were systematically evaluated. The results showed that the PS and CPS removal performance of magnetic biochar was improved by CTAB modification. These increases were assigned to the increase in the surface hydrophobicity of CMB. Due to the strong electrostatic repulsion between the nanoparticles, PS and CPS maintained high stability in alkaline conditions, resulting in a significant decrease in removal efficiency. The removal efficiency was decreased to 67.4% for PS and to 40.7% for CPS at pH 11. The inhibition effects of NaCl on the PS and CPS removal efficiencies were decreased gradually with the increase of NaCl concentration. Among the anions studied, H₂PO₄^- had the biggest impact on the removal performance of CMB. Besides, CMB could be used to remove PS and CPS in real surface water, and the removal efficiencies of PS and CPS were 95.3% and 97.8%, respectively. Particularly, the removal efficiencies of PS and CPS were 90.2% for PS and 94.8% for CPS when CMB was recycled five times. According to the characterization results of XRD, TGA, SEM, FTIR and XPS, PS and CPS nanoparticles were removed by CMB from water mainly through aggregation instead of adsorption. The efficient removal of PS and CPS by CMB via aggregation process offers new insight into the removal of NPs from aquatic environment.

Bacteria not fungi drive soil chemical quality index in banana plantations with increasing years of organic fertilizer application

BACKGROUND

Maintaining or improving soil chemical quality is critical for sustainable agricultural productivity and environmental safeguards. Organic fertilizer application, a common agricultural practice in banana cultivation, is often associated with greater microbial biomass and activity, which are linked to improvements in soil chemical quality. However, the effect of the duration of organic fertilizer application on soil chemical quality and whether it is microbially driven still needs to be investigated. We collected soil samples from banana plantations consistently applying organic fertilizers for 1 (Y1), 4 (Y4), 7 (Y7) and 10 (Y10) years. Soil chemical quality is expressed as total data set (TDS) and minimum data set (MDS) based on chemical indicators, and soil microorganisms are characterized by phospholipid fatty acid (PLFA).

RESULTS

Based on TDS and MDS, the soil chemical quality indices in Y7 and Y10 treatments were significantly higher than that in Y1 and Y4 treatments. Soil total PLFA concentrations and the proportional abundance of fungi and arbuscular mycorrhizal fungi increased with prolonged banana cultivation. Total PLFA concentrations were significantly positive correlation with the soil chemical quality index. Soil gram-positive bacteria (G+), bacteria, protozoa and ratio of G+ to gram-negative bacteria (G-) were major drivers of soil chemical quality.

CONCLUSION

The organic fertilizer application can significantly improve soil chemical quality, which is regulated by soil bacteria. Regular application of organic fertilizers is important in promoting soil quality and soil biological properties need to be incorporated into the assessment of soil health in banana plantations. © 2022 Society of Chemical Industry.

Occurrence and distribution of microplastics in coastal plain soils under three land-use types

Microplastic pollution is an issue of major environmental concern worldwide. Land-use type may affect the abundance, polymer types, and distribution characteristics of soil microplastics but their distribution remains unknown on the coastal plain of east China. Here, the abundance of microplastics in farmland (FL), plantation (P), and orchard/secondary forest (OSF) soils was determined on the east China coastal plain, and characteristics of the microplastics (shape, size, colour, and polymer composition) were analysed in soil samples collected from 33 sites. The average abundances of microplastics in FL, P, and OSF soils on the coastal plain of the east China coast were 185, 109, and 150 items kg^-1, respectively. Small particles, fibres and transparent particles were the main characteristics of the microplastics observed. The polymer types were mainly PP and PET. The abundance of microplastics in farmland was positively correlated with population density in the study area. Therefore, agricultural activities associated with high population density are the main factors leading to the high abundance of microplastics in farmland soil.

