Biogeochemical distribution of Pb and Zn forms in two calcareous soils affected by mycorrhizal symbiosis and alfalfa rhizosphere

Identification of cadmium-tolerant plant growth-promoting rhizobacteria and characterization of its Cd-biosorption and strengthening effect on phytoremediation: Development of a new amphibious-biocleaner for Cd-contaminated site

The use of plant growth-promoting rhizobacteria (PGPR) in decontaminating cadmium-contaminated soil and water is a sustainable and eco-friendly approach. This study aimed to isolate a PGPR strain from the rhizosphere soil of Solanum nigrum and evaluate its potential and mechanisms in remediating Cd-contaminated environments. The results showed that the isolated strain, Klebsiella sp. AW2, can tolerate 240 mg/L Cd2+. Batch biosorption experiments indicated that the optimal conditions for PGPR biosorption were a pH of 5.0, a biosorbent dosage of 1.0 g/L, and a Cd2+ concentration of 10 mg/L, resulting in a biosorption rate of 40.99%. Model fitting results revealed that the Cd biosorption process followed a uniform surface monolayer chemisorption mechanism, likely involving complexation with functional groups such as -NH, -OH, and -C=O, according to Fourier transform infrared spectrometer and desorption experiments. Furthermore, pot experiments demonstrated that PGPR application significantly enhanced the phytoremediation efficiency of Cd-contaminated soil, increasing the phytoextraction ratio by 32.41%. This improvement was primarily achieved by promoting S. nigrum growth and facilitating Cd horizontal transfer from rhizosphere soil to plants through influencing the rhizosphere soil physicochemical properties and Cd2+ influx in roots. In addition, the copy number of the 16S rRNA gene of the PGPR revealed that the PGPR was predominantly localized in the rhizosphere soil, directly leading to increased availability of Cd for plant uptake. Overall, these findings indicate that Klebsiella sp. AW2 is a promising biocleaner for Cd-contaminated environments and provide valuable insights into the application of biosorbents in phytoremediation efforts.

Plant growth promoting endophyte modulates soil ecological characteristics during the enhancement process of cadmium phytoremediation

Endophyte assisted phytoremediation of cadmium (Cd) contaminated soil represents a promising strategy. However, the precise soil ecological regulatory mechanisms by which endophyte enhance the Cd phytoextraction remain unclear. Here, we employed the plant growth promoting endophyte (PGPE) Pseudomonas sp. E3, which has been validated to effectively enhance Cd extraction in Solanum nigrum L., to investigate its regulatory mechanism on soil ecology. The results demonstrated that while PGPE inoculation resulted in minimal alterations to the physicochemical properties of the bulk soil, it led to a notable increase in acid phosphatase activity by 17.86% and urease activity by 24.85% in the rhizosphere soil. This, in turn, significantly raised the available nitrogen and phosphorus contents by 16.93% and 21.27%, respectively, in the rhizosphere soil. Additionally, PGPE inoculation effectively replenished the bioavailable fractions of Fe and Cd, which had been depleted due to root uptake. Importantly, the inoculation specifically augmented the abundance of biomarkers p_Patescibacteria, f_Saccharimonadales, and g_Saccharimonadales in the rhizosphere soil. These biomarkers exhibited a significant positive correlation with the available nutrient and metal element contents. Moreover, the co-occurrence network analysis demonstrated that the inoculation resulted in a simplified bacterial community network, which may have facilitated community synergism by displacing bacteria with a negative association. This regulation appears to occur independently of PGPE colonization. Overall, our findings suggested that PGPE also exerts a regulatory influence on soil ecological features, significantly aiding hyperaccumulators in nutrient acquisition and heavy metal accumulation.

