Biochar addition promotes soil organic carbon sequestration dominantly contributed by macro-aggregates in agricultural ecosystems of China.

Soil aggregates play pivotal roles in soil organic carbon (SOC) preservation and climate change. Biochar has been widely applied in agricultural ecosystems to improve soil physicochemical properties. However, the underlying mechanisms of SOC sequestration by soil aggregation with biochar addition are not well understood at a large scale. Here, we conducted a meta-analysis of 2335 pairwise data from 45 studies to explore how soil aggregation sequestrated SOC after biochar addition in agricultural ecosystems of China. Biochar addition markedly enhanced the proportions of macro-aggregates and aggregate stability, and the production of organic binding agents positively facilitated the formation of macro-aggregates and aggregate stability. Soil aggregate-associated organic carbon (OC) indicated a significantly increasement by biochar addition, which was attributed to direct and indirect inputs of OC from biochar and organic residues, respectively. Biochar stimulated SOC sequestration dominantly contributed by macro-aggregates, and it could be interpreted by a greater improvement in proportions and OC protection of macro-aggregates. Furthermore, the SOC sequestration of soil aggregation with biochar addition was regulated by climate conditions (mean annual temperature and precipitation), biochar attributes (biochar C/N ratio and pH), experimental practices (biochar addition level and duration), and agronomic managements (land type, cropping intensity, fertilization condition, and crop type). Collectively, our synthetic analysis emphasized that biochar promoted the SOC sequestration by improving soil aggregation in agricultural ecosystems of China.

Impact of co-inoculation with plant-growth-promoting rhizobacteria and rhizobium on the biochemical responses of alfalfa-soil system in copper contaminated soil.

The effects and regulatory mechanisms of co-inoculation of plant-growth-promoting rhizobacteria (PGPRs) and rhizobium in plant-soil systems remain unclear, despite numerous reports that PGPRs or rhizobium can alleviate metal toxicity. We used the co-inoculation of the PGPR Paenibacillus mucilaginosus and the metal-resistant rhizobium Sinorhizobium meliloti for exploring the physiological and biochemical responses of the plant-soil system in metal-contaminated soil. The co-inoculation with the PGPR and rhizobium significantly increased the nutrient (N, P, and K) contents in plant tissues and promoted plant growth in soil contaminated with copper (Cu). Stress from Cu-induced reactive oxygen species and lipid peroxidation were largely attenuated by the co-inoculation by increasing the activities of antioxidant enzymes. The contents and uptake of Cu in plant tissues increased significantly in the co-inoculation treatment compared with the uninoculated control and individual inoculation treatment. Co-inoculation with PGPR and rhizobium significantly increased soil microbial biomass, enzymatic activities, total nitrogen, available phosphorus, and soil organic matter contents compared with the uninoculated control. Interestingly, co-inoculation also affected the composition of the rhizospheric microbial community, and slightly increased rhizospheric microbial diversity. These improvements of the soil fertility and biological activity also had a beneficial impact on plant growth under Cu stress. Our results suggested that alfalfa co-inoculated with PGPR and rhizobium could increase plant growth and Cu uptake in metal-contaminated soil by alleviating plant Cu stress and improving soil biochemical properties. These results indicate that the co-application of PGPR and rhizobium can have a positive effect on the biochemical responses of alfalfa-soil systems in soil contaminated by heavy metals and can provide an efficient strategy for the phytomanagement of metal-contaminated land.

Application of signaling molecules in reducing metal accumulation in alfalfa and alleviating metal-induced phytotoxicity in Pb/Cd-contaminated soil.

