Dynamic responses of soil enzymes at key growth stages in rice after the in situ remediation of paddy soil contaminated with cadmium and arsenic

Impact of microplastics on microbial-mediated soil sulfur transformations in flooded conditions

As emerging environmental pollutants, microplastics have become a crucial focus in environmental science research. Despite this, the impact of microplastics on soil in flooding conditions remains largely unexplored. Addressing this gap, our study examined the influence of polystyrene (PS) and polyphenylene sulfide (PPS) on the microbial populations in black soil, meadow soil, and paddy soil under flooded conditions. Given the significant regulatory influence exerted by microorganisms on sulfur transformations, our study was primarily focused on evaluating the microbial contributions to alterations in soil sulfur species. Our findings revealed several notable trends: In black soil, both PS and PPS led to a marked increase in the abundance of γ-proteobacteria and Subgroup_6, while reducing Clostridia. Ignavibacteria were found to be lower under PPS compared to PS. In meadow soil, the introduction of PPS resulted in increased levels of KD4-96 and γ-proteobacteria, while α-proteobacteria decreased. Chloroflexia under PPS was observed to be lower than under PS conditions. In paddy soil, our study identified a significant rise in Bacteroidia and Ignavibacteria, accompanied by a decrease in α-proteobacteria and γ-proteobacteria. γ-proteobacteria levels under PPS were notably higher than those under PS conditions. These shifts in microbial communities induced by both PS and PPS had a direct impact on adenosine 5'-phosphosulfate reductase, sulfite reductase, and polysulfide dioxygenase. Consequently, these changes led to soil organic sulfur decrease and sulfide increase. This study not only offers a theoretical framework but also provides empirical evidence for understanding the effects of microplastics on soil microorganisms and their role in regulating nutrient cycling, particularly in flood-prone conditions. Furthermore, this study underscores the importance of ensuring an adequate supply of sulfur in agricultural practices, such as rice and lotus root cultivation, to support optimal crop growth in the presence of microplastic pollution.

The impact of microplastics on sulfur REDOX processes in different soil types: A mechanism study

Microplastics can potentially affect the physical and chemical properties of soil, as well as soil microbial communities. This could, in turn, influence soil sulfur REDOX processes and the ability of soil to supply sulfur effectively. However, the specific mechanisms driving these effects remain unclear. To explore this, soil microcosm experiments were conducted to assess the impacts of polystyrene (PS) and polyphenylene sulfide (PPS) microplastics on sulfur reduction-oxidation (REDOX) processes in black, meadow, and paddy soils. The findings revealed that PS and PPS most significantly decreased SO42- in black soil by 9.4%, elevated SO42- in meadow soil by 20.8%, and increased S2- in paddy soil by 20.5%. PS and PPS microplastics impacted the oxidation process of sulfur in soil by influencing the activity of sulfur dioxygenase, which was mediated by α-proteobacteria and γ-proteobacteria, and the oxidation process was negatively influenced by soil organic matter. PS and PPS microplastics impacted the reduction process of sulfur in soil by influencing the activity of adenosine-5'-phosphosulfate reductase, sulfite reductase, which was mediated by Desulfuromonadales and Desulfarculales, and the reduction process was positively influenced by soil organic matter. In addition to their impacts on microorganisms, it was found that PP and PPS microplastics directly influenced the structure of soil enzymes, leading to alterations in soil enzyme activity. This study sheds light on the mechanisms by which microplastics impact soil sulfur REDOX processes, providing valuable insights into how microplastics influence soil health and functioning, which is essential for optimizing crop growth and maximizing yield in future agricultural practices.

Remediation of arsenic-contaminated soil using nanoscale schwertmannite synthesized by persulfate oxidation with carboxymethyl cellulose stabilization

Schwertmannite (SCH) is a promising material for adsorbing inorganic arsenic (As). We synthesized SCH nanoparticles (nano-SCH) via a modified chemical oxidation method and investigated the application of nano-SCH for the remediation of As-contaminated soils. The production of nano-SCH was successfully prepared using the persulfate oxidation method with carboxymethyl cellulose stabilization. The spherical structure of the nano-SCH particles had an average hydrodynamic diameter of 296 nm with high specific surface areas (108.9 m2/g). Compared with SCH synthesized via the H2O2 oxidation method, the percentage of Fe3+ precipitation in nano-SCH synthesis increased from 63.2% to 84.1%. The inorganic As adsorption capacity of nano-SCH improved by 2.27 times at solution pH = 6. After remediation of heavily As-contaminated soils by using 5% nano-SCH, the leachability of inorganic As rapidly decreased to 0.01% in 30 d. Correspondingly, the immobilization efficiencies of inorganic As in soil reached >99.9%. The inorganic As fractions in treated soil shifted from specifically and nonspecifically bound forms to amorphous and crystalline hydrous oxide-bound fractions. After treatment with 5% nano-SCH for 60 d, soil pH slightly decreased from 5.47 to 4.94; by contrast, soil organic matter content increased by 20.9%. Simultaneously, dehydrogenase concentration in soil decreased by 22.4%-34.7% during the remediation process. These changes in soil properties and As immobilization jointly decreased microbial activity and initiated the re-establishment of bacterial communities in the soil. In summary, this study presents a novel and high-productivity technology for nano-SCH synthesis and confirms the high As immobilization effectiveness of nano-SCH in the remediation of As-contaminated soils.