Spraying or fertigation? looking for the better method in terms of the lower concentration of Pb and Ni in greenhouse cucumber

This study was conducted to investigate the status of Lead (Pb) and Nickel (Ni) in greenhouse cucumber affected by fertigation and spraying as a factorial split plot arrangement based on a randomized complete block design (RCBD) with three replications at Soil and Water Research Institute, Karaj, Iran in 2023. The main and sub-plots were respectively fertigation and spraying that was applied in two levels [the minimum permissible concentration of these metals in granular triple super phosphate fertilizer (five ppm Pb and five ppm Ni) as the control treatment, and the maximum permissible concentration of Pb and Ni in granular triple super phosphate fertilizer (50 ppm Pb and 100 ppm Ni)]. Spraying was done in three modes (spraying leaves, leaves + fruit, and fruit). Based on the results, the highest concentration of Pb in the fruit (fruit peel + fruit flesh) (0.295 mg/kg DW) was recorded in the fertigation + spraying with the maximum permissible concentrations of Pb and Ni treatments. A general comparison between the two fertilization methods showed that the spraying method accumulated a higher concentration of Pb in the fruit (i.e., peel + flesh) than the fertigation method. The reverse of this status happened for Ni, so the fertigation method accumulated a higher concentration of Ni in the fruit (skin + flesh) compared to the spraying method. On average, the order of Pb concentration in different organs was as follows: leaf (0.765 mg/kg) > fruit peel (0.232 mg/kg) > fruit peel + flesh (0.174 mg/kg) > fruit flesh (0.129 mg/kg). This order for Ni was as follows: fruit flesh (0.597 mg/kg) > fruit peel + flesh (0.345 mg/kg) > leaf (0.3 mg/kg) > fruit peel (0.175 mg/kg). These orders show that the status of the heavy metals in plant tissues is related to the nature of the metal, the type of the organ, and the method of fertilization.

Ecotoxic effect in Allium cepa due to sphalerite weathering arising in calcareous conditions

The ecotoxic effect of Zn species arising from the weathering of the marmatite-like sphalerite ((Fe, Zn)S) in Allium cepa systems was herein evaluated in calcareous soils and connected with its sulfide oxidation mechanism to determine the chemical speciation responsible of this outcome. Mineralogical analyses (X-ray diffraction patterns, Raman spectroscopy, scanning electron microscopy and atomic force microscopy), chemical study of leachates (total Fe, Zn, Cd, oxidation-reduction potential, pH, sulfates and total alkalinity) and electrochemical assessments (chronoamperometry, chronopotentiometry, cyclic voltammetry, and electrochemical impedance spectroscopy) were carried out using (Fe, Zn)S samples to elucidate interfacial mechanisms simulating calcareous soil conditions. Results indicate the formation of polysulfides (Sn2-), elemental sulfur (S0), siderite (FeCO3)-like, hematite (Fe2O3)-like with sorbed CO32- species, gunningite (ZnSO4·H2O)-like phase and smithsonite (ZnCO3)-like compounds in altered surface under calcareous conditions. However, the generation of gunningite (ZnSO4·H2O)-like phase was predominant bulk-solution system. Quantification of damage rates ranges from 75 to 90% of bulb cells under non-carbonated conditions after 15-30 days, while 50-75% of damage level is determined under neutral-alkaline carbonated conditions. Damage ratios are 70.08 and 30.26 at the highest level, respectively. These findings revealed lower ecotoxic damage due to ZnCO3-like precipitation, indicating the effect of carbonates on Zn compounds during vegetable up-taking (exposure). Other environmental suggestions of the (Fe, Zn)S weathering and ecotoxic effects under calcareous soil conditions are discussed.

