Search

Periodic redox condition changes in acidic paddy soils substantially induce the biogeochemical redox cycling, and consequently affect Cd availability. However, the underlying biogeochemical mechanisms of the complicated redox processes in paddy soil remain poorly understood. Thus, we investigated the dynamics of Cd fractions under anoxic (0–40 days) and oxic (40–55 days) conditions. The available Cd content was evaluated using the diffusive gradients in thin film (DGT) technique, and the results show it decreased from 6.3 μg L–1 to below 0.1 μg L–1 under anoxic conditions, but rapidly increased to 13.5 μg L–1 under oxic conditions. Both sequential extraction procedures and X-ray absorption spectroscopy (XAS) analyses found that majority of available Cd transformed to organic matter complex and Fe-Mn oxides fractions, rather than bounded with sulfides. The soil chemical properties further evaluated, and the correlation analysis and principal component analysis results suggested that the key factors in soils for the available Cd was the SO 4 2–, pH, and surface site concentration. The reduction of SO 4 2– could generate sulfides which may precipitate with dissolved Cd. The biogeochemical redox cycling of Fe/N/S determined the changes of soil pH and consequently influenced adsorption behavior of Cd. The breakdown of the soil aggregations changed the surface site concentration, which may affect the immobilization of Cd. A process-based kinetic model was established, and it found that iron biogeochemical redox cycling dominated the changes of soil pH, and thereby contributed to majority Cd retention by Fe-Mn oxides (34.4 %) and organic matter (33.7 %). In addition, sulfur biogeochemical redox cycling only contributed to 13.6 % of the total Cd contents, because of relatively low Cd solubility in contaminated soils and the competition with other cations for limited sulfides. The findings provide robust targets for interpreting the iron and sulfur biogeochemical processes for Cd availability and would be helpful for developing the precise and effective remediation strategies in Cd-contaminated soils. [ABSTRACT FROM AUTHOR]

• More Cd was immobilized in the flooding period than that in the drainage period. • ZVI reduced the release of Cd and promoted Cd immobilization during drainage process. • Adding ZVI was more conducive to Cd immobilization than directly increasing soil pH. • Adding ZVI in the flooding stage immobilized more Cd than that in the drainage stage. Zero-valent iron (ZVI) is a promising material for the remediation of Cd-contaminated paddy soils. However, the effects of ZVI added during flooding or drainage processes on cadmium (Cd) retention remain unclear. Herein, Cd-contaminated paddy soil was incubated for 40 days of flooding and then for 15 days of drainage, and the underlying mechanisms of Cd immobilization coupled with Fe/S/N redox processes were investigated. The addition of ZVI to the flooding process was more conducive to Cd immobilization. Less potential available Cd was detected by adding ZVI before flooding, which may be due to the increase in paddy soil pH and newly formed secondary Fe minerals. Moreover, the reductive dissolution of Fe minerals promoted the release of soil colloids, thereby increasing significantly the surface sites and causing Cd immobilization. Additionally, the addition of ZVI before flooding played a vital role in Cd retention after soil drainage. In contrast, the addition of ZVI in the drainage phase was not conducive to Cd retention, which might be due to the rapid decrease in soil pH that inhibited Cd adsorption and further immobilization on soil surfaces. The findings of this study demonstrated that Cd availability in paddy soil was largely reduced by adding ZVI during the flooding period and provide a novel insight into the mechanisms of ZVI remediation in Cd-contaminated paddy soils. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Readily soluble and occluded P were transformed into Ca bound P along pyrolysis. • Organic P was decomposed into abundant orthophosphate and trace pyrophosphate along pyrolysis. • Exogenous Ca ion promoted the transformation of polymerized P to monomeric P. • Doping Ca ion promoted the formation of Ca 5 (PO 4) 3 Cl and Ca 10 (PO 4) 6 (OH) in biochar. • Ca-composite biochar offered more available P that benefited to the growth of rice. Transformation of phosphorus (P) species during pyrolytic production of biochar from P-rich biowastes with a subsequent soil amendment is important to P reclamation. Aiming at increasing the content of plant-available P and restraining the formation of easily mobile P in pyrolysis product, this study used exogenous calcium ions (20 wt% CaCl 2) addition prior to pyrolysis to regulate the pyrolytic transformation of P chemical fractions from sewage sludge and bone dreg. Results showed that active Ca catalyzed the decomposition of organic P to transform into inorganic orthophosphate. Based on Hedley's sequential extraction method, this study found that addition of Ca ions remarkably reduced the content of soluble P, exchange P, Fe/Al bound P, and occluded P in biochar, while increased Ca bound P from 78 to 85% to 85–96%. Liquid 31P NMR indicated that exogenous Ca induced the crack of the P-O-P bond in pyrophosphate to become orthophosphates. It also explained why new orthophosphates including chlorapatite (Ca 5 (PO 4) 3 Cl) and calcium hydroxyapatite (Ca 10 (PO 4) 6 (OH) 2) appeared in the Ca-composite biochar compared to pristine biochar. Combined with rapid P-release test in paddy soil (pH 6.27) and 30-days rice seedling growth test under flooded condition (10 wt% biochar addition ratio), it was confirmed that compared to pristine biochar, Ca-composite biochar released more P in paddy soil, but also promoted more P to be taken in by rice root and stalk. These results suggested that pretreating biowaste with Ca ion was a friendly approach to enhance P reclamation during biochar formation, making it a promising P fertilizer. [ABSTRACT FROM AUTHOR]

• The Cr transformation kinetics was evaluated under alternating redox conditions. • A kinetic model was established to assess the relationship between Cr and Fe. • The release or retention of Cr was driven by Fe redox transformation. • The Cr availability was limited in paddy soil with high geological background. The flooding and drainage of paddy fields has great effects on the transformation of heavy metals, however, the transformation of Cr in basalt-derived paddy soil with high geological background values was less recognized. The typical basalt-derived paddy soil was incubated under alternating redox conditions. The Cr fractions and the dynamics of Fe/N/S/C were examined. The HCl-extractable Cr increased under anaerobic condition and then decreased during aerobic stage. The UV-vis spectra of the supernatant showed that amounts of colloids were released under anaerobic condition, and then re-aggregated during aerobic phase. The scanning transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) revealed that Fe oxides were reduced and became dispersed during anaerobic stage, whereas Fe(II) was oxidized and recrystallized under aerobic condition. Based on these results, a kinetic model was established to further distinguish the relationship between the transformation of Cr and Fe. During anaerobic phase, the reduction of Fe(III) oxides not only directly released the structurally bound Cr, but also enhanced the breakdown of soil aggregation and dissolution of organic matter causing indirect mobilization of Cr. During aerobic phase, the oxidation of Fe(II) and further recrystallization of newly formed Fe(III) oxides might induce the re-aggregation of soil colloids and further incorporation of Cr. In addition, the kinetic model of Cr and Fe transformation was further verified in the pot experiment. The model-based findings demonstrated that the Cr transformation in the basalt-derived paddy soil with high geological background values was highly driven by redox sensitive iron cycling. JES-21-03056 Graphical Abstract [Display omitted]. [ABSTRACT FROM AUTHOR]