The utilization of forages grown on metal-contaminated soil can increase the risk of heavy metals entering the food chain and affecting human health because of elevated toxic metal concentrations. Meanwhile, hydrogen sulfide (H2S) and nitric oxide (NO) as signaling molecules are known to promote plant growth in metal-contaminated soils. However, the regulatory mechanisms of such molecules in plant physiology and soil biochemistry have not been well-documented. Hence, we investigate the role of the exogenous application of H2S and NO on alfalfa growth in lead/cadmium (Pb/Cd)-contaminated soil. Our results indicate that the signaling molecules increase the alfalfa chlorophyll and biomass content and improve alfalfa growth. Further, H2S and NO reduce the translocation and bioconcentration factors of Pb and Cd, potentially reducing the risk of heavy metals entering the food chain. These signaling molecules reduce metal-induced oxidative damage to alfalfa by mitigating reactive oxygen species accumulation and increasing antioxidant enzyme activities. Their exogenous application increases soil enzymatic activities, particularly of catalase and polyphenol oxidase, without significantly changing the composition and structure of rhizosphere bacterial communities. Interestingly, H2S addition enriches the abundance of plant-growth-promoting rhizobacteria in soil, including Nocardioides, Rhizobium, and Glycomyces. H2S is more effective than NO in improving alfalfa growth and reducing heavy-metal contamination of the food chain. These results provide new insights into the exogenous application of signaling molecules in alleviating metal-induced phytotoxicity, including an efficient strategy for the safe use of forages.

Reveal the response of enzyme activities to heavy metals through in situ zymography.

Enzymes in the soil are vital for assessing heavy metal soil pollution. Although the presence of heavy metals is thought to change the soil enzyme system, the distribution of enzyme activities in heavy metal polluted-soil is still unknown. For the first time, using soil zymography, we analyzed the distribution of enzyme activities of alfalfa rhizosphere and soil surface in the metal-contaminated soil. The results showed that the growth of alfalfa was significantly inhibited, and an impact that was most pronounced in seedling biomass and chlorophyll content. Catalase activity (CAT) in alfalfa decreased with increasing heavy metal concentrations, while malondialdehyde (MDA) content continually increased. The distribution of enzyme activities showed that both phosphatase and ß-glucosidase activities were associated with the roots and were rarely distributed throughout the soil. In addition, the total hotspot areas of enzyme activities were the highest in extremely heavy pollution soil. The hotspot areas of phosphatase were 3.4%, 1.5% and 7.1% under none, moderate and extremely heavy pollution treatment, respectively, but increased from 0.1% to 0.9% for ß-glucosidase with the increasing pollution levels. Compared with the traditional method of enzyme activities, zymography can directly and accurately reflect the distribution and extent of enzyme activity in heavy metals polluted soil. The results provide an efficient research method for exploring the interaction between enzyme activities and plant rhizosphere.

Unveiling the impacts of microplastic pollution on soil health: A comprehensive review.

Soil contamination by microplastics (MPs) has emerged as a significant global concern. Although traditionally associated with crop production, contemporary understanding of soil health has expanded to include a broader range of factors, including animal safety, microbial diversity, ecological functions, and human health protection. This paradigm shifts underscores the imperative need for a comprehensive assessment of the effects of MPs on soil health. Through an investigation of various soil health indicators, this review endeavors to fill existing knowledge gaps, drawing insights from recent studies conducted between 2021 and 2024, to elucidate how MPs may disrupt soil ecosystems and compromise their crucial functions. This review provides a thorough analysis of the processes leading to MP contamination in soil environments and highlights film residues as major contributors to agricultural soils. MPs entering the soil detrimentally affect crop productivity by hindering growth and other physiological processes. Moreover, MPs hinder the survival, growth, and reproductive rates of the soil fauna, posing potential health risks. Additionally, a systematic evaluation of the impact of MPs on soil microbes and nutrient cycling highlights the diverse repercussions of MP contamination. Moreover, within soil-plant systems, MPs interact with other pollutants, resulting in combined pollution. For example, MPs contain oxygen-containing functional groups on their surfaces that form high-affinity hydrogen bonds with other pollutants, leading to prolonged persistence in the soil environment thereby increasing the risk to soil health. In conclusion, we succinctly summarize the current research challenges related to the mediating effects of MPs on soil health and suggest promising directions for future studies. Addressing these challenges and adopting interdisciplinary approaches will advance our understanding of the intricate interplay between MPs and soil ecosystems, thereby providing evidence-based strategies for mitigating their adverse effects.

Phosphorus acquisition strategies of wheat are related to biochar types added in cadmium-contaminated soil: Evidence from soil zymography and root morphology.