Applicability of the sigmoid model to estimate heavy metal uptake in maize and sorghum as affected by organic acids

Although assisted phytoremediation using chemical treatments is a suitable technique for the removal of heavy metals (HMs), the estimation of this process using simple models is also crucial. For this purpose, a greenhouse trial was designed to evaluate the effectiveness of citric, oxalic, and tartaric acid on Cd, Pb, Ni, and Zn phytoremediation by maize and sorghum and to estimate this process using sigmoid HMs uptake model. Results showed that mean values of root and shoot dry weight and metals uptake, translocation factor (TF) of Pb and Zn, and uptake efficiency (UE) of Cd in maize were higher than sorghum but the TF of Cd and the phytoextraction efficiency (PEE) and UE of Pb in sorghum were higher than maize. Citric, oxalic, and tartaric acid significantly increased the UE of Pb by 17.7%, 22.5%, and 32.5%, respectively. Tartaric acid significantly increased the mean values of shoot dry weight, shoot Cd, Pb, and Ni uptake, and PEE of Pb and Ni, but decreased TF of Zn. The R2, NRMSE, and KM values indicated the ability of sigmoid HM uptake model in estimating HMs uptake in maize and sorghum treated with organic acids. Thus, tartaric acid was more effective than citric and oxalic acids to enhance phytoremediation potential. Sigmoid HM uptake model is suitable to estimate the HMs uptake in plants treated with organic acids at different growth stages.

Characterization of the Rhizosphere Bacterial Microbiome and Coffee Bean Fermentation in the Castillo-Tambo and Bourbon Varieties in the Popayán-Colombia Plateau

BACKGROUND

The microbial biodiversity and the role of microorganisms in the fermentation of washed coffee in Colombia were investigated using the Bourbon and Castillo coffee varieties. DNA sequencing was used to evaluate the soil microbial biota and their contribution to fermentation. The potential benefits of these microorganisms were analyzed, including increased productivity and the need to understand the rhizospheric bacterial species to optimize these benefits.

METHODS

This study used coffee beans for DNA extraction and 16 S rRNA sequencing. The beans were pulped, samples were stored at 4ºC, and the fermentation process was at 19.5ºC and 24ºC. The fermented mucilage and root-soil samples were collected in duplicate at 0, 12, and 24 h. DNA was extracted from the samples at a concentration of 20 ng/µl per sample, and the data obtained were analyzed using the Mothur platform.

RESULTS

The study demonstrates that the coffee rhizosphere is a diverse ecosystem composed primarily of microorganisms that cannot be cultured in the laboratory. This suggests that the microbial community may vary depending on the coffee variety and play an essential role in fermentation and overall coffee quality.

CONCLUSIONS

The study highlights the importance of understanding and optimizing the microbial diversity in coffee production, which could have implications for the sustainability and success of coffee production. DNA sequencing techniques can help characterize the structure of the soil microbial biota and evaluate its contribution to coffee fermentation. Finally, further research is needed to fully understand the biodiversity of coffee rhizospheric bacteria and their role.

Nitrogen and sulfur fertilizers promote the absorption of lead and cadmium with Salix integra Thunb. by increasing the bioavailability of heavy metals and regulating rhizosphere microbes

Fertilization is an effective agronomic strategy to improve the efficiency of phytoextraction by Salix integra Thunb. However, the specific effects of the simultaneous application of nitrogen (N) and sulfur (S) fertilizers in the rhizosphere remain unclear. We investigated the bioavailability of lead (Pb) and Cadmium (Cd) along with the microbial metabolic functions and community structure in the rhizosphere soil of S. integra after the application of N (0, 100, and 200 kg·ha⁻¹·year⁻¹) and S (0, 100, and 200 kg·ha⁻¹·year⁻¹) fertilizers for 180 days. The simultaneous application of N and S fertilizers significantly enhanced the absorption of Pb and Cd by S. integra, whereas this effect was not observed for the single application of N or S fertilizer. The contents of acid-soluble Pb and Cd in the rhizosphere soil significantly increased after either single or combined fertilize applications. The microbial metabolic activity was enhanced by the N and S fertilizers, whereas the microbial diversity markedly decreased. The metabolic patterns were mainly affected by the concentration of N fertilizer. The dominant fungi and bacteria were similar under each treatment, although the relative abundances of the dominant and special species differed. Compared to the N200S100 and N200S200 treatments, the N100S100 and N100S200 treatments resulted in fewer pathogenic fungi and more rhizosphere growth-promoting bacteria, which promoted phytoremediation by S. integra. Redundancy analysis indicated that the pH and nitrate content were the key factors affecting the structure of the microbial community. Collectively, the results suggest interactive effects between N and S fertilizers on the rhizosphere soil, providing a potential strategy for plant-microbial remediation by S. integra.