[Display omitted] • LIBS was employed to capture soil heterogeneity at mesoscale. • PCA method and ternary plots explored main elements distribution. • RGB method was applied to qualify soil heterogeneity. • Controlled release urea altered the paddy soil heterogeneity at mesoscale. Farmland soil shows strong heterogeneity due to crop rotations and fertilizer inputs, and characterization of soil heterogeneity will benefit understanding soil management. In this study, the technique of laser induced breakdown spectroscopy (LIBS) was used to investigate the heterogeneity of paddy soil in-situ at mesoscale (20 μm ∼ 2 mm) under long term input of controlled release nitrogen. Principal component analysis (PCA) of LIBS spectra interpreted elements distribution in the first three PCs. PC1 contained higher proportions of K, O, Al, Si, Na, Ca and Mg information (more than 50 %), PC2 contained more proportions of Li, Ti, Fe, K, Mg and Si information (40 %), and PC3 contained more proportions of Ca, Mg, Mo, Ti and Pb information (35–40 %). The Red-Green-Blue composite using PC1, PC2 and PC3 as color codes was constructed. Soil heterogeneity was in-situ visualized by color blending maps at mesoscale and the hierarchical cluster analysis showed that increasing input of controlled released nitrogen altered soil heterogeneity. Therefore, LIBS provided a method to achieve a digital description of soil heterogeneity at mesoscale, which could be an alternative option to capture spatio-temporal soil information for evaluating the effects of agricultural management on soil quality. [ABSTRACT FROM AUTHOR]

Fertilization exerts a profound influence on the formation, turnover and stabilization of soil organic carbon (SOC). However, how long-term fertilization regulates the contribution of plant residues and microbial necromass to SOC sequestration in acidic paddy soil remains unclear. This study aimed to elucidate these mechanisms by determining plant and microbial biomarkers, i.e., lignin phenols and amino sugars, as well as carbon degradation genes in both topsoil (0–15 cm) and subsoil (15–30 cm) within the context of a 10-year paddy field experiment. We investigated four fertilization regimes: no fertilizers (control), chemical fertilizers applied alone (NPK), NPK combined with cattle manure (NPKM), and NPK combined with rice straw (NPKS). Compared to the control, integrated organic-inorganic fertilization increased SOC by 15–21 % and 14–19 % in the topsoil and subsoil, respectively. Specifically, NPKM and NPKS did not alter the contribution of total lignin phenols to SOC in the topsoil but increased that in the subsoil by 63 % and 26 %, respectively. SOC accumulation exhibited a positive correlation with the concentration of total lignin phenols but a negative correlation with the acid to aldehyde ratio of vanillyl, indicating selective preservation of vanillyls. However, NPKM and NPKS decreased the contribution of total microbial residual carbon (MRC) to SOC in the topsoil (13–16 %) but didn't alter that in the subsoil. The decreased MRC contribution of topsoil was mainly reflected in fungal MRC, implying that fungi played a pivotal role in regulating topsoil carbon accumulation. Correlation analyses indicated that soil biotic and abiotic factors jointly affected the sequestration of plant lignin phenols and microbial necromass, and topsoil carbon accumulated with the abundance ratio of hydrolytic to oxidative enzyme-coding genes increasing (r = 0.62, p < 0.05). Combined with Partial Least Squares Path Model, our study revealed that the accumulation of organic carbon in the topsoil was more influenced by plant-derived carbon, whereas in the subsoil, it was co-affected by both plant- and microbially-derived carbon. • Topsoil OC accumulated with the abundance ratio of hydrolytic to oxidative enzyme coding genes increasing; • Organic amendments decreased microbially-derived C to topsoil OC but increased plant-derived C to subsoil OC; • Soil abiotic and biotic factors jointly affected the accumulation of plant- or microbially-derived C; • Topsoil OC was more influenced by plant-derived C, while plant- and microbially-derived C co-affected subsoil OC. [ABSTRACT FROM AUTHOR]

Iron (Fe) oxides have a strong adsorption affinity for Cd and organic carbon (SOC). However, under alternate wet-dry (IF) condition,the influences of Fe oxides on the speciation and disrtribution of Cd and SOC in soil aggregates are unkown. In the present study, soils untreated (S), removed (S-Fe) or added (S+Fe) Fe oxide soils were blended with cadmium chloride solution and cultivated for 56 days under different moisture management practices. Compared with the S-Fe soil, the IF treatment increased the contents of Fe oxide-bound SOC (Fe-OC) and Fe/Mn oxide-bound Cd (Fe/Mn-Cd) by 18.5–29.8-fold and 1.45–2.45-fold, repectively, in the S and S+Fe soils, corresponding to a 36 %−42 % increase in the recalcitrant C pool (RCP) and a 53 %−87 % decrease in the exchangeable Cd content. These results could be attributed to soil particle aggregation and Fe redistribution. Fe addition promoted the transfer of Cd/SOC accumulated in microaggregates to macroaggregates and increased the variable negative charge content in macroaggregates and the adsorption capacity of macroaggregates for Cd/SOC. More Cd/SOC accumulated in macroaggregates in Fe oxide-bound form, which reduced the risk of Cd migration and Cd availability and increased the physical protection of SOC. Therefore, Fe oxide has great potential to simultaneously reduce carbon emissions and cadmium toxicity in paddy soil. [Display omitted] • Fe distribution in aggregates affected the adsorption capacity of soil for Cd and SOC. • Fe oxides-bounded Cd immobilized in aggregate (0.053-2 mm) reduced Cd availability. • Fe oxides-associated SOC content was positively correlated with RCP content in aggregates. • RCP content in 0.25-2 mm and < 0.053 mm fractions determined SOC stability in bulk soil. • Addition of Fe in paddy soil simultaneously reduced Cd availability and carbon emissions. [ABSTRACT FROM AUTHOR]

Methane, the primary substrate for aerobic methanotrophs, regulates the rate of methanotrophic activity and shapes the composition of the methane-oxidizing community. Given that methane-derived carbon may fuel the food web in the soil, methane availability can potentially be a key determinant, structuring the network of the interacting methane-oxidizing community. Here, we determined the response of the methane-driven interaction network to different methane concentrations (∼1.5 % v/v , 3 % v/v , and 7 % v/v), indicative of different levels of energy flow through the soil food web, using a stable isotope probing approach with 13C-methane coupled to a co-occurrence network analysis in a microcosm study. The accumulated 13C-atom fraction in the total carbon content increased from 1.08 % (background level) to an average of 7.2 % in the incubation under 7 % v/v methane, indicating that the carbon-flow via the methanotrophs can significantly contribute to the total carbon in the rice paddy soil. The 13C-enriched 16 S rRNA gene sequencing analysis revealed the predominance of gammaproteobacterial methanotrophs and Methylocystis. The composition of the actively growing (13C-labelled) bacterial community was dissimilar in the incubation under ∼3 % v/v than under 1.5 % v/v and 7 % v/v methane. This was also reflected in the co-occurrence network analysis, where the topological properties indicated a more complex and connected network in the incubation under 3 % v/v methane. It thus appears that moderate methane concentrations fostered closer associations among members of the methane-oxidizing community. Overall, our research findings showed that the methanotrophs can contribute to the total soil carbon, and methane concentrations not only shifted the bacterial community, including the methanotrophic composition, but also affected bacterial interactions. [ABSTRACT FROM AUTHOR]

