Dominant factor affecting Pb speciation and the leaching risk among land- use types around Pb-Zn mine

Soil lead (Pb) pollution around the mining area has severely threaten human health. However, Pb leaching risk in soils with different land uses and which is the proper land use are still unknown. In this work, Pb speciation characteristics and the dominant soil factors affecting Pb speciation in three land uses (farmland, woodland, and grassland) surrounding the Pb-Zn mine in Feng Country, Shaanxi province were investigated. Moreover, the Pb leaching risk and associated determining factors were evaluated by the combination of leached Pb concentration and structural equation model (SEM). The results showed that farmland presented the highest total Pb content (410.1 mg kg(-1)) among three land use types. The reducible fraction of Pb (Fe-Mn oxides bound) was the major speciation ( > 50%) in all tested soils of three land-use types. Soil total phosphorus (TP), water content (WC), and pH play major role in regulating Pb speciation. Though soil biological properties, like microbial communities, catalase, and microbial biomass nitrogen (MBN) exhibited distinct responses to three different land uses, they showed minor influence on Pb speciation. More interestingly, SEM analysis indicated that Pb leaching risk was directly linked with bacteria abundance, total Pb content, clay content, and C/N. Grassland presented the higher predicted Pb leaching concentration (85.03 mg kg(-1)), compared with that in woodland, suggesting that grassland was the worst land-use type to buffer the Pb toxicity. Woodland could be recommended as the proper native land use to alleviate environmental risk. Overall, our results demonstrated the dominant factor to regulate Pb speciation and pointed out the proper land-use in relieving Pb leaching risk around Pb-Zn mine. These finding provides the new strategies to the remediation and management of metal-contaminated soil

Rhizobacteria inoculation benefits nutrient availability for phytostabilization in copper contaminated soil:Drivers from bacterial community structures in rhizosphere

Plant growth-promoting rhizobacteria (PGPR) and rhizobia are potentially advantageous in improving plant growth in heavy metal contaminated soils. However, only limited information is available in literature on the manner through which the co-inoculation of PGPR and rhizobia can potentially supply nutrients to benefit plant growth in heavy metal contaminated soil. Accordingly, this study investigated the effects of Paenibacillus mucilaginosus (PGPR) and Sinorhizobium meliloti (rhizobia) co-inoculation on soil nutrients, enzyme activities, and microbial biomass in copper (Cu) contaminated soil planted with alfalfa (Medicago sativa). Moreover, we assessed soil bacterial community structure using high-throughput Illumina sequencing of 16S rRNA genes. Results showed that PGPR and/or rhizobia inoculation improved alfalfa growth. In particular, we found that this co-inoculation approach decreased Cu accumulation (48.6%) in shoots compared to the control (uninoculated). Both partial least squares path modeling (PLS-PM) and the relative importance of regressors in the linear models identified that enzyme activities, microbial biomass, and microbial community structure in Cu contaminated soil were major controlling variables of soil nutrient availability. The co-inoculation treatment significantly increased soil carbon (C) and nitrogen (N) concentrations by increasing urease (55.6%), saccharase (29.5%), and β-glucosidase (31.4%) activities compared to the control. Furthermore, the rhizosphere microbial community structure in the co-inoculation treatment was mainly regulated by soil N concentrations (i.e., both total N and available N) while altering alpha diversity (α-diversity). The relative abundances of Firmicutes (including biomarkers of the Bacillus genus) and Acidobacteria were enriched in the co-inoculated treatment, which can potentially improve soil nutrient availability and subsequently benefit plant growth. These findings indicated that the co-inoculation of PGPR and rhizobia plays an important role in promoting plant growth in Cu contaminated soil. This is because this approach can increase soil nutrient availability by enhancing soil enzyme activities and regulating rhizosphere microbial community structure

Reduction of Cu and nitrate leaching risk associated with EDDS-enhanced phytoextraction process by exogenous inoculation of plant growth promoting rhizobacteria