Vermicomposting of sewage sludge with organic bulking materials to improve its properties

Adverse properties of sewage sludge such as high concentration of toxic metals and the presence of pathogens have limited its use in agriculture. In this study, the effect of vermicomposting on some important chemical properties of sewage sludge was investigated as a factorial arrangement based on a completely randomized design with three replications. The studied factors were bulking agents in three concentrations (0, 5, and 10% v/v), earthworm (Eisenia fetida) in 2 different concentrations (0 and 50 adult earthworms/6 kg sewage sludge), and retention times in three levels (30, 60, and 90 days). Sewage sludge was obtained from the lagoons of Ghahdarijan refinery in Isfahan province in Iran. The results showed that vermicomposting significantly (p < 1%) reduced the number of coliforms and concentrations of Ni, Cr, and Pb and increased the organic carbon percent and N of the sewage sludge. When comparing different treatments, vermicomposting of sewage sludge with rice husk 10% v/v during retention times of 60 and 90 days were superior treatments in reducing the concentrations of Ni, Cr, and Pb and reducing the number of coliforms. Furthermore, the highest %OC was measured in the rice husk 10% + earthworm in the retention time of 90 days which was about 41% more than the control treatment. In general, it is concluded that vermicomposting significantly reduced the adverse properties of sewage sludge for using in agriculture as an organic fertilizer.

Increase of P and Cd bioavailability in the rhizosphere by endophytes promoted phytoremediation efficiency of Phytolacca acinosa

The information about the spatial distribution of bioavailable phosphorus (P) and heavy metal (HM) in the rhizosphere could aid in the precise phytoremediation regulation. In this study, a rhizobox system was adapted to study soil-root interactions and used to access the endophyte inoculation variation on bioavailable P and cadmium (Cd) spatial distribution during phytoremediation of Cd contaminated soils. Results showed that endophyte PE31 Bacillus cereus inoculation enhanced Cd uptake of P. acinosa by 52.70% and 46.73% in low and high Cd contaminated soils, increasing the phytoremediation hotspot area from 45.78% and 15.29% to 60.97% and 21.80%, respectively. Available P and Cd significantly diminished because root activities depleted large amounts of bioavailable P and Cd concentrations. However, PE31 increased bioavailable P and Cd concentration in the rhizosphere soil. The bioavailable P enhancement in the rhizosphere was positively correlated to plant growth and Cd accumulation. Overall, endophyte inoculation compensated the diminution of bioavailable P and Cd in the rhizosphere to improve plant biomass and HM absorption, and thus promote phytoremediation efficiency. This study helped to better understand bioavailable P and Cd spatial distribution under endophyte inoculation, which could provide effective management strategies for the precise regulation of phytoremediation.

Investigation of factors affecting phytoremediation of multi-elements polluted calcareous soil using Taguchi optimization