How to determine the optimal dosage of phosphorus (P) fertilizer input for an agricultural field is important to maintain soil quality and crop production while minimizing environmental impact. In this study, we set up a 5-year rice-wheat rotation with contrasting P fertilization treatments (0, 25, 50, 75, 100, and 150 kg P 2 O 5 ha−1, hereafter, P 0 , P 25 , P 50 , P 75 , P 100 , and P 150 , respectively) per season to explore the relationship between the amount of P input and crop yield, P use efficiency (PUE), balance of P accumulation and loss, ecosystem multifunctionality (EMF), and soil quality. Our results indicate that increased P amounts significantly boosted rice and wheat production of both straw and grain, but the tendency slowed down when the input was over 75 kg P 2 O 5 ha−1. The PUE declined with increased P input and soil P balance of 50 kg P 2 O 5 ha−1 for wheat and 100 kg P 2 O 5 ha−1 for rice. Runoff emerges as the main pathway for soil P loss and escalates with higher P application rates. We emphasize increasing ridge height and controlling water input for basal fertilizer to minimize P loss. The application of P fertilizer increased the soil P pool, with labile P (L-P) and moderately labile P (M-P) increasing by 13–114 % and 23–111 %, respectively, compared to P 0. The transformation of M-P to L-P in paddy soil is associated with an increased abundance of Actinobacteria. Low P applications (P 25 and P 50) increased EMF by 3.27 and 3.58 times, while high P applications (P 75 , P 100 , and P 150) decreased EMF. Furthermore, P application significantly improved the soil quality index (SQI) compared to P 0. The impact of abiotic factors on yield and P loss is more significant than that of biotic elements, with the SQI serving as a dependable indicator for predicting yield. Central to minimizing P loss while maximizing yield is the reduction of Resin-P content and the maintenance of NaOH-P i levels, suggesting that organic materials may be a good alternative strategy. These findings provide valuable data and theoretical support for optimizing P application in rice-wheat cropping systems, promoting a mutually beneficial scenario for agricultural production and ecological protection. [Display omitted] • Balance P annually with 100 and 50 kg P 2 O 5 ha−1 in rice and wheat seasons. • Control runoff and prolong retention time in the field to minimize P loss. • The soil quality index (SQI) is a reliable predictor of crop yield. • Reduce Resin-P and maintain NaOH-P i to enhance SQI and decrease P loss. [ABSTRACT FROM AUTHOR]

Microbially mediated N fixation is widespread in rice paddy ecosystems and crucial in maintaining soil fertility. However, our understanding of the factors determining the distribution of free-living diazotrophic microorganisms that perform this process in paddy fields is limited. This study investigated the spatial distribution and factors influencing presence and potential activity of free-living microorganisms capable of N 2 fixation in addition to dissimilatory nitrate reduction to ammonium (DNRA), anaerobic ammonium oxidation (anammox), and denitrification in 50 paddy soils across China. Using 15N isotope tracing in laboratory incubations and microbial community analysis via metagenomics, we demonstrate that paddy soils may represent a previously underappreciated hotspot for N 2 fixation with mean potential rates of 24.4 ± 17.8 nmol N g−1 h−1, 10-fold higher than DNRA (2.55 ± 0.4 nmol N g−1 h−1), and could counterbalance a portion of N 2 losses through anammox and denitrification (9.24 ± 1.1 nmol N g−1 h−1). Site longitude and organic carbon (C) concentrations, as well as the diazotrophic community composition, were the dominant abiotic and biotic factors accounting for regional variations in potential N 2 fixation rates. The N 2 metabolic pathways predicted from the metagenome-assembled genomes (MAGs) revealed significant co-occurrence of the diazotroph marker gene nifH with denitrification-associated genes (nirS/K and nosZ) and organic C oxidation-related genes (yiaY and galM). Furthermore, enzymes involved in organic C oxidation, particularly glycoside hydrolases and glycosyltransferases, were not only phenotypically correlated with free-living N 2 fixation rates but were also identified in nifH -containing MAGs, indicating the heterotrophic capabilities of diazotrophs in paddy soils. Collectively, our results underscore the substantial contribution of free-living N 2 fixation to soil N fertility in paddy fields, and highlight the importance of coupling organic C oxidation with nitrate reduction to enhance N 2 fixation. [Display omitted] • Free-living N 2 fixation is a substantial contributor to N fertility in paddy soils. • Longitude and SOC drive geographical variations in N 2 fixation in paddy soils. • Diazotroph diversity was the key biotic factor influencing N 2 fixation rates. • The metabolic versatility of diazotroph aid efficient N turnover in paddy soils. [ABSTRACT FROM AUTHOR]

Rice (Oryza sativa L.) is known to accumulate inorganic arsenic (iAs) and dimethylarsenate (DMA) in its grains, which threatens both human health and rice yield. Although schwertmannite, a metastable Fe (Ⅲ)-oxyhydroxysulfate mineral with extremely high adsorption capacity for iAs, has been proposed to remediate paddy soil to decrease As accumulation in rice, it remains unclear whether the phase transformation of schwertmannite would occur in flooded paddy soil and how its phase transformation changes the soil microbial processes that impact the accumulation of iAs and DMA in grains. Here, we found that amending As-contaminated paddy soil with 0.5%–1% (w/w) schwertmannite decreased the accumulation of iAs and DMA in grains by 37.41%–43.29% and 50.60%–73.89%, respectively, even though schwertmannite has transformed to goethite and secondary FeS was formed in both rhizosphere and bulk soils. The phase transformation of schwertmannite released a considerable amount of SO 4 2− into porewater, thereby increasing the abundances of both sulfate-reducing bacteria and the dsrB gene but decreasing the abundance of iron-reducing bacteria. This result suggested that schwertmannite phase transformation has promoted sulfate-reducing process and weakened iron-reducing process in flooded soil. Such promoted sulfate-reducing process and weakened iron-reducing process in paddy soil can decrease the reductive dissolution of As-bearing (oxyhydr)oxides, increase the formation of secondary FeS mineral for decreasing porewater As concentration, and strengthen the role of Fe plaque as a barrier for As absorption by rice. Additionally, the application of schwertmannite has decreased the abundance of arsM gene and weakened As methylation process in soil. Therefore, the effective decrease of iAs and DMA accumulation in rice grains by schwertmannite can not only be ascribed to the adsorption capacity of schwertmannite for As and the adsorption or incorporation of As by transformation products, but also contributed by the promoted sulfate-reducing process and the weakened iron-reducing process in flooded paddy soil. [Display omitted] • Transformation of schwertmannite into goethite occurs in flooded paddy soil. • Phase transformation alters iron-reduction and sulfate-reduction processes. • Schwertmannite decreases arsM abundance and weakens arsenic methylation process. • Schwertmannite simultaneously decreases the accumulation of iAs and DMA in rice grains. [ABSTRACT FROM AUTHOR]