Biochar application for the remediation of cadmium (Cd)-contaminated soils may result in a relative deficiency of phosphorus (P) due to the disruption of soil nutrient balance. However, the P acquisition strategies of plants in such situation are still unclear. In this study, analyses on soil zymography and root morphology were combined for the first time to investigate the effects of pristine and P-modified biochars from apple tree branches on the P acquisition strategies of wheat under Cd stress. The results show that the application of pristine biochar exacerbated the soil's relative P deficiency. Wheat was forced to improve foraging for P by forming longer and thinner roots (average diameter 0.284 mm) as well as releasing more phosphatase to promote P mobilization in the soil. Moreover, bioavailable Cd affected the P acquisition strategies of wheat through stimulating the release of phosphatase from roots. The P-modified biochar maintained high levels of Olsen-P (>100 mg kg-1) in the soil over time by slow release, avoiding the creation of relative P deficiency in the soil; and increased the average root diameter (0.338 mm) and growth performance index, which promoted shoot growth (length and biomass). Furthermore, the P-modified biochar reduced DTPA-extracted Cd concentration in soils by 79.8 % (pristine biochar by 26.9 %), and decreased the Cd translocation factor from root to shoot as well as Cd concentration in the shoots. Therefore, P-modified biochar has a great potential to regulate the soil element balance (carbon, nitrogen, and P), promote wheat growth, and remediate the Cd-contaminated soil.

Synergistic interplay between Azospirillum brasilense and exogenous signaling molecule H2S promotes Cd stress resistance and growth in pak choi (Brassica chinensis L.).

Inoculation with growth-promoting rhizobacteria inoculation and the addition of exogenous signaling molecules are two distinct strategies for improving heavy metal resistance and promoting growth in crops through several mechanisms. However, whether rhizobacteria and phyllosphere signaling molecules can act synergistically alleviate heavy metal stress and promote growth and the mechanisms underlying these effects remain unclear. Here, a novel strategy involving the co-application of growth-promoting rhizobacteria and an exogenous signaling molecule was developed to reduce cadmium (Cd) phytotoxicity and promote pak choi growth in Cd-contaminated soil. We found that the co-application of Azospirillum brasilense and hydrogen sulfide (H2S) resulted in significant improvements in shoot biomass and antioxidant enzyme content and a decline in the levels of Cd translocation factors. In addition, this co-application significantly improved pak choi Cd resistance. Furthermore, we observed a significant negative correlation between abscisic acid concentration and Cd content of pak choi and a positive correlation between H2S concentration and biomass. These findings revealed that the co-application of rhizobacteria and exogenous signaling molecules synergistically promoted the growth of vegetable crops subjected to heavy metal stress. Our results may serve as a guide for improving the food safety of crops grown in soil contaminated with heavy metals.

Efficient removal of Cd(II) by phosphate-modified biochars derived from apple tree branches: Processes, mechanisms, and application.

Phosphate (P)-modified biochar is a good material for cadmium (Cd) immobilization, and the pore-forming effect of potassium ions (K+) can favor the P loading on biochar. However, few studies have been done specifically on Cd(II) removal by composites of potassium phosphates with biochar, and the removal potential and mechanisms are not clear. Herein, apple tree branches, a major agricultural waste suitable for the development of porous materials, were pyrolyzed individually or together with KH2PO4, K2HPO4·3H2O, or K3PO4·3H2O to obtain biochars to remove Cd(II), denoted as pristine BC, BC-1, BC-2, and BC-3, respectively. The results showed that the orthophosphates containing more K+ enlarged the specific surface area, total pore volume and phosphorus loading of biochar. Co-pyrolysis of apple tree branches and P promoted the thermochemical transformation of P species. Only weak signal of orthophosphate was observed in the pristine BC, while the presence of orthophosphate, pyrophosphate and metaphosphate were detected in BC-1, and BC-2 and BC-3 showed the presence of orthophosphate and pyrophosphate. The maximum Cd(II) adsorption capacities of pristine BC, BC-1, BC-2 and BC-3 were 10.4, 88.5, 95.8, and 116 mg·g-1, respectively. Orthophosphate modification enhanced the Cd(II) adsorption capacity due to the formation of Cd-P-precipitates, namely Cd5(PO4)3Cl, Cd5(PO4)3OH, Cd3(PO4)2, Cd2P2O7, and Cd(PO3)2. Furthermore, higher cation exchange efficiencies between Cd(II) and K+ in P-modified biochars also contributed to their high Cd(II) adsorption capacity. Cd(II) removal by BC-3 from artificially polluted water bodies showed more than 99.98% removal rates. Application of BC-3 also reduced the diethylene triamine pentaacetic acid-extracted Cd(II) in soil by 69.1%. The co-pyrolysis of apple tree branches and potassium phosphates shows great prospect in Cd(II) wastewater/soil treatment and provide a promising solution for agricultural waste utilization and carbon sequestration.