Investigating the impact of microplastics on sulfur mineralization in different soil types: A mechanism study

Microplastics can affect the physicochemical properties of soil and soil microorganisms, potentially resulting in changes in the soil sulfur mineralization and its capacity to supply available sulfur. However, the specific mechanisms underlying these effects remain unclear. We performed soil microcosm experiments, in which the effects of polystyrene (PS) and polyphenylene sulfide (PPS) microplastics on sulfur mineralization were examined in black, meadow, and paddy soils under flooded and dry conditions. Under dry condition, the presence of PS and PPS microplastics impeded sulfur (S) mineralization in black and paddy soils, but promoted sulfur mineralization in meadow soil. The size of microplastics was identified as the primary factor influencing sulfur mineralization in black soil, while in meadow soil, it was influenced by the microplastics type. In the case of paddy soil, the concentration of microplastics was the key factor affecting sulfur mineralization. During the flooding period, PS and PPS microplastics in black and paddy soils curtailed sulfur mineralization, however expedited sulfur mineralization in meadow soil, with PS enhancing soil sulfur mineralization more pronouncedly than PPS in black soil. The type and concentration of microplastics were identified as the main factors affecting sulfur mineralization in black soil, while in paddy soil, it was influenced by the size of microplastics. The principal regulating factors of soil sulfur mineralization were the sulphatase and arylsulfatase enzymes produced by Actinobacteria, Xanthomonadales, and Rhizobiales microorganisms, while organic matter and Olsen-P also had an influential role. Additionally, microplastics directly affected the structure of soil enzymes, thereby altering soil enzyme activities. This study provided insights into the mechanism by which microplastics affect soil sulfur mineralization, offering significant implications for assessing the influence of microplastics on soil sulfur availability and making informed decisions about sulfur application in future agricultural practices.

Effects of Straw Returning and New Fertilizer Substitution on Rice Growth, Yield, and Soil Properties in the Chaohu Lake Region of China

Recently, replacing chemical fertilizers with straw returning and new fertilizers has received considerable attention in the agricultural sector, as it is believed to increase rice yield and improve soil properties. However, less is known about rice growth and soil properties in paddy fields with the addition of different fertilizers. Thus, in this paper, we investigated the effects of different fertilizer treatments, including no fertilization (CK), optimized fertilization based on the medium yield recommended fertilizer amount (OF), 4.50 Mg ha-1 straw returning with chemical fertilizers (SF), 0.59 Mg ha-1 slow-release fertilizer with chemical fertilizers (SRF), and 0.60 Mg ha-1 water-soluble fertilizer with chemical fertilizers (WSF), on rice growth, yield, and soil properties through a field experiment. The results show that compared with the OF treatment, the new SF, SRF, and WSF treatments increased plant height, main root length, tiller number, leaf area index, chlorophyll content, and aboveground dry weight. The SF, SRF, and WSF treatments improved rice grain yield by 30.65-32.51% and 0.24-1.66% compared to the CK and OF treatments, respectively. The SRF treatment increased nitrogen (N) and phosphorus (P) uptake by 18.78% and 28.68%, the harvest indexes of N and P by 1.75% and 0.59%, and the partial productivity of N and P by 2.64% and 2.63%, respectively, compared with the OF treatment. However, fertilization did not significantly affect the average yield, harvest indexes of N and P, and partial productivity of N and P. The contents of TN, AN, SOM, TP, AP, and AK across all the treatments decreased significantly with increasing soil depth, while soil pH increased with soil depth. The SF treatment could more effectively increase soil pH and NH4+-N content compared to the SRF and WSF treatments, while the SRF treatment could greatly enhance other soil nutrients and enzyme activities compared to the SF and WSF treatments. A correlation analysis showed that rice yield was significantly positively associated with tiller number, leaf area index, chlorophyll, soil NO3--N, NH4+-N, SOM, TP, AK, and soil enzyme activity. The experimental results indicate that SRF was the best fertilization method to improve rice growth and yield and enhance soil properties, followed by the SF, WSF, and OF treatments. Hence, the results provide useful information for better fertilization management in the Chaohu Lake region of China.