Growing environmental concern regarding multi elements-contaminated soils reveals the necessity of paying more attention to environmentally friendly remediation techniques such as phytoremediation. A large number of factors influences phytoremediation of potentially toxic elements (PTEs) and investigation on a variety of these factors need appropriate statistical approaches such as "Taguchi optimization" which effectively decreases time and cost of experiments. In the present study, based on the Taguchi optimization method, the effects of several biological (plant type and mycorrhizal fungi (AMF)) and chemical (chelating agents, surfactants and organic acids) factors, on the phytoremediation of soils contaminated with zinc (Zn), lead (Pb), cadmium (Cd) and nickel (Ni) were investigated. The goal was to find out the most effective factors as well as the best level for each factor. The values of dry weights in roots and aerial parts of the studied plants were in orders of maize > sorghum > sunflower and sorghum > maize > sunflower, respectively. AMF was the main factor in increasing dry weight of shoots. Inoculation of AMF caused increases in root and shoot uptake of some PTEs. RESULTS: showed that phytoremediation of PTEs is element-dependent; as Zn showed the highest translocation factor (TF) and bioconcentration factor (BCF) values, while Ni showed the lowest ones and the intermediate values belonged to Pb and Cd. These results show the diverse distribution of elements in plant parts, as Zn and Ni were mostly accumulated in shoot and root, respectively. Although different factors caused impacts on phytoremediation criteria, the role of plant type in the phytoremediation of PTEs was at the first rank. Mean TF of PTEs in sunflower was 6.3 times that of maize. Sunflower showed high TF value for the four elements and translocated most of the PTEs from root to the aerial parts demonstrating phytoextraction as the main mechanism in this plant. Maize and sorghum, however, showed low TF and accumulated most of PTEs in their roots revealing phytostabilization as the main mechanism. In general, it can be concluded that plant type was the most influential factor in the phytoremediation of PTEs followed by EDTA and AMF. Taguchi optimization revealed the appropriateness and significance of different chemical and biological treatments on phytoremediation criteria of different elements.

Indigenous Bacteria Have High Potential for Promoting Salix integra Thunb. Remediation of Lead-Contaminated Soil by Adjusting Soil Properties

Salix integra Thunb., a fast-growing woody plant species, has been used for phytoremediation in recent years. However, little knowledge is available regarding indigenous soil microbial communities associated with the S. integra phytoextraction process. In this study, we used an Illumina MiSeq platform to explore the indigenous microbial composition after planting S. integra at different lead (Pb) contamination levels: no Pb, low Pb treatment (Pb 500 mg kg^–1), and high Pb treatment (Pb 1500 mg kg^–1). At the same time, the soil properties and their relationship with the bacterial communities were analyzed. The results showed that Pb concentration was highest in the root reaching at 3159.92 ± 138.98 mg kg^–1 under the high Pb treatment. Planting S. integra decreased the total Pb concentration by 84.61 and 29.24 mg kg^–1, and increased the acid-soluble Pb proportion by 1.0 and 0.75% in the rhizosphere and bulk soil under the low Pb treatment compared with unplanted soil, respectively. However, it occurred only in the rhizosphere soil under the high Pb treatment. The bacterial community structure and microbial metabolism were related to Pb contamination levels and planting of S. integra, while the bacterial diversity was only affected by Pb contamination levels. The dominant microbial species were similar, but their relative abundance shifted in different treatments. Most of the specific bacterial assemblages whose relative abundances were promoted by root activity and/or Pb contamination were suitable for use in plant-microbial combination remediation, especially many genera coming from Proteobacteria. Redundancy analysis (RDA) showed available nitrogen and pH having a significant effect on the bacteria relating to phytoremediation. The results indicated that indigenous bacteria have great potential in the application of combined S. integra-microbe remediation of lead-contaminated soil by adjusting soil properties.