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.

A review of microplastics in soil: Occurrence, analytical methods, combined contamination and risks.

Microplastics (MPs) pollution is becoming a serious environmental issue of global concern. Currently, the effects of MPs on aquatic ecosystems have been studied in detail and in depth from species to communities. However, soils, the largest reservoir of MPs, have been less studied, and little is known about the occurrence, environmental fate and ecological impacts of MPs. Therefore, based on the existing knowledge, this paper firstly focused specifically on the main sources of soil MPs pollution and explored the main reasons for their strong heterogeneity in spatial distribution. Secondly, as a primary prerequisite for evaluating MPs contamination, we systematically summarized the analytical methods for soil MPs and critically compared the advantages and disadvantages of the different methods in the various operational steps. Furthermore, this review highlighted the combined contamination of MPs with complex chemical contaminants, the sorption mechanisms and the associated factors in the soil. Finally, the risks posed by MPs to soil, plants, the food chain and even humans were outlined, and future directions for soil MPs research were proposed, while the urgent need for a unified approach to MPs extraction and identification was emphasized. This study provides a theoretical reference for a comprehensive understanding of the separation of soil MPs and their ecological risk as carriers of pollution.

Microplastics addition reduced the toxicity and uptake of cadmium to Brassica chinensis L.

The coexistence of microplastics (MPs) and toxic metal contaminants in soils is becoming increasingly common, thereby posing serious threat to soil-plant systems. Cadmium (Cd) is the most common metal contaminant in soil and can easily combine with MPs, thereby altering its bioavailability. However, few studies have focused on the co-pollution of MPs and Cd, particularly the complex phytotoxicity caused by their interaction and the effect of co-exposure on Cd uptake in plants. We conducted pot experiments to compare the effects of exposure to polystyrene (PS) and Cd, as well as the effects of co-exposure (PS + Cd), on the physiological characteristics of Brassica chinensis L. and explored the regulatory factors of MPs on Cd uptake in plant tissues. The results showed that plant biomass, photosynthetic parameters, and chlorophyll content significantly decreased (p < 0.05) with increasing PS doses under treatment with MPs alone. Although the negative effects of PS and Cd co-exposure on plants were higher than those of PS alone, however, the addition of MPs reduced the toxicity effects of Cd on plants and decreased the uptake and accumulation of Cd by plants compared with the Cd treatment alone. Furthermore, plants can resist the increased malondialdehyde content and oxidative stress induced by PS and Cd exposure by increasing the activities of superoxide dismutase and peroxidase. Under the PS + Cd treatment, linear models showed that soil organic carbon and sucrase activity were the key variables affecting Cd uptake by plant shoots and roots, respectively. The results of the partial least squares path modeling further showed that PS indirectly affected Cd uptake by B. chinensis by significantly affecting the physicochemical properties of soil, Cd concentration, and enzyme activity. Our results provide a new perspective and an important reference for further understanding the effects of MPs on the bioavailability and fate of heavy metals.

A critical review of microplastics in the soil-plant system: Distribution, uptake, phytotoxicity and prevention.