Biodegradable chelant (S,S)-N,N '-ethylenediaminedisuccinic acid (EDDS) has the more advantages of enhanced metal mobility, rapid degradation, environmental friendliness, and ammonium release. However, the risk of metal and/or nitrate residues and leaching within EDDS biodegradation remains as the bottleneck for the widespread application of EDDS-induced phytoremediation. This study aims to explore if the inoculation of plant growth-promoting rhizobacteria (PGPRs) can eliminate the risk associated with the short-term application of EDDS by investigating Cu phytoextraction and soil nitrate content. Results showed that EDDS application significantly increased the copper (Cu) concentration in shoots, soil total Cu, NH4+-N and NO3--N content, but decreased plant biomass. The inoculation of PGPRs in the soil showed a strong ability to increase plant biomass, Cu phytoextraction and soil NH4+-N content, and decrease soil Cu and NO3--N content. Moreover, bacterial dominant taxa were found to be the largest contributors to soil NH4+-N and NO3--N variation, and the abundance of denitrifying bacteria (Bacteroidetes and Stenotrophomonas) decreased in the treatment with PGPRs. The risk of residual Cu and nitrate leaching was reduced by the inoculation of PGPRs without significantly changing the stability of the bacterial community. These new findings indicate that the exogenous application of beneficial rhizobacteria can provide an effective strategy to reduce the risk in metal-contaminated soils of chelant-assisted phytoextraction.</p

A novel extracellular enzyme stoichiometry method to evaluate soil heavy metal contamination: Evidence derived from microbial metabolic limitation

Heavy metal contaminates have become a significant threat to soil ecosystems due to their chronicity and universality in soil. Soil microbial metabolism plays a vital role in biogeochemical cycles and soil functions. However, the response of microbial metabolism to heavy metal contamination in soil remains elusive despite potentially offering important insight into the health and ecological consequences of soil ecosystems under such contamination. This study used extracellular enzyme stoichiometry models to identify the response of microbial metabolism to various heavy metal contaminants, while also revealing potential implications of heavy metal contaminates in soil ecosystems. Results showed that microbial metabolism was restricted by soil carbon (C) and phosphorus (P) within a heavy metal polluted area in Northwest China. Heavy metal stress significantly increased microbial C limitation while decreasing microbial P limitation. However, microbial C and P limitations both responded consistently to different heavy metals (i.e., Cd, Pb, Zn, and Cu). Heavy metals had the greatest effect on microbial C limitation (i.e., 0.720 of the total effects) compared to other soil properties, and soil with the lowest heavy metal concentration exhibited the lowest microbial C limitation, and vice versa. These results indicated that microbial metabolic limitation can robustly and sensitively reflect the degree of heavy metals pollution in soil. Additionally, increased microbial C limitation caused by heavy metal contaminants could potentially escalate C release by promoting soil C decomposition as well as increasing investments in enzyme production and the maintenance of metabolic processes. Consequently, potential C loss induced by heavy metal pollution on soil eco-systems may be extensive and significant. Generally, our results suggest the usefulness of extracellular enzyme stoichiometry as a new method from which to evaluate heavy metal soil pollution, while microbial metabolic limitation could potentially be a promising indicator. (C) 2020 Elsevier B.V. All rights reserved

Soil moisture mediates microbial carbon and phosphorus metabolism during vegetation succession in a semiarid region

Revegetation of semiarid lands depends upon soil microbial communities to supply nutrients for successive plant species, but microbial activity can be constrained by insufficient water. The objective of this study was to quantify the metabolic limitation of microbes by extracellular enzymatic stoichiometry, and to determine how this affected microbial carbon use efficiency (CUE) with biogeochemical equilibrium model. The study occurred in long-term revegetation experiment with seven successional stages (0, 11, 35, 60, 100, 130 and 150 years) in the Loess Plateau, China. Microbes maintained stoichiometric homeostasis in all successional stages, but plants did not. Microbial metabolism was limited by low soil phosphorus (P) concentration throughout the succession, whereas plants were limited by low soil P during the late successional stages (from 60 to 150 years) only. An increase in soil moisture during succession was associated with greater P limitation in microbes and plants. There was less microbial P limitation at the 35-year successional stage, and the greatest microbial P limitation occurred at the 130-year successional stage. The microbial C limitation followed a unimodal pattern through the vegetation succession and reached a maximum at 100 years of succession (the early forest stage). This coincided with the lowest microbial CUE at 100 years of succession (CUE was from 0.24 to 0.41), suggesting a change in the physiological responses from microbes (such as enzyme synthesis and the priming effect), that tended to reduce soil C sequestration. Our results indicate that soil moisture regulated microbial C and P metabolism during the vegetation succession in this semiarid region, which has implications for understanding how microbial metabolism affects soil C dynamics under vegetation restoration