Microbial stoichiometry is pivotal in the soil elements cycle within terrestrial ecosystems. However, the impact of microbial stoichiometry on the phosphorus (P) pool transformation in low-P paddy soil, especially with manure addition, remains poorly understood. This study aimed to elucidate the response mechanism of microbial stoichiometry in regulating P pool transformation in two low-P paddy soils during a 60-day flooding-drought incubation. The results demonstrated that pig manure and vermicompost application significantly increased soil Olsen-P by 202–309 %, and microbial biomass P (MBP) by 54.4–79.3 % compared to no fertilization. Additionally, vermicompost treatment increased moderately labile organic P (MLP o) by 133–257 % and decreased fulvic acidassociated organic P (FAP o) by 10.5–25.4 % in Acrisol-flooding, Acrisol-drought, and Ultisol-drought, indicating that manure application improved the transformation of FAP o to MLP o. The microbial biomass carbon (MBC)/MBP ratio was lowest under Acrisol-flooding and highest under Ultisol-drought, suggesting that microorganisms adjust high ratios for stoichiometric stability and enhanced MBP utilization under deficient resource conditions. Manure treatments increased alkaline phosphatase (ALP) by 5.33–12.9 % under flooding conditions, indicating microorganisms facilitate the mineralization of soil organic P (P o). Compared to Acrisol-flooding, both ALP and β-1,4-glucosidase (BG) significantly increased by 103 % and 259 %, respectively, under Ultisol-drought, along with a positive correlation between BG and MLP o , implying that microorganisms enhance soil organic matter mineralization in resource-limited conditions by increasing C-acquiring enzymes and releasing P o. Additionally, the microbial community composition shifted from r -strategists to K -strategists, primarily by decreasing Proteobacteria and increasing Acidobacteria under resource deficiency and drought. The r -strategists directly mineralize P o by maintaining a low MBC/MBP and high ALP, while K -strategists indirectly mineralize P o by maintaining a high MBC/MBP and high BG. The findings suggest that manure application altered the resource status of low-P paddy soils, changed microbial stoichiometry, and influenced soil P availability through adjustments in microbial activity and extracellular enzyme production. [Display omitted] • Microorganisms improve soil available P to MBP by increasing the MBC/MBP ratio. • The r -strategists directly mineralize P o by maintaining low MBC/MBP and high ALP. • The K -strategists indirectly mineralize P o by maintaining high MBC/MBP and high BG. [ABSTRACT FROM AUTHOR]

[Display omitted] • Residue amendments altered paddy topsoil OC pools and SOM composition. • Biochar amendment increased topsoil OC via promoted aggregation and char input. • The OC pool ratio of POM to MAOM increased with all the residue amendments. • Residue amendments augmented the contribution of plant-derived lipids to SOM. In agricultural ecosystems, incorporation of crop residues has been practiced as a recycling approach for sustaining soil organic matter (SOM) and soil fertility. However, how crop residue amendments in different forms (direct straw return, converted as manure and pyrolyzed as biochar) affect soil organic carbon (SOC) pools and SOM composition is not well known. In this study, a short-term (2015–2019) field experiment in paddy soil was conducted with a one-time maize residue amendment, consistently in a single doses of 10 Mg ha−1 organic carbon (OC) equivalent, in three forms: air-dried straw (CS), cattle manure (CM) and biochar (CB). No residue amendment (CK) was used as control. Topsoil organic matter changes were analysed using 13C isotopic tracing, biomarker analysis, and solid-state 13C nuclear magnetic resonance spectroscopy in combination with soil aggregate density/size fractionation. After four cropping cycles following amendment, SOC content was unchanged under CS and CM but increased by 24% under CB; However, the OC pool ratio of particulate organic matter (POM) to mineral-associated organic matter (MAOM) significantly increased under all amendments compared to CK. The δ13C values indicated that maize-derived OC was preserved the most in the POM within the macroaggregates, particularly under CB. Regarding the molecular composition, all residue amendments increased the abundance ratio of plant- to microbe-derived lipids. Plant-derived lipids were primarily concentrated in macroaggregates, whereas microbial lipids were more prevalent in the silt–clay fractions. Lignin phenols were significantly enriched only in microaggregates under CS relative to CK. Overall, SOM changes in the paddy topsoil following crop residue amendment in different forms were depicted by OC pool redistribution and molecular composition alteration. The study highlighted that biochar amendment, instead of straw or manure, greatly enhanced SOC accumulation by promoting macro-aggregation, which in turn preserved plant-derived carbon through the direct input of the persistent char in a rice paddy. [ABSTRACT FROM AUTHOR]

Organic carbon (C) can greatly affect soil microbial activity and thus alter the nutrient availability in soil. However, little is known about the microbial-mediated P transformation processes with organic C inputs in flooded soils. This study used high-throughput sequencing technology to investigate the effects of labile (glucose) and recalcitrant (lignin) organic C inputs on P availability and microbial communities in the bulk and rhizosphere soil under rice growth, aiming to clarify the key microorganisms involved in soil P transformation. Glucose addition enhanced microbial P immobilization and decreased P availability in the rhizosphere soil, while lignin amendment increased available P concentration in bulk soil through increased soil pH and altered bacterial community. Organic C inputs stimulated the growth and cooperation of specific P-solubilizing bacteria (e.g. , Acetobacter and Phenylobacterium), facilitating the transformation of non-labile P to labile P in bulk soils, but also stimulated the growth of Clostridium_sensu_stricto , immobilizing labile P into their biomass. Irrespective of C supply, soil P availability was correlated positively with the abundances of Phenylobacterium and Desulfosporosinus but negatively with those of Acetobacter , Clostridium_sensu_stricto , Clostridium_IV , Gluconacetobacter and Syntrophomonas. This study emphasized the contrasting effects of labile and recalcitrant C on microbial mechanisms of P transformation in a flooded paddy soil. These findings provide potential strategies for microbially mediated P management in paddy fields. [Display omitted] • Effects of labile and non-labile C on microbial P transformation were compared. • Glucose decreased rhizosphere P availability due to microbial P immobilization. • Lignin mobilized P in bulk soils via elevated pH and altered microbiota. • Organic C inputs enhanced interactions between P-solubilizing bacteria in the soil. [ABSTRACT FROM AUTHOR]