Microplastics (MPs) are creating an emerging threat on the soil ecosystems and are of great global concern. However, the distribution in soil-plant system, as well as the phytotoxicity and impact mechanisms of MPs remain largely unexplored so far. This study introduced the diverse sources of MPs and showed the significant spatial variation in the global geographic distribution of MPs contamination based on data collected from 116 studies (1003 sampling sites). We systematically discussed MPs phytotoxicity, such as plant uptake and migration to stems and leaves, delaying seed germination, impeding plant growth, inhibiting photosynthesis, interfering with nutrient metabolism, causing oxidative damage, and producing genotoxicity. We further highlighted the alterations of soil structure and function by MPs, as well as their self and load toxicity, as potential mechanisms that threaten plants. Finally, this paper provided several preventive strategies to mitigate soil MPs pollution and presented research gaps in the biogeochemical behavior of MPs in soil-plant systems. Meanwhile, we recommended that methods for the quantitative detection of MPs accumulated in plant tissues should be explored and established as soon as possible. This review will improve the understanding of the environmental behavior of MPs in soil-plant systems and provide a theoretical reference to better assess the ecological risk of MPs.

A global meta-analysis of heavy metal(loid)s pollution in soils near copper mines: Evaluation of pollution level and probabilistic health risks.

With the rapid development of the mining industry, the pollution of heavy metal(loid)s in soils near copper (Cu) mining sites is a significant concern worldwide. However, the pollution status and probabilistic health risks of heavy metal(loid)s of soils associated with Cu mines, have rarely been studied on a global scale. In this study, eight heavy metal(loid) concentrations in soil samples taken near 102 Cu mining sites worldwide were obtained through a literature review. Based on this database, the heavy metal(loid) pollution and ecological risk in soils near Cu mines were evaluated. Most of the study sites exceeded the moderately to heavily polluted levels of Cu and Cd; compared to other regions, higher pollution levels were observed at sites in Oman, China, Australia, and the United Kingdom. Soil pollution by Cd, Pb, and Zn at agricultural sites was higher than that in non-agricultural sites. In addition, these heavy metal(loid)s produced a high ecological risk to soils around Cu mining sites in which the contribution of Cd, Cu, and As reached up to 46.5%, 21.7%, and 18.4%, respectively. The mean hazard indices of the eight heavy metal(loid)s were 0.209 and 0.979 for adults and children, respectively. The Monte Carlo simulation further predicted that 1.40% and 29.9% of non-carcinogenic risk values for adults and children, respectively, exceeded the safe level of 1.0. Moreover, 84.5% and 91.0% of the total cancer risk values for adults and children, respectively, exceeded the threshold of 1E-04. Arsenic was the main contributor to non-carcinogenic risk, while Cu had the highest exceedance of carcinogenic risk. Our findings indicate that the control of Cu, Cd, and As should be prioritized because of their high incidence and significant risks in soils near Cu mines. These results provide valuable inputs for policymakers in designing effective strategies for reducing the exposure of heavy metal(loid)s in this area worldwide.

Microbial metabolic limitation of rhizosphere under heavy metal stress: Evidence from soil ecoenzymatic stoichiometry.

Slow nutrient turnover and destructed soil function were the main factors causing low efficiency in phytoremediation of heavy metal (HM)-contaminated soil. Soil ecoenzymatic stoichiometry can reflect the ability of soil microorganisms to acquire energy and nutrients, and drive nutrient cycling and carbon (C) decomposition in HM-contaminated soil. Therefore, for the first time, we used the enzymatic stoichiometry modeling to examine the microbial nutrient limitation in rhizospheric and bulk soil of different plants (Medicago sativa, Halogeton arachnoideus and Agropyron cristatum) near the Baiyin Copper Mine. Results showed that the main pollutants in this area were Cu, Zn, Cd, and Pb, while Cd and Zn have the greatest contribution according to the analysis of pollution load index (PLI). The activities of soil C-, nitrogen (N)-, and phosphorus (P)-acquiring enzymes in the rhizosphere of plants were significantly greater than that in bulk soil. Moreover, microbial C and P limitations were observed in all plant treatments, while the lower limitation was generally in the rhizosphere compared to bulk soil. The HM stress significantly increased microbial C limitation and decreased microbial P limitation, especially in the rhizospheric soil. The partial least squares path modeling (PLS-PM) further indicated that HM concentration has the greatest effects on microbial P limitation (-0.64). In addition, the highest enzyme activities and the lowest P limitation were observed in the rhizospheric and bulk soil of M. sativa, thereby implying that soil microbial communities under the remediation of M. sativa were steadier and more efficient in terms of their metabolism. These findings are important for the elucidation of the nutrient cycling and microbial metabolism of rhizosphere under phytoremediation, and provide guidance for the restoration of HM-contaminated soil.