Potential mobilization of cadmium and zinc in soils spiked with smithsonite and sphalerite under different water management regimes

Particulate cadmium (Cd) and zinc (Zn) are ubiquitous in agricultural soils of Pb-Zn mining regions. Water management serves as an important agronomic measure altering the bioavailability of Zn and Cd in soils, but how this affects particulate Cd and Zn and the underlying mechanisms remain largely unknown. Microcosm soil incubation combined with spectroscopic and microscopic characterization was conducted. During a two-year-long incubation period we observed that the concentrations of soil CaCl₂-extractable Zn and Cd increased 3-10 times in sphalerite-spiked soils and 1-2 times in smithsonite-spiked soils under periodic flooding conditions due to the long-term dissolution of sphalerite (SP) and smithsonite (SM). However, the increase in the concentration of CaCl₂-extractable metals (Zn: from 0.607 mg kg^-1 to 1.051 mg kg^-1 and Cd: from 0.047 mg kg^-1 to 0.119 mg kg^-1) was found only in SP-treatment under continuous flooding conditions, indicating the mobilization of metals. Ultrafiltration analysis shows that the nanoparticulate fraction of Zn and Cd in soil pore water increased 5 and 7 times in SP-treatments under continuous flooding conditions, suggesting the increment of metal pools in soil pore water. HRTEM-EDX-SAED further reveals that these nanoparticles were mainly crystalline ZnS and Zn-bearing sulfate nanoparticles in the SP-treatment and amorphous ZnCO₃ and ZnS nanoparticles in the SM-treatment. Therefore, the formation of the stable crystalline Zn-bearing nanoparticles in the SP-treatment may explain the elevation of the concentration of soil CaCl₂-extractable Zn and Cd under continuous flooding. The potential mobility of particulate metals should therefore be expected in scenarios of continuous flooding such as paddy soils and wetland systems.

Legacy of contamination with metal(loid)s and their potential mobilization in soils at a carbonate-hosted lead-zinc mine area

Chemical weathering of carbonate-hosted Pb-Zn mines via acid-promoted or oxidative dissolution generates metal-bearing colloids at neutral mine drainage sites. However, the mobility and bioavailability of the colloids associated with metals in nearby soils are unknown. Here, we monitored the mobility of metal(loid)s in soils affected by aeolian deposition and river transport in the vicinity of a carbonate-hosted Pb-Zn mine. Using chemical extraction, ultrafiltration, and microscopic and spectroscopic analysis of metals we find that contamination levels of the soil metals cadmium (Cd), lead (Pb) and zinc (Zn) were negatively correlated with metal extractability. However, nano-scale characterization indicates that colloid-metal(loid) interactions induced potential mobilization and increased risk from metal(loid)s. Dynamic light scattering (DLS) and HRTEM-EDX-SAED analysis further indicate that organic matter (OM)-rich nano-colloids associated with calcium (Ca), silicon (Si) and iron (Fe) precipitates accounted for the majority of the dissolved metal fractions in carbonate-hosted Pb-Zn mine soils. More stable nano-crystals (ZnS, ZnCO₃, Zn-bearing sulfates, hematite and Al-Si-Fe compounds) were present in the pore water of aeolian-impacted upland soils rather than in river water-impacted soils. Our results suggest that future work should consider the possibility that potential mobilization of metal(loid)s induced by the weathering and transformation of these metal-bearing nano-crystals to metal-bearing amorphous colloids, potentially elevating metal mobility and/or bioavailability in river water-impacted agricultural soils.

Cadmium isotope fractionation during transport processes within agricultural soil profiles in a mining area: Implications for source tracing