Cadmium (Cd) contamination in paddy soils poses a serious threat to agricultural production and human health. In this study, cysteine was selected as the leaching agent for remediating Cd-contaminated paddy soils because of its effective, low cost and ecofriendly characteristics. Microcosmic simulation experiment was using to investigate the fate of Cd and elucidate the transformation mechanism of Cd in the key components of paddy soil under cysteine leaching. Results showed that the total Cd removal rate of paddy soils reached 46.8–87.4% under the optimal conditions. This was mainly attributed to the decrease of exchangeable fraction (F1), carbonate bound fraction (F2), iron/manganese-bound fraction (F3) and organic matter bound fraction (F4) of Cd. After cysteine leaching, soil fertility was promoted due to the increase in nitrate nitrogen (NN), ammonia nitrogen (AN), available phosphorus (AP) and organic matter (OM). pH, CaCO 3 , available potassium (AK), NN, AP, amorphous iron oxides (Fe o) were the main contributors to Cd fractionation transformation. The desorption of Cd from ferrihydrite (Fh) and goethite (Gt) surfaces is hypothesized to stem from dual mechanisms: the dissolution due to the low solution pH and the reducibility of cysteine, which facilitated the conversion of Fh and Gt to hematite (Hm). And the -SH complexation also ascribed to the activation of Cd bound by Fe-OH in Fh and Gt and oxygen-containing functional groups in humic acid (Ha). For chlorite (Ch), the primary driver for Cd desorption was identified as an ion-exchange process, whereby electrostatically adsorbed Cd2+ were replaced by H+ produced by cysteine protonation. [Display omitted] • The key influencing parameters of reduce remediation of Cd polluted paddy soil by cysteine leaching were investigated. • The removal of Cd mainly attributed to the decrease of F1, F2, F3, and F4 fractions. • Cd was desorbed from Fh and Gt owing to the low solution pH and reducibility of cysteine. • The -SH complexation caused the activation of Cd bound by Fe-OH in Fh and Gt and the oxygen functional groups in Ha. • Cd desorption in chlorite was achieved by ion exchange between electrostatically adsorbed Cd and H+ of cysteine solution. [ABSTRACT FROM AUTHOR]

Contamination of paddy soils with arsenic (As) poses imminent threat to the environment and public health. This research work explored the effect of goethite-modified biochar (GMBC) on As immobilization in paddy soil and subsequent accumulation in rice grains. The results showed that the soil suppiementation with GMBC significantly improved the biomass of rice plants. In addition, the GMBC application effectively decreased the As content in rice grains (0.72-0.16 mg kg--1). Compared with the control, GMBC 1.5% treatment augmented the iron plague (Fe-plague) buildup on rice roots and efficiently sequestered the As by 174%, and reduced its uptake in rice tissues. Soil supplementation with GMBC 1.5% greatly enhanced the activities of soil peroxidase (POD) and catalase (CAT) by 90% and 40%, respectively, compared to the control. Moreover, GMBC amendments improved the relative abundance of the soil bacterial communities and minimized the As mobility in the soil. GMBC 1.5% significantly enhanced the abundance of acidobacteria and Firmicutes by 211% and 95% while that of Chloroflexi decreased by 25%, respectively. The findings of the present investigation demonstrated that GMBC could be used as an environment-friendly approach to remediate As polluted paddy soils and minimize its accumulation in rice grains for mitigation of food security risks and protect public health. [ABSTRACT FROM AUTHOR]

While the transformation of antimony (Sb) in paddy soil has been previously investigated, the biogeochemical processes of highly chemical active Sb in the soil remain poorly understood. In addition, there is a lack of quantitative understanding of Sb transformation in soil. Therefore, in this study, the kinetics of exogenous Sb in paddy soils were investigated under anaerobic and aerobic incubation conditions. The dissolved Sb(V) and the Sb(V) extracted by diffusive gradient technique decreased under anaerobic conditions and then increased under aerobic conditions. The redox reaction of Sb occurred, and Sb bioavailability significantly decreased after 55 days of incubation. The kinetics of Fe and the scanning transmission electron microscopy analysis revealed that the Fe oxides were reduced and became dispersed under anaerobic conditions, whereas they were oxidized and re-aggregated during the aerobic stage. In addition, the redox processes of sulfur and nitrogen were detected under both anaerobic and aerobic conditions. Based on these observations, a simplified kinetic model was established to distinguish the relative contributions of the transformation processes. The bioavailability of Sb was controlled by immobilization as a result of S reduction and by mobilization as a result of Fe reductive dissolution and S oxidation, rather than the pH. These processes coupled with the redox reaction of Sb jointly resulted in the complex behavior of Sb transformation under anaerobic and aerobic conditions. The model-based method and findings of this study provide a comprehensive understanding of the Sb transformation in a complex soil biogeochemical system under changing redox conditions. [Display omitted] [ABSTRACT FROM AUTHOR]

The aim of this study is to investigate conversion of nitrogen and COD in enriched paddy soil by nitrification coupled with anammox process in a dual chamber bioelectrochemical system. The paddy soil was enriched for denitrification coupled with anammox by microbial consortia and was acclimatized in the cathodic chamber of microbial fuel cells (MFCs). The bioelectrochemical systems were treated with different ammonium concentrations in the cathodic chamber: the MFC with low concentration ammonium (LA-MFC, 50 mg/L ammonium), the MFC with medium concentration ammonium (MA-MFC, 500 mg/L ammonium), and MFC with high concentration ammonium (HA-MFC, 1000 mg/L ammonium), and the initial COD in the anodic chamber was 1200 mg/L. The CK treatments were conducted with 1000 mg/L ammonium under the same conditions, except without inoculum in the cathode chamber. The consumption rate of ammonium in the cathodic chambers of CK, LA-MFC, MA-MFC, and HA-MFC were 9%, 64%, 84%, and 84%, respectively. The degradation rate for COD achieved in the anode chambers of CK, LA-MFC, MA-MFC, and HA-MFC were 70%, 86%, 93%, and 93%, respectively. The analysis of the microbial community of three treated MFCs in the cathode chamber indicated that the nitrification-denitrification process occurs in the cathode chamber. The dominant species for nitrification was Nitrospira, and the dominant species for denitrification were Denitratisoma, Dechloromonas, and Candidatus_Competibacter. Moreover, anammox process also observed in the cathode chamber. The functional genes nirS/K, hzsB, and 16S rDNA were assessed by qPCR analysis, and the results confirmed the presence of denitrification-coupled anammox in the cathodic chamber. [ABSTRACT FROM AUTHOR]