Effects of selenium combined with zinc amendment on zinc fractions and bioavailability in calcareous soil

Selenium (Se) and zinc (Zn) are two important trace elements for human being and animals. The interaction between Se and Zn on the bioavailability of Zn in soil is still unclear. Therefore, pot experiments exposed to different dosages of zinc sulfate (ZnSO₄) (0, 20, and 50 mg/kg soil) and sodium selenite (Na₂SeO₃) (0, 0.5, 1.0, and 2.5 mg/kg soil) were conducted to investigate the effects of selenite application on Zn bioavailability in calcareous soil and its related mechanisms. The total Zn content of different tissues (roots and shoots) of pak choi (Brassica chinensis L.) and the changes in Zn fraction distribution in soil before planting and after harvest were determined, and the mobility factor (M_F) and distribution index (D_I) of Zn in soils were calculated. In addition, the Pearson correlation and path analysis were conducted to clarify the relationships between Zn fractions in soil and the Zn uptake of pak choi. Results showed that Se amendment elevated soil Zn bioavailability at appropriate levels of Se and Zn. When 1.0 and 2.5 mg/kg of Se and 20 mg/kg of Zn were applied in soil, the proportion of exchangeable Zn (Ex-Zn) and Zn weakly bound to organic matter (Wbo-Zn) to the total content of Zn was significantly increased by 28.14%-82.52% compared with that of the corresponding single Zn treatment. Therefore, the Zn concentration in the shoots of pak choi was significantly increased by 27.2%-31.1%. High Zn (50 mg/kg) and Se co-amended treatments showed no significantly beneficial effect on the bioavailability of Zn. In addition, the potential available Zn content in soil (weakly bound to organic matter and carbonate bound Zn) and M_F and D_I values were all positively correlated with the Zn concentrations in pak choi, indicating that these indexes can be used to predict the bioavailability of Zn in soil. This study can provide a good reference for Se and Zn biofortification of plants in calcareous soil.

Soil organic matter prevails over heavy metal pollution and vegetation as a factor shaping soil microbial communities at historical Zn-Pb mining sites

This study examined the effects of soil heavy metals, macronutrients, texture and pH as well as plant species richness and composition on soil respiration, enzymatic activity, microbial biomass, metabolic quotient (qCO₂) and arbuscular mycorrhizal fungi (AMF) at sites of historical Zn-Pb mining. The study was conducted both on a large scale (65 heaps scattered over the area of 750 km²) and on a small scale (25 plots along two 48 m transects extending from heaps to adjacent fallow fields). Total concentrations of metals exceeded 400 (Cd), 20,000 (Pb) and 80,000 (Zn) mg kg^-1 at the most polluted sites. Although they decreased along the heap-fallow direction, they still remained above environmental standards in fallow soils. In contrast, some soluble metal forms increased with the increasing distance from heaps. Soil organic matter had the strongest positive effect on most microbial parameters. Total and/or available heavy metals exhibited significant negative effects on microbial biomass, enzymatic activity and AMF, and a positive effect on qCO₂. Organic matter alleviated negative effects of heavy metals on microorganisms; they were not observed where the increase in the contamination was accompanied by the increase in organic matter content. Plant species richness affected positively enzymatic activity and mycorrhization level. Plant species composition possibly contributed to the formation of soil microbial communities, but its effect was entangled in that of heavy metals as plant communities changed along pollution gradients (from metal-tolerant grasslands dominated by Festuca ovina to calcareous grasslands and ruderal communities at less polluted sites).

Arbuscular mycorrhiza fungi and related soil microbial activity drive carbon mineralization in the maize rhizosphere

This research is aimed to investigate the effect of arbuscular mycorrhiza (AM) fungi on soil microbial activity and carbon mineralization in the maize rhizosphere under potted condition. Glomus etunicatum was used for our experiment. Results showed that AM symbiosis increased the levels of microorganism in the maize rhizosphere soil, and enhanced activity of soil microbial enzymes. After inoculating AM fungi, the contents of dissolved organic carbon (DOC), microbial biomass carbon (MBC) and readily oxidizable carbon (ROC) in the rhizosphere soil of maize increased with varying degrees. We obtained strong evidence that higher contents of MBC, DOC, ROC, superior number of microbes and stronger soil enzyme activities could be responsible for the higher rate of carbon mineralization in AM fungi treatment. AM fungi inoculation was confirmed to be effective to improve the soil quality for larger-scale ecoengineering.