Cadmium (Cd) isotope fractionation patterns within soil profiles and the underlying mechanisms remain unclear and poorly documented. Here, Cd concentrations and isotope compositions of metal ore, surface soils and soil profile samples around a lead-zinc mine in southwest China were determined, and the relationships between soil properties and Cd isotope fractionation within the soil profiles were investigated. Cadmium concentrations of eleven surface soil samples were 0.49-66.1 mg kg^-1 and the samples with high Cd concentrations had Cd isotope compositions similar to the metal ore (δ^114/110Cd = 0.02‰), indicating that mining activity was the main Cd source at the study areas. Within three soil profiles with different Cd pollution levels the δ^114/110Cd values gradually increased with increasing depth from 0 to 40 cm (Δ^114/110Cd = 0.08-0.18‰), reaching a maximum at 30-40 cm depth, and then remained fairly constant or decreased with increasing soil depth below 40 cm. Soil δ^114/110Cd values were negatively correlated with free iron and manganese oxides contents, which decreased at 0-40 cm depth then increased below 40 cm. This indicates that light Cd isotopes within 0-40 cm depth preferentially migrated downward with free iron and manganese oxides, leaving the soils at a depth of 0-40 cm enriched in heavy Cd isotopes. At 40-90 cm depth the preferential retention of heavy Cd isotopes by hydroxides may be responsible for the gradual decrease in δ^114/110Cd values with increasing soil depth. These observations demonstrate that the vertical migration of Cd can induce detectable isotope fractionation within soil profiles and alter the δ^114/110Cd values including those of the surface soils. Our study highlights the need to consider Cd mobilization and transport in soil profiles when tracing metal sources using isotope techniques.

Arbuscular mycorrhizal colonization increases plant above-belowground feedback in a northwest Chinese coal mining-degraded soil by increasing photosynthetic carbon assimilation and allocation to maize

A three-compartment culture system was used to study the mechanism by which the AM fungus Funneliformis mosseae influences host plant growth and soil organic carbon (SOC) content in a northwest China coal mining area. A ¹³CO₂ pulse tracing technique was used to trace the allocation of maize photosynthetic C in shoots, roots, AM fungus, and soil. Carbon accumulation and allocation in mycorrhizal (inoculated with Funneliformis mosseae) and non-mycorrhizal treatments were detected. AM fungal inoculation significantly increased the ¹³C concentration and content in both above- and below-ground plant parts and also significantly enhanced anti-aging ability by increasing soluble sugars and catalase activity (CAT) in maize leaves while reducing foliar malondialdehyde content (MDA) and leaf temperature and promoted plant growth. AM fungi also increased P uptake to promote maize growth. Soil organic carbon (SOC), glomalin, microbial biomass carbon (MBC), and nitrogen (MBN) contents increased significantly after inoculation. A mutually beneficial system was established involving maize, the AM fungus and the microbiome, and the AM fungus became an important regulator of C flux between the above- and below-ground parts of the system. Inoculation with the AM fungus promoted plant growth, C fixation and allocation belowground to enhance soil quality. A positive above-belowground feedback appeared to be established.

High bacterial diversity and siderophore-producing bacteria collectively suppress Fusarium oxysporum in maize/faba bean intercropping

Beyond interacting with neighboring plants, crop performance is affected by the microbiome that includes pathogens and mutualists. While the importance of plant–plant interactions in explaining overyielding in intercropping is well known, the role of the microbiome, in particular how the presence of microbes from heterospecific crop species inhibit pathogens of the focal plants in affecting yield remains hardly explored. Here we performed both field samplings and pot experiments to investigate the microbial interactions in the maize/faba bean intercropping system, with the focus on the inhibition of Fusarium oxysporum in faba bean plants. Long-term field measurements show that maize/faba bean intercropping increased crop yield, reduced the gene copies of F. oxysporum by 30–84% and increased bacterial richness and Shannon index compared to monocropping. Bacterial networks in intercropping were more stable with more hub nodes than the respective monocultures. Furthermore, the observed changes of whole microbial communities were aligned with differences in the number of siderophore-producing rhizobacteria in maize and pathogen abundances in faba bean. Maize possessed 71% more siderophore-producing rhizobacteria and 33% more synthetases genes abundance of nonribosomal peptides, especially pyochelin, relative to faba bean. This was further evidenced by the increased numbers of siderophore-producing bacteria and decreased gene copies of F. oxysporum in the rhizosphere of intercropped faba bean. Four bacteria (Pseudomonas spp. B004 and B021, Bacillus spp. B005 and B208) from 95 isolates antagonized F. oxysporum f. sp. fabae. In particular, B005, which represented a hub node in the networks, showed particularly high siderophore-producing capabilities. Intercropping increased overall bacterial diversity and network complexity and the abundance of siderophore-producing bacteria, leading to facilitated pathogen suppression and increased resistance of faba bean to F. oxysporum. This study has great agronomic implications as microorganisms might be specifically targeted to optimize intercropping practices in the future.