[Display omitted] Impact of environmental perturbations i.e., nitrogen (N), phosphorus (P), potassium (K), and rice straw (Rs) on the dynamics of soil bacterial and archaeal communities are multifactor dependent and seeks a contemporary approach to study underlying mechanisms. The current study investigates the effect of pure and mixed fertilizers on soil physicochemical properties, the microbial community structure, and their functional metabolic predictions. It involved amendments with distinct combinations of N as C(H 2 N) 2 O, P and K as KH 2 PO 4 , K as KCl, and Rs in paddy soil microcosms with concentrations common in rice fields agriculture. Soil pH, electrical conductivity (EC), total carbon (TC), total nitrogen (TN), organic matter (OM), available K (AK), and total extractable P (TEP) were evaluated. To comprehend community variation and functional predictions, 16S rRNA-based high throughput sequencing (HTS) and phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt) were employed, respectively. Our findings showed enhanced community richness and diversity in all amendments compared to control. Proteobacteria, Actinobacteria, and Firmicutes were dominant bacterial phyla. Regarding relative abundance, Chloroflexi, Bacteroidetes, and Verrucomicrobia showed positive while Actinobacteria, Acidobacteria, and Gemmatimonadetes showed negative trends compared to controls. Thaumarchaeota and Euryarchaeota were dominant archaeal phyla and exhibited increasing and decreasing trends, respectively. The PICRUSt analysis indicated functional prediction more towards amino acid, carbohydrate, energy, and lipid metabolism while less towards others. Concerning energy metabolism, most and least responsive treatments were KP and controls, respectively. These outcomes enhanced our understanding regarding soil quality, fertilizer composition and application, and functional metabolomics of archaea and bacteria. [ABSTRACT FROM AUTHOR]

• Effects of natural organic matter on cadmium mobility in paddy soil are reviewed. • Adding OM can adjust soil pH and therefore alter Cd adsorption. • Cd mobility in paddy soil is closely linked to iron and sulfur redox reactions. • Natural OM does not often decrease Cd mobility in soil or the Cd content in rice grains. Cadmium (Cd) contamination in paddy soil has caused public concern. The uptake of Cd by rice plants depends on soil Cd mobility, which is in turn substantially influenced by organic matter (OM). In this review, we first summarize the fate of Cd in soil and the role of OM. We then focus on the effects of OM on Cd mobility in paddy soil and the factors influencing the remedial effectiveness of OM amendments. We further discuss the performance of straw incorporation in the remediation of Cd-contaminated paddy soils reported in laboratory and field studies. Considering the huge production of organic materials (such as straw) in agriculture, the use of natural OM for soil remediation has obvious appeal due to the environmental benefits and low cost. Although there have been successful application cases, the properties of OM amendments and soil can significantly affect the remedial performance of the OM amendments. Importantly, straw incorporation alone does not often decrease the mobility of Cd in soil or the Cd content in rice grains. Careful evaluation is required when considering natural OM amendments, and the factors and mechanisms that influence their remedial effectiveness need further investigation in paddy soil with realistic Cd concentrations. [Display omitted] [ABSTRACT FROM AUTHOR]

Cadmium (Cd) contamination in agricultural soil has been an elevated concern due to the high health risks associated with the transfer through the soil-food chain, particularly in the case of rice. Recently, there has numerous researches on the use of nanoparticle-loaded materials for heavy metal-polluted soil remediation, resulting in favorable outcomes. However, there has been limited research focus on the field-scale application and recovery. This study was aimed to validate the Cd reduction effect of the nano-FeS loaded lignin hydrogel composites (FHC) in mildly polluted paddies, and to propose a field-scale application method. Hence, a multi-site field experiment was conducted in southern China. After the application for 94–103 days, the FHC exhibited a high integrity and elasticity, with a recovery rate of 91.90%. The single-round remediation led to decreases of 0.42–31.72% in soil Cd content and 1.52–49.11% in grain Cd content. Additionally, this remediation technique did not adversely impact rice production. Consequently, applying FHC in the field was demonstrated to be an innovative, efficient, and promising remediation technology. Simultaneously, a strategy was proposed for reducing Cd levels while cultivating rice in mildly polluted fields using the FHC. [Display omitted] • FHC retained its excellent mechanical strength and exhibited a high recovery rate. • FHC reduced Cd in grain and soil effectively after one to two rounds remediation. • FHC did not negatively impact the grain yield or biomass of rice. • This study proposed a strategy for reducing Cd in polluted paddy fields using FHC. [ABSTRACT FROM AUTHOR]

Paddy soil is an important source of methane (CH 4) emissions, and the nitrite-driven anaerobic oxidation of methane (AOM) mediated by Candidatus Methylomirabilis bacteria is a crucial process for CH 4 mitigation. However, the vertical and temporal fluctuations in nitrite-driven AOM activity, as well as the abundance and composition of Methylomirabilis bacteria in paddy soil, remain poorly understood. We investigated the potential rate of nitrite-driven AOM and the composition of Methylomirabilis bacteria at varying depths (0–40 cm, at 10 cm intervals) within a paddy soil during different growth stages of rice, including tillering, jointing, flowering, and milky. The potential rate of nitrite-driven AOM ranged from 0.24 to 20.6 nmol CO 2 g−1 (dry soil) d−1, and the abundance of Methylomirabilis bacterial 16S rRNA genes varied from 5.8 × 106 to 4.4 × 107 copies g−1 dry soil. The 10–20 cm soil depth showed higher potential activity and bacterial abundance compared to other depths, and the tillering and jointing stages exhibited greater activity and abundance. The composition of Methylomirabilis bacteria showed significant variation across different soil depths but remained stable throughout the growth stages of rice. Soil organic carbon content was positively correlated with potential nitrite-driven AOM rate, and nitrite content was negatively correlated with the abundance of Methylomirabilis bacteria. Soil organic carbon and ammonium content had a significant impact on the composition of Methylomirabilis bacteria. The results underscore the substantial influence of soil depth and the growth stage of rice on nitrite-driven AOM in paddy ecosystems. • Nitrite-driven AOM activity and the bacterial abundance peaked at 10–20 cm soil depth. • Early rice growth stage exhibited higher activity and abundance than late stage. • Composition of Methylomirabilis bacteria varied significantly with soil depth. • Organic carbon content was significantly correlated with nitrite-driven AOM activity. [ABSTRACT FROM AUTHOR]

Arsenic and cadmium (As/Cd) are toxic elements which negatively impact the health and productivity of rice plants by subjecting them to stress. To address these issues, we developed three biochar amendment materials derived from plant-based corn-stover with iron-magnesium modification (FM@CB), char sourced from animal-based eggshells with iron-magnesium modification (FM@EB), and a blend (1:1, w/w) of both (FM@CEB). Batch adsorption experiment revealed that FM@CB exhibited a Langmuir sorption capacity of 77 mg g−1 for As(III) and 107 mg g−1 for Cd(II). In contrast, FM@EB showed values of 63 mg g−1 for As(III) and 115 mg g−1 for Cd(II), both observed in mixed systems. The exothermic dual adsorption mechanisms could include ion exchange, cation-anion bridging, complexation with surface functional groups, and precipitation processes. Pot trials were conducted, and the findings unveiled a substantial enhancement in rice grain yield with a 2% application of FM@CB, FM@EB, and FM@CEB. Specifically, there was a notable increase of 21.9%, 34.9%, and 37.6%, respectively. Simultaneously, the application led to a reduction in grain As levels by 34.4%, 53.2%, and 42.6%, and grain Cd levels by 48.7%, 71.2%, and 59.9%, respectively, in comparison to the no-amendment control condition. Our study observed a transformation in the soil's microbial community, as beneficial bacterial genera such as Anaeromyxobacter , Citrifermentans , and Desulfovibrio emerged to assist in the detoxification of these metal contaminants. Concurrently, the application of these materials led to a reduction in harmful bacterial genera like Pseudomonas , Flavobacterium , and Massilia. In addition, these amendments decreased the bioavailable As/Cd fractions in the soil and promoted the root-iron-plaque, which effectively sequesters these contaminants, rendering rice quality improvement. Ultimately, the use of these amendments, particularly FM@CEB, holds the promise of protecting rice from harmful metal(loid)s contaminants, increasing yields, and revitalizing the soil. The presence of these unique bacterial genera, especially those capable of detoxifying As/Cd, plays a crucial role in ensuring a healthier and more secure environment for all living organisms. [Display omitted] • Fe–Mg modified plant/animal-based FM@CB, FM@EB, and FM@CEB were prepared. • Biochars' sorption capacity: 63−77 mg g−1 for As(III), 107−115 mg g−1 for Cd(II). • Sorption mechanisms: ion exchange, cation-anion bridge, complexation, precipitation. • Beneficial bacteria aid As/Cd detoxification in treated soil. • 2% amendments significantly reduce bioavailable As/Cd in soil and rice grain. [ABSTRACT FROM AUTHOR]