Rhizobium Inoculation Enhances the Resistance of Alfalfa and Microbial Characteristics in Copper-Contaminated Soil.

Some studies have reported the importance of rhizobium in mitigating heavy metal toxicity, however, the regulatory mechanism of the alfalfa-rhizobium symbiosis to resist copper (Cu) stress in the plant-soil system through biochemical reactions is still unclear. This study assessed the effects of rhizobium (Sinorhizobium meliloti CCNWSX0020) inoculation on the growth of alfalfa and soil microbial characteristics under Cu-stress. Further, we determined the regulatory mechanism of rhizobium inoculation to alleviate Cu-stress in alfalfa through plant-soil system. The results showed that rhizobium inoculation markedly alleviated Cu-induced growth inhibition in alfalfa by increasing the chlorophyll content, height, and biomass, in addition to nitrogen and phosphorus contents. Furthermore, rhizobium application alleviated Cu-induced phytotoxicity by increasing the antioxidant enzyme activities and soluble protein content in tissues, and inhibiting the lipid peroxidation levels (i.e., malondialdehyde content). In addition, rhizobium inoculation improved soil nutrient cycling, which increased soil enzyme activities (i.e., ß-glucosidase activity and alkaline phosphatase) and microbial biomass nitrogen. Both Pearson correlation coefficient analysis and partial least squares path modeling (PLS-PM) identified that the interactions between soil nutrient content, enzyme activity, microbial biomass, plant antioxidant enzymes, and oxidative damage could jointly regulate plant growth. This study provides comprehensive insights into the mechanism of action of the legume-rhizobium symbiotic system to mitigate Cu stress and provide an efficient strategy for phytoremediation of Cu-contaminated soils.

Associated soil aggregate nutrients and controlling factors on aggregate stability in semiarid grassland under different grazing prohibition timeframes.

Grazing prohibition is an effective measure in improving soil stability and ecological quality. However, only a limited number of studies have been published on the dominant factors that impact soil aggregate stability and their associated effects on nutrient distribution for different size soil aggregates under long-term grazing prohibition management. In this study, we investigated variation in soil aggregate stability and nutrient distribution characteristics in semiarid grassland sites under different grazing prohibition timeframes (0 years [GP0], 11 years [GP11], 26 years [GP26], and 36 years [GP36]). Results showed that organic carbon (C) and total nitrogen (TN) concentrations in soil aggregates decreased at GP11 before progressively increasing and reaching its highest value at GP36, and the total phosphorus (TP) concentration did not change significantly. Most nutrients accumulated in macroaggregates (> 0.25 mm) under grazing prohibition, and the nutrient stoichiometry in soil aggregates increased after 26 years. Compared to the control (GP0), the mean weight diameter (MWD) value of the soil stability index increased at GP11 (21.7%) and decreased at GP26 (18.9%). Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) also showed that the proportion of stable organic C-related functional groups (i.e., alkene-C and aromatic-C) in macroaggregates were higher at GP11 and GP36 than at GP26. Furthermore, principal component analysis (PCA), partial least squares path modeling (PLS-PM), and the relative importance of regressors all showed that glomalin-related soil proteins (GRSP) and nutrients indirectly improved aggregate stability in semiarid grassland through their influence on the GRSP accumulation potential and nutrient stoichiometry. Generally, after 26 years grazing prohibition had a positive effect on soil aggregate stability and nutrient accumulation in the semiarid grassland sites investigated for this study. Results from this study provide a theoretical basis to select appropriate grazing prohibition timeframes under grassland management initiatives to optimize ecological quality measures in semiarid regions.

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 ecosystems 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.

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.