Zero-valent iron-induced successive chemical transformation and biodegradation of lindane in historically contaminated soil: An isotope-informed metagenomic study

Zero-valent iron (ZVI) is widely used to mitigate environmental pollutants such as chlorinated pesticides through reductive reactions accompanied by extensive impacts on the soil microbial community. However, whether and how ZVI changes the biodegradation of target compounds remain poorly understood. Here, we monitor the fate of lindane using a ¹⁴C-labled tracer and evaluate the growth and functions of the bacterial community in ZVI-stressed conditions in a historically γ-hexachlorocyclohexane (lindane)-contaminated soil using a combination of isotopic (¹⁸O-H₂O) and metagenomic methods. ZVI promoted the biomineralization of lindane in a dose-dependent manner. Soil bacteria were inhibited by amendment with ZVI during the initial stages of incubation (first three days) but recovered during the subsequent six weeks. Metagenomic study indicates that the todC1/bedC1 genes involved in the oxidation of dechlorinated lindane intermediates were upregulated in the ¹⁸O-labeled bacterial community but the presence of the lin genes responsible for lindane dechlorination was not confirmed. In addition, the benzoate biodegradation pathway that links to downstream catabolism of lindane was enhanced. These findings indicate successive chemical and biological degradation mechanisms underlying ZVI-enhanced lindane mineralization and provide a scientific basis for the inclusion of an extended bioremediation stage in the environmental application of ZVI materials.

Effects of a soil collembolan on the growth and metal uptake of a hyperaccumulator: Modification of root morphology and the expression of plant defense genes

Soil collembolans live in close proximity to plant roots and may have a role in the phytoextraction of potentially toxic metals from contaminated soils but the underlying mechanisms remain poorly investigated. We hypothesize that soil collembolans may change the root morphology of hyperaccumulators by regulating plant physiological characteristics. Here, a pot experiment was conducted in which a cadmium (Cd) and zinc (Zn) hyperaccumulator (Sedum plumbizincicola) was grown with or without a collembolan (Folsomia candida), and plant transcriptome and hormones as well as the root characteristics of S. plumbizincicola were analyzed. F. candida promoted the growth and Cd/Zn uptake of S. plumbizincicola, the root and shoot biomass increasing by 53.3 and 34.4%, and the uptake of Cd and Zn in roots increased by 83.2 and 65.4%, respectively. Plant root morphology, total root length, root tip number and lateral root number increased significantly by 40.7, 37.2 and 33.8%, respectively, with the addition of F. candida. Transcriptome analysis reveals that the expression levels of defense-related genes in S. plumbizincicola were significantly up-regulated. In addition, the defensive plant hormones, i.e. salicylic acid in the roots, increased significantly by 338%. These results suggest that the plant in defense of the action of F. candida regulated the expression of the corresponding genes and increased the defensive plant hormones, thus modifying root morphology and plant performance. Overall, this study highlights the importance of the regulation by collembolans of plant growth and metal uptake by interaction with hyperaccumulator roots.