Biochar could reshape microbial communities, thereby altering methylmercury (MeHg) concentrations in rice rhizosphere and seeds. However, it remains unclear whether and how biochar amendment perturbs microbe-mediated MeHg production in mercury (Hg) contaminated paddy soil. Here, we used pinecone-derived biochar and its six modified biochars to reveal the disturbance. Results showed that selenium- and chitosan-modified biochar significantly reduced MeHg concentrations in the rhizosphere by 85.83% and 63.90%, thereby decreasing MeHg contents in seeds by 86.37% and 75.50%. The two modified bicohars increased the abundance of putative Hg-resistant microorganisms Bacillus , the dominant microbe in rhizosphere. These reductions about MeHg could be facilitated by biochar sensitive microbes such as Oxalobacteraceae and Subgroup_7. Pinecone-derived biochar increased MeHg concentration in rhizosphere but unimpacted MeHg content in seeds was observed. This biochar decreased the abundance in Bacillus but enhanced in putative Hg methylator Desulfovibrio. The increasing MeHg concentration in rhizosphere could be improved by biochar sensitive microbes such as Saccharimonadales and Clostridia. Network analysis showed that Saccharimonadales and Clostridia were the most prominent keystone taxa in rhizosphere, and the three biochars manipulated abundances of the microbes related to MeHg production in rhizosphere by those biochar sensitive microbes. Therefore, selenium- and chitosan-modified biochar could reduce soil MeHg production by these microorganisms, and is helpful in controlling MeHg contamination in rice. [Display omitted] • Effects of six modified biochars on the enrichment of MeHg in rice were compared • Selenium and chitosan modified biochars greatly reduced MeHg contents in rice. • Bacillus altered soil MeHg production by MeHg degradation under biochar amendment. • Saccharimonadales and Clostridia could increase MeHg contents in rhizosphere. [ABSTRACT FROM AUTHOR]

Soil microbial carbon metabolic activity is critical to the carbon balance of terrestrial ecosystems. How the microbial interactions induce microbial carbon metabolic activity in rice-based ecosystems at different fertility levels are unclear. Here, we investigated the association of microbial carbon metabolic activity with soil physicochemical properties, microbial diversity and interactions at three fertilization practices and four soil profile levels (0–10 cm, 10–20 cm, 20–40 cm, 40–60 cm) in two paddy soils with different fertility levels. Our results found that microbial carbon metabolic activity and content of soil nutrient increased with fertilizer application practices but decreased with increasing soil depth. We provide an effective tool for studying microbial interactions across the soil profiles named cross-profile network model. The cross-profile showed that 10–20 cm had the highest average degree of edges and nodes in the co-occurrence network. The network indicated that the combination of organic and inorganic fertilizers increased the number of nodes and edges, while the single application of organic fertilizers decreased the number of nodes and edges in fertile soil. The taxonomic diversity of microorganisms and the carbon-related functional diversity were significantly correlated with microbial interactions by linear/non-linear fitted models. Partial least squares path modeling showed that microbial carbon metabolic activity in fertile soil was directly affected by dissolved organic carbon. However, soil physicochemical properties indirectly affected the carbon metabolic activity by mediating microbial diversity and interactions in infertile soil. For the first time, we incorporated potential microbial interactions between soil layers into a network model, emphasizing the importance of microbial diversity and interactions in regulating carbon metabolism activities. Our results pave the way for the integration of bacterial community diversity and interactions to the predict soil organic carbon dynamics. [Display omitted] [ABSTRACT FROM AUTHOR]

• Phytoliths dissolved three times faster in paddy soils than in non-paddy soils. • Phytolith morphology and surface chemistry were not affected by paddy management. • Laboratory dissolution rate decreased with time in soil and with surface Al. • Field dissolution was unrelated to changes in surface chemistry. • Field dissolution is overwhelmingly governed by water regime. Phytoliths are an important component in the cycling of silicon (Si) in rice cultivation, yet little is known about their medium to long-term stability. While it is commonly accepted that phytolith solubility in soil decreases with time, the mechanisms that cause this decrease remain unclear. Most studies on phytolith aging to date have been conducted under laboratory conditions and field studies are rare. Here, we present a comprehensive field study of phytolith aging in paddy and non-paddy fields in the Philippines and Vietnam. Phytoliths extracted from rice straw were placed in mesh bags and exposed to ambient soil conditions for up to 550 days. Following exposure, total mass loss and changes in physical (particle size, specific surface area, ζ potential) and chemical (surface chemical composition by acid digestion and X-ray photoelectron spectroscopy) properties as well as dissolution kinetics (56 days in 0.1 M CaCl 2 at pH 6.5) were determined. Phytolith dissolution in the field was rapid (up to 30 % mass loss) and three times faster in paddy than in non-paddy soils. Differences in phytolith properties were unexpectedly little between paddy and non-paddy soils. Laboratory and field-derived dissolution rates decreased with exposure time but were one order of magnitude lower in the field. While laboratory dissolution rates were negatively related to surface Al contents this was not observed for field-exposed phytoliths. Our findings suggest that under natural field conditions, phytolith dissolution is overwhelmingly governed by water regime, while the influence of protective surface coatings is only minor. We conclude that reliable estimates of phytolith dissolution can be best obtained in field experiments. [ABSTRACT FROM AUTHOR]