Co-inoculation effect of plant-growth-promoting rhizobacteria and rhizobium on EDDS assisted phytoremediation of Cu contaminated soils

Chelants application can increase the bioavailability of metals, subsequently limiting plant growth and reducing the efficiency of phytoremediation. Plant growth-promoting rhizobacteria (PGPRs) and rhizobium have substantial potential to improve plant growth and plant tolerance to metal stress. We evaluated the effects of co-inoculation with a PGPR strain (Paenibacillus mucilaginosus) and a Cu-resistant rhizobium strain (Sinorhizobium meliloti) on the efficiency of biodegradable chelant (S,S-ethylenediaminedisuccinic acid; EDDS) assisted phytoremediation of a Cu contaminated soil using alfalfa. The highest total Cu extraction by alfalfa was observed in the EDDS-treated soil upon co-inoculation with the PGPR and rhizobium strains, which was 1.2 times higher than that without co-inoculation. Partial least squares path modeling identified plant oxidative damage and soil microbial biomass as the key variables influencing Cu uptake by alfalfa roots. Co-inoculation significantly reduced the oxidative damage to alfalfa by mitigating the accumulation of malondialdehyde and reactive oxygen species, and improving the antioxidation capacity of the plant in the presence of EDDS. EDDS application decreased microbial diversity in the rhizosphere, whereas co-inoculation increased microbial biomass carbon and nitrogen, and microbial community diversity. Increased relative abundances of Actinobacteria and Bacillus and the presence of Firmicutes taxa as potential biomarkers demonstrated that co-inoculation increased soil nutrient content, and improved plant growth. Co-inoculation with PGPR and rhizobium can be useful for altering plant-soil biochemical responses during EDDS-enhanced phytoremediation to alleviate phytotoxicity of heavy metals and improve soil biochemical activities. This study provides an effective strategy for improving phytoremediation efficiency and soil quality during chelant assisted phytoremediation of metal-contaminated soils. (C) 2020 Elsevier Ltd. All rights reserved

Ecoenzymatic stoichiometry reveals microbial phosphorus limitation decreases the nitrogen cycling potential of soils in semi-arid agricultural ecosystems

Variations in soil microbial metabolism currently represent one of the greatest areas of uncertainty with regard to soil nutrient cycles and the control of terrestrial carbon (C) and nitrogen (N) loss and are poorly understood in agricultural ecosystems with intensive farming practices. In this study, extracellular enzymatic stoichiometry models and quantitative PCR techniques were used to examine microbial metabolic limitation and its relationship with N-cycling gene expression in semi-arid agricultural ecosystems considering four N fertilization levels (N 0, N 100, N 250, and N 400 kg N ha(-1)) and two agronomic strategies (film mulching and no mulching). Film mulching increased microbial C limitation (reflecting microbial C metabolism size; 0.189 of the total effects), while very small effects on microbial phosphorus (P) limitation were observed (-0.007 of the total effects). N fertilization increased the microbial demand for P (microbial P limitation; 0.504 of the total effects). Increased microbial C metabolism was mainly attributed to increased soil moisture content after film mulching, which enhanced microbial decomposition of organic C (high C-acquiring enzyme activities). Changes in nutrient stoichiometry and the increase in N availability due to N fertilization were largely responsible for increased microbial P limitation. Furthermore, microbial P limitation negatively affected the abundance of AOA amoA, AOB amoA (involved in nitrification), nirK, nirS, nosZ (involved in denitrification) genes, strongly inhibiting nitrification and denitrification potential (-0.743 and -0.761 of the total effects, respectively). The present results suggest that agricultural ecosystems with film mulching are conducive to organic residue decomposition, while appropriate P limitation under N fertilization could reduce the loss of N due to nitrification and denitrification in soil. This study highlights the importance of elemental stoichiometry-driven microbial metabolic variation in understanding soil nutrient cycles and optimizing agricultural practices