Potential environmental risk of natural particulate cadmium and zinc in sphalerite- and smithsonite-spiked soils

Cadmium (Cd)-bearing sphalerite and smithsonite ore particles are ubiquitous in soils near metal-mining areas. Previous studies indicate that smithsonite is more readily dissolved in acidic waters and soils than sphalerite but the mobility of Cd and zinc (Zn) derived from these ores in soils is unknown. Using microcosm incubation experiments and microscopic and spectroscopic analysis, we found that the mobility of Cd and Zn derived from smithsonite is higher than from sphalerite. The mobilization rates of Cd (16.6%) and Zn (13.7%) released from smithsonite in soils after 30-day incubation experiments were higher than those from sphalerite (Cd, ~ 1.42%; Zn, ~ 0.75%). Moreover, the percentages of Cd²⁺ and Zn²⁺ in soil pore water showed a dynamic increase in smithsonite-spiked treatments but a decrease in sphalerite-spiked treatments. HRTEM-EDX-SAED analysis further indicates the occurrence of dynamic transformation of amorphous Cd and Zn species in soil pore water to crystalline ZnS and iron oxides in sphalerite-spiked soil but crystalline ZnCO₃ nanoparticles were dynamically transformed to amorphous metal-bearing species in smithsonite-spiked soil. The opposite transformation trends in pore water of Zn ore-spiked soils provide new insights into the Cd environmental risks in soils affected by Zn mining.

Zinc uptake and replenishment mechanisms during repeated phytoextraction using Sedum plumbizincicola revealed by stable isotope fractionation

Improving phytoremediation techniques requires a thorough understanding of the mechanisms of plant uptake and the replenishment of the bioavailable pool of the target element, and this may be effectively explored using stable isotope methods. A repeated phytoextraction experiment over five successive crops of cadmium (Cd) and zinc (Zn) hyperaccumulator Sedum plumbizincicola X.H. Guo et S.B. Zhou ex L.H. Wu (Crassulaceae) was conducted using four agricultural soils differing in soil pH and clay content. The isotopic composition of total Zn and NH₄OAc-extractable Zn in soils before phytoextraction and after the fifth crop were determined, together with Zn in shoot samples in the first crop. S. plumbizincicola preferentially took up light Zn isotopes from the NH₄OAc-extractable pool (Δ⁶⁶Zn_{shoot-extract} = -0.42 to -0.16‰), indicating the predominance of Zn low-affinity transport. However, after long-term phytoextraction NH₄OAc-extractable Zn became isotopically lighter than prior to phytoextraction in three of the soils (Δ⁶⁶Zn_{extract: P5-P0} = -0.39 to -0.10‰). This was resulted from the equilibrium replenishment of Zn bound to iron (Fe) and manganese (Mn) oxides based on Zn isotopic and chemical speciation analysis. Zinc showed opposite fractionation patterns to Cd in the same plant-soil system with heavy Cd isotope enrichment in S. plumbizincicola (Δ^114/110Cd_{shoot-extract} = 0.02-0.17‰) and in the NH₄OAc-extractable pool after repeated phytoextraction (Δ^114/110Cd_{extract: P5-P0} = 0.07-0.18‰). This indicates different mechanisms of membrane transport (high-affinity transport of Cd) and supplementation of the bioavailable pool in soil (Cd supplied mainly through complexation with root-derived organic ligands) of the two metals. The combination of chemical speciation and stable Zn isotope ratios in the plant and the bioavailable soil pool reveal that the Zn pool related to Fe and Mn oxides became increasingly bioavailable with increasing crop generations. Capsule: Stable isotope analysis indicates that soil Fe- and Mn-oxide bound Zn replenishment boosted Zn uptake by the hyperaccumulator Sedum plumbizincicola during long-term remediation.

Hydrogen peroxide combined with surfactant leaching and microbial community recovery from oil sludge

The remediation effects of hydrogen peroxide (H₂O₂) oxidation and surfactant-leaching alone or in combination on three typical oilfield sludges were studied. The removal efficiency of total petroleum hydrocarbons (TPHs) of Jidong, Liaohe and Jiangsu oil sludges by hydrogen peroxide oxidation alone was very poor (6.5, 6.8, and 3.4 %, respectively) but increased significantly (p < 0.05), especially of long-chain hydrocarbons, by combining the use of H₂O₂ with surfactants (80.0, 79.8 and 82.2 %, respectively). Oxidation combined with leaching may impair microbial activity and organic manure was therefore added to the treated sludges for biostimulation and the composition and function of the microbial community were studied. The addition of manure rapidly restored sludge microbial activity and significantly increased the relative abundance of some salt-tolerant and alkali-tolerant petroleum-degrading bacteria such as Corynebacterium, Pseudomonas, Dietzia and Jeotgalicoccus. Moreover, the relative abundance of two classic petroleum-degrading enzyme genes, alkane 1-monooxygenase and catechol 1, 2-dioxygenase, increased significantly.