Physiological tolerance of black locust (Robinia pseudoacacia L.) and changes of rhizospheric bacterial communities in response to Cd and Pb in the contaminated soil

Woody plants possess great potential for phytoremediation of heavy metal-contaminated soil. A pot trial was conducted to study growth, physiological response, and Cd and Pb uptake and distribution in black locust (Robinia pseudoacacia L.), as well as the rhizosphere bacterial communities in Cd and Pb co-contaminated soil. The results showed that R. pseudoacacia L. had strong physiological regulation ability in response to Cd and Pb stress in contaminated soil. The total chlorophyll, malondialdehyde (MDA), soluble protein, and sulfhydryl contents, as well as antioxidant enzymes (superoxide dismutase, peroxidase, catalase) activities in R. pseudoacacia L. leaves under the 40 mg·kg-1 Cd and 1000 mg·kg-1 Pb co-contaminated soil were slightly altered. Cd uptake in R. pseudoacacia L. roots and stems increased, while the Pb content in the shoots of R. pseudoacacia L. under the combined Cd and Pb treatments decreased in relative to that in the single Pb treatments. The bacterial α-diversity indices (e.g., Sobs, Shannon, Simpson, Ace, and Chao) of R. pseudoacacia L. rhizosphere soil under Cd and Pb stress were changed slightly relative to the CK treatment. However, Cd and Pb stress could significantly (p < 0.05) alter the rhizosphere soil microbial communities. According to heat map and LEfSe (Linear discriminant analysis Effect Size) analysis, Bacillus, Sphingomonas, Terrabacter, Roseiflexaceae, Paenibacillus, and Myxococcaceae at the genus level were notably (p < 0.05) accumulated in the Cd- and/or Pb-contaminated soil. Furthermore, the MDA content was notably (p < 0.05) negatively correlated with the relative abundances of Isosphaeraceae, Gaiellales, and Gemmatimonas. The total biomass of R. pseudoacacia L. was positively (p < 0.05) correlated with the relative abundances of Xanthobacteraceae and Vicinamibacreraceae. Network analysis showed that Cd and Pb combined stress might enhance the modularization of bacterial networks in the R. pseudoacacia L. rhizosphere soil. Thus, the assembly of the soil bacterial communities in R. pseudoacacia L. rhizosphere may improve the tolerance of plants in response to Cd and/or Pb stress.

Protective effects of polymer amendment on specific metabolites in soil and cotton leaves under cadmium contamination

Polymer materials have great potential for soil heavy metal contamination remediation, but the metabolic mechanism by which polymer amendments regulate the responses of soil-plant systems to cadmium (Cd) stress is still unclear. To clarify the metabolic mechanism by which a self-developed soluble polymer amendment (PA) remediates Cd contamination in cotton fields, the common and differential metabolites in soil and cotton leaves were analyzed during the critical period of cotton growth (flowering and bolling stage) in a field experiment. The results showed that Cd stress increased Cd concentration in the soil-cotton system, and reduced enzyme activity in soil and cotton leaves. Besides, Cd stress also reduced the abundance of α-linolenic acid in soil and the abundance of 2-Oxoarginine and S-Adenosylmethionine in cotton leaves. These ultimately led to reductions in weight, boll number, yield, and fiber elongation. However, the application of PA to the Cd-contaminated soil significantly reduced the soil exchangeable Cd (Ex-Cd) concentration by 41.43%, and increased the boll number, yield, and fiber strength by 14.17%, 21.04%, and 19.89%, respectively compared with the Cd treatment. The results of metabolomic analysis showed that PA application mainly affected the Nicotinate and nicotinamide metabolism pathway, Lysine degradation pathway, and Arginine and proline metabolism pathway in cotton leaves and soil. Besides, in these metabolic pathways, succinic acid semialdehyde of cotton leaves, saccharopine of soil, and S-Adenosylmethionine of soil and cotton had the most significant response to PA application. Therefore, the application of PA to Cd-contaminated soil can increase soil and cotton leaf enzyme activity and cotton yield (boll number and seed cotton yield) and quality (fiber strength), and maintain soil-plant material balance by regulating the distribution of Cd ions and key metabolites in the soil-cotton system. This study will deepen our understanding of the metabolic mechanism of PA remediating Cd-contaminated cotton fields, and provide a technical reference for the remediation of heavy metal contamination in drip-irrigated cotton fields in arid areas.