• Kinetics of sb in paddy soil with adding nitrate was controlled multiple effects. • Proton consumption via denitrification could enhance Sb(V) mobility. • The redox cycling of fe and s could diversely affect sb availability. • Arsenite-oxidizing marker gene aoxB may be involved in Sb(III) oxidation. Nitrate (NO 3 −) is known to be actively involved in the processes of mineralization and heavy metal transformation; however, it is unclear whether and how it affects the bioavailability of antimony (Sb) in paddy soils and subsequent Sb accumulation in rice. Here, the effects of NO 3 − on Sb transformation in soil-rice system were investigated with pot experiments over the entire growth period. Results demonstrated that NO 3 − reduced Sb accumulation in brown rice by 15.6% compared to that in the control. After amendment with NO 3 −, the Sb content in rice plants increased initially and then gradually decreased (in roots by 46.1%). During the first 15 days, the soil pH increased, the oxidation of Sb(III) and sulfides was promoted, but the reduction of iron oxide minerals was inhibited, resulting in the release of adsorbed and organic-bound Sb from soil. The microbial arsenite-oxidizing marker gene aoxB played an important role in Sb(III) oxidation. From days 15 to 45, after NO 3 − was partially consumed, the soil pH decreased, and the reductive dissolution of Fe(III)-bearing minerals was enhanced; consequently, iron oxide-bound Sb was transformed into adsorbed and dissolved Sb species. After day 45, NO 3 − was completely reduced, Sb(V) was evidently reduced to Sb(III), and green rust was generated gradually. Thus, the available Sb decreased due to its enhanced affinity for iron oxides. Moreover, NO 3 − inhibited the reductive dissolution of iron minerals, which ultimately caused low Sb availability. Therefore, NO 3 − can chemically and biologically reduce the Sb availability in paddy soils and alleviate Sb accumulation in rice. This study provides a potential strategy for decreasing Sb accumulation in rice in the Sb-contaminated sites. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

In this study, we investigated the microscale behavior of arsenic (As) in an oxic-anoxic interface in laboratory-incubated paddy soil by combining multiple techniques, micro X-ray fluorescence spectrometry combined with micro X-ray absorption fine structure (μXRF-μXAFS), microelectrodes, and quantitative polymerase chain reaction (qPCR) analyses. A steep redox gradient with depth was observed in the topsoil millimeter layer (depth: 0–4.6 mm) after 15 days, oxidative condition in the surface soil and reductive condition in deeper soil, due to the limited oxygen supply caused by flooding. Microscale elemental mapping of the top soil layer by μXRF showed that a large amount of the As (about 10% of total) in the system strongly accumulated within the top 3 mm soil layer after 15 days of incubation. Direct As speciation by extended X-ray absorption fine structure (EXAFS) indicated that the host phases of As in the accumulation layer were the Fe(III) hydroxides included in the soil, which were the dominant Fe species in the layer. Direct As speciation by micro X-ray absorption near edge structure (μXANES) showed that the dominant As species in the As accumulation and depletion zones were clearly changed. Specifically, the oxidized species As(V) increased in the As accumulation (top surface) layer, and the reduced species As(III) decreased in the As depletion (deeper) layer. Direct Fe speciation also showed that the most abundant fraction of Fe in the accumulation layer was the Fe(III) hydroxides such as ferrihydrite, the most preferable sorbent for As. These findings suggest that the As accumulations observed in the study are significantly associated with As reduction and oxidation in the oxic-anoxic interface of paddy soil. Specifically, more labile As(III) formed in the reductive deeper layer migrates to the upper layer, the migrated As(III) is oxidized to As(V) in oxidative surface millimeter layer, and the As(V) is then scavenged by Fe(III) hydroxides that have high affinity for As(V); this is a possible mechanism for the strong accumulation of As observed in the topsoil layer in this study. In addition, manganese (Mn) and iron (Fe) μXANES analyses and microbial As(III) oxidase gene (aioA) quantification by qPCR suggested that As(III) oxidation in the top layer, which acted as a trigger reaction for As accumulation, might be induced by both chemical and microbial oxidation processes. The findings in this study indicate that the paddy soil system could potentially have a strong ability to prevent As migration to the floodwater. Our findings also indicate that the millimeter-scale topsoil layer in the flooded paddy soil could biogeochemically influence the whole As behavior in the paddy fields. Thus, to fully understand and predict As fate and migration in the paddy fields, we should investigate localized physical, chemical, and, biological properties of paddy soil. [ABSTRACT FROM AUTHOR]

Electrotrophs are microbes that can receive electrons directly from cathode in a microbial electrolysis cell (MEC). They not only participate in organic biosynthesis, but also be crucial in cathode-based bioremediation. However, little is known about the electrotrophic community in paddy soils. Here, the putative electrotrophs were enriched by cathodes of MECs constructed from five paddy soils with various properties using bicarbonate as an electron acceptor, and identified by 16S rRNA-gene based Illumina sequencing. The electrons were gradually consumed on the cathodes, and 25%–45% of which were recovered to reduce bicarbonate to acetic acid during MEC operation. Firmicutes was the dominant bacterial phylum on the cathodes, and Bacillus genus within this phylum was greatly enriched and was the most abundant population among the detected putative electrotrophs for almost all soils. Furthermore, several other members of Firmicutes and Proteobacteria may also participate in electrotrophic process in different soils. Soil pH, amorphous iron and electrical conductivity significantly influenced the putative electrotrophic bacterial community, which explained about 33.5% of the community structural variation. This study provides a basis for understanding the microbial diversity of putative electrotrophs in paddy soils, and highlights the importance of soil properties in shaping the community of putative electrotrophs. Unlabelled Image [ABSTRACT FROM AUTHOR]

In paddy fields, periodic flooding and drainage phases can significantly affect the availability of antimony (Sb), but the underlying mechanisms remain unclear. In this study, Sb-contaminated paddy soil was incubated under anaerobic (40 day) and subsequently aerobic (40–55 day) conditions. The Sb fractions was investigated and a kinetic model was established to quantitatively evaluate the main processes controlling Sb transformation. Under anaerobic conditions, the reductive dissolution of iron (Fe) (oxyhydr)oxides, the release of soil colloids, and dissolved organic carbon (DOC) could facilitate the release of Sb(V), while newly released Sb(V) were synchronously reduced to Sb(III) that could be incorporated into the solid phase (34.1%, 40 day) or precipitated as Sb 2 S 3 (9.7%, 40 day). After soil aeration, a significant increase in dissolved and extracted Sb(V) (34.7%, 45 day) was observed due to the Sb(III) oxidization by the reactive oxygen species (ROS) generated from Fe(II) oxidization. The dissolved and extracted Sb(V) were simultaneously incorporated into the solid phase as the re-aggregation of soil colloids and DOC, and only contributed to 17.1% of the total Sb content at the end of aerobic phase (55 day). Our results elucidated the mechanisms about how biogeochemical Fe/S/C cycling jointly controlled Sb transformation in paddy systems. [Display omitted] ● Sb(V) released from iron oxides and organic matter was reduced to Sb(III). ● Sb(III) was subsequently immobilized in solid phase under reducing conditions. ● A significant mobilization of Sb(V) was observed following Sb(III) oxidization. ● Sb(III) oxidization was dominated by ROS through chemical processes. ● A kinetics model was used to quantitatively evaluate Sb transformation. [ABSTRACT FROM AUTHOR]