Multigenerational exposure of the collembolan Folsomia candida to soil metals: Adaption to metal stress in soils polluted over the long term

Multigenerational tests provide a comprehensive assessment of the long-term toxicity of pollutants. Here, the multigenerational effects of soil metal contamination on Folsomia candida were investigated over five generations (generations 1-5: F1-F5). Nine soils with varying physicochemical properties and degrees of metal pollution were studied. The selected endpoints were survival, reproduction, body size and body metal concentrations. F. candida was cultured only up to the fifth generation with high reproduction in contaminated acid soils where reproduction was at least 5 times that in neutral soils and 20 times that in calcareous soils. Correlation analysis indicated that soil pH (68.9% contribution) and cation exchange capacity (CEC, 15.4% contribution) were more important factors than pollution level affecting the reproduction of F. candida. No significant difference was observed in adult survival or adult length over five generations. The highest collembolan body Cd concentrations in soils A1-A3 were 3.15, 2.93 and 3.23 times those in F1, with similar results for body Pb. A similar trend in reproduction and juvenile length was observed with an initial decrease (p < 0.05) and then an increase (p < 0.05) over the generations in each acid soil; the opposite trend occurred in the changes in body cadmium (Cd) and lead (Pb) concentrations which increased initially (p < 0.05) and then decreased (p < 0.05) compared to the original concentrations of the first generation. The results indicate that F. candida can adapt to soil metal stress during multigenerational exposure and the adaption energy may be related to a tradeoff between reproduction or growth of juveniles and the detoxification of metals accumulated in the body. Soil properties, especially pH and CEC, had a substantial influence on the long-term survival of the collembolan in the metal-polluted soils.

Soil Metal Immobilization in Agricultural Land Contaminated with Cadmium and Lead: A Case Study of Effectiveness Evaluation in Lanping, Southwest China

The growth of edible crops on land that is highly polluted with potentially toxic elements is prohibited in many developed countries, but the growth of fiber or energy crops may be permitted. Here, we have evaluated metal immobilization in a maize field polluted with cadmium (Cd) and lead (Pb) to determine the thresholds of soil CaCl₂-extractable Cd and Pb and to assess management options designed to maximize food safety. Based on geographical and statistical methods we found that when the soil pH was increased from 5.24 to 6.24, the soil CaCl₂-extractable Cd and Pb values decreased by 47.8 and 74.7%, respectively. Soil CaCl₂-extractable Pb concentrations need to be < 2.14 mg kg^-1 in order to comply with the Chinese maximum permissible grain Pb concentration (< 0.2 mg kg^-1). Immobilization increased the percentage of samples that were below permissible levels from 77.4% to 96.2% (grain Cd) and 90.6% to 96.2% (grain Pb) during the period 2017 to 2019. To avoid excessive or inadequacy immobilization, the spatial distribution of correlation coefficients of soil pH, CaCl₂-extractable or grain Cd/Pb may be helpful in the precise management of immobilization for long-term remediation.