Significant difference in the efficacies of silicon application regimes on cadmium species and environmental risks in rice rhizosphere

Silicon (Si) is commonly applied as base-fertilizer or foliar-topdressing to palliate the uptake-translocation-accumulation of cadmium (Cd) in rice through Si-Cd antagonism. However, little is known about the fate of Cd in rice rhizosphere soil and its eco-environmental effects under different Si treatments. Here, systematic works had been carried out to elucidate the Cd species, soil properties, and environmental risks in rice rhizosphere driven by different Si soil-fertilization regimes including CK (without Si-addition), TSi (added before transplanting stage), JSi (added at jointing stage), and TJSi (split into two equal parts, added half before transplanting and another half at jointing stage). Results showed that TJSi outperformed the rest of fertilization regimes. The solid-phase-Cd concentrations treated with TSi, TJSi and JSi were increased by 4.18%, 5.73% and 3.41%, respectively, when compared to CK. The labile Cd (F1+F2) proportion of TJSi was reduced by 16.30%, 9.30% and 6.78%, respectively, when compared to CK, TSi, and JSi. Simultaneously, the liquid-phase-Cd concentration was appreciably suppressed by TJSi throughout the rice lifecycle, while TSi mainly abated Cd dissociation during the vegetative period, and JSi attenuated it during the grain-filling stage. The mobility factor of Cd treated with TJSi was the lowest, which was significantly lower than that of TSi (9.30%) and JSi (6.78%), respectively. Similarly, the oral exposure risk of TJSi was reduced by 4.43% and 32.53%; and the food-chain exposure risk of TJSi was decreased by 13.03% and 42.78%. Additionally, TJSi was the most effective in promoting enzyme activities and nutrient content in rhizosphere soil. Overall, TJSi is more positive and sustainable than TSi and JSi in reconstructing Cd-contaminated rhizosphere environments and abating the environmental risks of Cd. Agronomic practices in Cd-contaminated paddy soils can be informed by applying Si-fertilizer separately before transplanting and at jointing stage to achieve soil welfare and food security.

Microbiological mechanism for "production while remediating" in Cd-contaminated paddy fields: A field experiment

Security utilization measures (SUMs) for "production while remediating" in moderate and mild Cd-polluted paddy fields had been widely used. To investigate how SUMs drove rhizosphere soil microbial communities and reduced soil Cd bioavailability, a field experiment was conducted using soil biochemical analysis and 16S rRNA high-throughput sequencing. Results showed that SUMs improved rice yield by increasing the number of effective panicles and filled grains, while also inhibiting soil acidification and enhancing disease resistance by improving soil enzyme activities. SUMs also reduced the accumulation of harmful Cd in rice grains and transformed it into FeMn oxidized Cd, organic-bound Cd, and residual Cd in rhizosphere soil. This was partly due to the higher degree of soil DOM aromatization, which helped complex the Cd with DOM. Additionally, the study also found that microbial activity was the primary source of soil DOM, and that SUMs increased the diversity of soil microbes and recruited many beneficial microbes (Arthrobacter, Candidatus_Solibacter, Bryobacter, Bradyrhizobium, and Flavisolibacter) associated with organic matter decomposition, plant growth promotion, and pathogen inhibition. Besides, special taxa (Bradyyrhizobium and Thermodesulfovibrio) involved in sulfate/sulfur ion generation and nitrate/nitrite reduction pathway were observably enriched, which effectively reduced the soil Cd bioavailability through adsorption and co-precipitation. Therefore, SUMs not only changed the soil physicochemical properties (e.g., pH), but also drove rhizosphere microbes to participate in the chemical species transformation of soil Cd, thus reducing Cd accumulation in rice grains.

Different composites inhibit Cd accumulation in grains under the rice-oilseed rape rotation mode of karst area: A field study