Cadmium and Lead Pollution Characteristics of Soils, Vegetables and Human Hair Around an Open-cast Lead-zinc Mine

Atmospheric deposition of cadmium (Cd) and lead (Pb) was investigated together with the accumulation, distribution and health risks from potentially toxic metals in soils, vegetables and human hair at a mining area in southwest China. Annual atmospheric deposition of Cd and Pb were 41.1 and 192 g ha^- 1, respectively, and consisted mainly of dry deposition. Agricultural soils experienced high levels of metal pollution around the mine, with 66.4% and 57.3 % of vegetable samples grown on these polluted fields exceeding maximum permissible Cd and Pb concentrations, particularly the leafy vegetables. Residents living near the mining area had high Cd (0.75 mg kg^- 1) and Pb (6.87 mg kg^- 1) concentrations in their hair, and the maximum values occurred in occupationally exposed individuals. Long-term mining activities have resulted in high health risks to the local population due to Cd and Pb deposition and accumulation from the atmosphere, soils and vegetables.

Microplastics in an agricultural soil following repeated application of three types of sewage sludge: A field study

Microplastics were investigated in an agricultural soil to which three types of sludge were repeatedly applied: fresh municipal sludge (FSS1), fresh mixed sludge (mainly industrial sludge) (FSS2), and dry heat-treated municipal sludge (DSS). The percentages of microplastics <1.0 mm were 24.3 and 28.7-59.1 % in unamended and amended soils, respectively. Particles of this size accounted for 47.1-60.0 % of microplastics in different sludges and polymers of particle size <100 μm occurred in all soil samples and sludges examined. Fibers were the commonest microplastic type, ranging from 66.7 to 82.5 % in soil and 89.4-97.2 % in sludges. Polyester (PES) and polypropylene (PP) accounted for ~80 % of the total microplastics found in soil and poly-(styrene:acrylate) (PS-AC) microspheres were found in all sludge-amended soil samples examined. There was also a pronounced weathering effect on the surfaces of the microplastics in soil. Nine years of repeated sludge application led to the accumulation of microplastics in the soil. The abundance of microplastics was significantly higher in the municipal sludge (149.2 ± 52.5 particles kg^-1) than in the mixed (68.6 ± 21.5 particles kg^-1) or dried (73.1 ± 15.4 particles kg^-1) sludge and this was related to the microplastic abundance in the sludges. This field study confirms that sludges are drivers of soil microplastic pollution and measures are required to minimize the inputs of microplastics to agricultural land.

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.

Three-dimensional macroscopic aminosilylated nanocellulose aerogels as sustainable bio-adsorbents for the effective removal of heavy metal ions

Designing an environmentally benign bio-adsorbent for the removal of heavy metal ions from aqueous medium was a sustainable strategy to ensure water safety. Herein, three-dimensional macroscopic aminosilyated nanocellulose aerogels (APTM_S-modified TO-NFC) for the removal of heavy metal ions in water were successfully synthesized from bamboo-derived TEMPO-oxidized nanofibrillated cellulose (TO-NFC) and aminopropyltrimethoxysilane (APTMs) via a facile freeze-drying process. Owing to a relatively high BET surface area (129.32 m² g^-1), high porosity (99.14%) as well as high substitution degree of amino groups (0.41), the resulting APTM_S-modified TO-NFC aerogel exhibited good adsorption capacity of 99.0, 124.5, and 242.1 mg g^-1 for Cu²⁺, Cd²⁺ and Hg²⁺, respectively. Furthermore, the crosslinked and three-dimensionally porous architecture imparted it with relatively high compression strength, good excellent stability in water, and ease of recyclability from water after the usage. The pH value of the solution had a great influence on adsorption efficiency of the aerogel adsorbent, and optimal adsorption efficiency could be achieved at pH 3-7. Thermodynamic parameters suggested the spontaneous and endothermic nature of adsorption process. This work provides a facile method for preparing sustainable bio-adsorbent for effective heavy metal ions removal from aqueous medium.

Combined inoculation with dark septate endophytes and arbuscular mycorrhizal fungi: synergistic or competitive growth effects on maize?

BACKGROUND

Effects on maize were assessed of dual inoculation with arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE) isolated from other plant species.

METHODS

Suspensions of DSE isolated from Stipa krylovii were prepared at different densities (2, 4, and 8 × 10⁵ CFU mL^- 1) and inoculated separately (AMF or DSE) or together (AMF + DSE), to explore their effects on maize growth.

RESULTS

Inoculation with AMF or medium and high densities of DSE and combined inoculation (AMF + DSE) increased plant above-ground growth and altered root morphology. Differences in plant growth were attributable to differences in DSE density, with negative DSE inoculation responsiveness at low density. AMF promoted plant above-ground growth more than DSE and the high density of DSE promoted root development more than AMF. Combined inoculation might lead to synergistic growth effects on maize at low density of DSE and competitive effects at medium and high DSE densities.

CONCLUSIONS

AMF and DSE co-colonized maize roots and they had positive effects on the host plants depending on DSE density. These findings indicate the optimum maize growth-promoting combination of AMF and DSE density and provide a foundation for further exploration of potentially synergistic mechanisms between AMF and DSE in physiological and ecological effects on host plants.

Soil microbial community and association network shift induced by several tall fescue cultivars during the phytoremediation of a petroleum hydrocarbon-contaminated soil

Biodegradation of soil contaminants may be promoted near plant roots due to the "rhizosphere effect" which may enhance microbial growth and activity. However, the effects of different plant cultivars within a single species on degradation remains unclear. Here, we evaluated the removal of soil total petroleum hydrocarbons (TPHs) by ten different cultivars of tall fescue grass (Festuca arundinacea L.) and their associated rhizosphere microbiomes. TPH removal efficiency across the ten different cultivars was not significantly correlated with plant biomass. Rhizing Star and Greenbrooks cultivars showed the maximum (76.6%) and minimum (62.2%) TPH removal efficiencies, respectively, after 120 days. Significant differences were observed between these two cultivars in the composition of rhizosphere bacterial and fungal communities, especially during the early stages (day 30) of remediation but the differences decreased later (day 90). Putative petroleum-degrading bacterial and fungal guilds were enriched in the presence of tall fescue. Moreover, the complexity of microbial networks declined in treatments with higher TPH removal efficiency. The relative abundances of saprotrophic fungi and putative genes alkB and C12O in bacetria involved in petroleum degradation increased, especially in the presence of Rhizing Star cultivar, and this was consistent with the TPH removal efficiency results. These results indicate the potential of tall fescue grass cultivars and their associated rhizosphere microbiomes to phytoremediate petroleum hydrocarbon-contaminated soils.

Enhanced biomass and cadmium accumulation by three cadmium-tolerant plant species following cold plasma seed treatment

Cold plasma seed treatment can promote plant growth and enhance the resistance of agricultural crops to adverse stress. However, the effects of plasma seed treatment on the growth and phytoextraction response of plants to cadmium (Cd) remain poorly documented. Here, we have investigated the feasibility of using plasma seed treatment to enhance the biomass and Cd accumulation of three Cd-tolerant species, namely Bidens pilosa L, Solanum nigrum L. and Trifolium repens L, under different plasma treatment conditions. Possible enhancement mechanisms are also proposed according to the levels of organic acids in the roots and the Cd fractions in rhizosphere soil following different plasma treatment conditions. The optimum plasma power was 100 W (B. pilosa) or 500 W (S. nigrum and T. repens). The optimum plasma exposure time for all three species was 60 s. Plasma seed treatment under the optimum treatment conditions enhanced plant dry biomass by ~17.3-45.0% and Cd accumulation by 8.8-54.4% across all three species compared to the controls. Furthermore, the phytoremediation efficiencies, bioaccumulation factors and transfer factors of the three species also increased significantly after seed plasma treatment. The promotion of plasma treatment on the biomass and Cd accumulation of three species might be due to increased exudation of organic acids from the roots into the rhizosphere soil, thus increasing the concentrations of acid-soluble Cd to form Cd-organic acid complexes that facilitated the uptake and translocation of Cd by the plants. Results of this study revealed that cold plasma seed treatment is an environmentally friendly, economical and efficient means to develop the application of phytoremediation for Cd-contaminated soils.