Ensuring the safe production of food and oil crops in soils with elevated cadmium (Cd) content in karst regions is crucial. We tested a field experiment to examine the long-term remediation effects of compound microorganisms (CM), strong anion exchange adsorbent (SAX), processed oyster shell (POS), and composite humic acids (CHA) on Cd contamination in paddy fields under a rice-oilseed rape rotation system. In comparison to the control group (CK), the application of amendments significantly increased soil pH, cation exchange capacity (CEC), and soil organic matter (SOM) content while markedly decreasing the content of available Cd (ACd). During the rice cultivation season, Cd was predominantly concentrated in the roots. Relative to the control (CK), the Cd content in each organ was significantly reduced. The Cd content in brown rice decreased by 19.18-85.45%. The Cd content in brown rice following different treatments exhibited the order of CM > POS > CHA > SAX, which was lower than the Chinese Food Safety Standard (GB 2762-2017) (0.20 mg/kg). Intriguingly, during the oilseed rape cultivation season, we discovered that oilseed rape possesses potential phytoremediation capabilities, with Cd mainly accumulating in roots and stems. Notably, CHA treatment alone significantly decreased the Cd content in oilseed rape grains to 0.156 mg/kg. CHA treatment also maintained soil pH and SOM content, consistently reduced soil ACd content, and stabilized Cd content in RSF within the rice-oilseed rape rotation system. Importantly, CHA treatment not only enhances crop production but also has a low total cost (1255.230 US$/hm2). Our research demonstrated that CHA provides a consistent and stable remediation effect on Cd-contaminated rice fields within the crop rotation system, as evidenced by the analysis of Cd reduction efficiency, crop yield, soil environmental change, and total cost. These findings offer valuable guidance for sustainable soil utilization and safe production of grain and oil crops in the context of high Cd concentrations in karst mountainous regions.

Responsive change of crop-specific soil bacterial community to cadmium in farmlands surrounding mine area of Southeast China

In arable soils co-influenced by mining and farming, soil bacteria significantly affect metal (Cadmium, Cd) bioavailability and accumulation. To reveal the soil microecology response under this co-influence, three intersection areas (cornfield, vegetable field, and paddy field) were investigated. With a similar nutrient condition, the soils showed varied Cd levels (0.31-7.70 mg/kg), which was negatively related to the distance from mining water flow. Different soils showed varied microbial community structures, which were dominated by Chloroflexi (19.64-24.82%), Actinobacteria (15.49-31.96%), Acidobacteriota (9.46-20.31%), and Proteobacteria (11.88-14.57%) phyla. A strong correlation was observed between functional microbial taxon (e. g. Acidobacteriota), soil physicochemical properties, and Cd contents. The relative abundance of tolerant bacteria including Vicinamibacteraceae, Knoellia, Ardenticatenales, Lysobacter, etc. elevated with the increase of Cd, which contributed to the enrichment of heavy metal resistance genes (HRGs) and integration genes (intlI), thus enhancing the resistance to heavy metal pollution. Cd content rather than crop species was identified as the dominant factor that influenced the bacterial community. Nevertheless, the peculiar agrotype of the paddy field contributed to its higher HRGs and intlI abundance. These results provided fundamental information about the crop-specific physiochemical-bacterial interaction, which was helpful to evaluate agricultural environmental risk around the intersection of farmland and pollution sources.

Effects of Phosphate, Red Mud, and Biochar on As, Cd, and Cu Immobilization and Enzymatic Activity in a Co-Contaminated Soil

Arsenic (As), cadmium (Cd), and copper (Cu) are the primary inorganic pollutants commonly found in contaminated soils. The simultaneous stabilization of the three elements is a preferred approach for mixture-contaminated soils which has received extensive research attention. However, few studies have focused on the immobilization efficiency of a single amendment on the three elements. In this study, phosphate, red mud, and biochar were used to remediate As (237.8 mg kg−1), Cd (28.72 mg kg−1), and Cu (366.5 mg kg−1) co-contaminated soil using a 180-day incubation study. The BCR (European Community Bureau of Reference) extraction method, NH4H2PO4–extractable As, and diethylenetriamine penta-acetic acid (DTPA)–extractable Cd and Cu were analyzed at different time intervals. The results indicated that the application of red mud and biochar significantly reduced soil DTPA–Cd and Cu concentrations during the incubation, while the decrease in soil NH4H2PO4–As was much less than that of soil DTPA–Cd and Cu. After 180 days of incubation, the concentrations of NH4H2PO4–As in red mud and biochar treatments decreased by 2.15~7.89% and 3.01~9.63%, respectively. Unlike red mud and biochar, phosphate significantly reduced the concentration of soil DTPA–Cd and Cu, but failed to lower that of As. The BCR extraction method confirmed that red mud and biochar addition increased the reducible fraction of As due to the surface complexes of As with Fe oxide. Canonical correspondence analysis (CCA) demonstrated that soil pH in addition to available As, Cd, and Cu concentrations were the primary factors in driving the changes in soil enzymatic activity. Soil pH showed positive correlation with soil urease and catalase activities, while negative correlation was observed between soil-available As, Cd, and Cu, and soil enzyme activities. This study revealed that it is difficult to simultaneously and significantly reduce the bioavailabilities of soil As, Cd, and Cu using one amendment. Further research on modifying these amendments or applying combined amendments will be conducted, in order to develop an efficient method for simultaneously immobilizing As, Cd, and Cu.