Paddy Soil Research Articles

Diversity and transcription of genes involved in respiratory As(V) reduction and As(III) methylation in Japanese paddy soils

BackgroundArsenic (As) metabolism by soil microorganisms has an impact on As geochemical cycling in paddy soils, which in turn affects As uptake in rice. However, little is known about the key microorganisms involved in this process in Japanese paddy soil.ResultsTotal RNA was extracted from Japanese paddy soils with different levels of dissolved As under flooded conditions, and the transcription of As metabolic genes (arrA, ttrA and arsM) was analyzed via a metatranscriptomic approach. The results showed that ttrA was the predominant respiratory arsenate reductase gene transcribed in these soils rather than arrA, suggesting that ttrA contributes to the reductive dissolution of As. The predominant taxa expressing ttrA differed among soils but were mostly associated with genera known for their iron- and/or sulfate-reduction activity. In addition, a wide variety of microorganisms expressed and upregulated arsM approximately 5.0- to 13.2-fold at 9 d compared with 3 d of incubation under flooded conditions in flasks.ConclusionsOur results support the involvement of microbial activity in the geochemical cycling of As in Japanese paddy soils and suggest that ttrA may be one of the key genes involved in the formation of arsenite, an inorganic species taken up by rice.

Decoupled fungal and bacterial functional responses to biochar amendment drive rhizosphere priming effect on soil organic carbon mineralization

The application of biochar to soil is widely recognized as a promising strategy for enhancing the accumulation and stability of soil organic carbon (SOC), which is crucial in mitigating climate change. However, the influence of interactions between plants and biochar on soil microbial communities and their involvement in SOC mineralization and stability remains unclear. This understanding is essential for optimizing carbon (C) sequestration in systems involving plants, soil, and biochar. In this study, employing a 13C natural abundance approach, we investigated the effect of biochar on the maize rhizosphere priming effect (RPE) in paddy soil. We also examined alterations in microbial communities and functional genes related to C degradation and fixation. Over the 99 days of maize growth, biochar application increased RPE and total SOC while decreasing dissolved organic C. It also elevated soil pH, resulting in shifts in fungal and bacterial community structure, favoring oligotrophic species. Fungal and bacterial assemblies were dominated by deterministic and stochastic processes, respectively. While the abundance of fungal guilds varied irregularly, bacterial guilds were uniformly enriched under biochar-plant interactions. Functional traits such as ecoenzymatic activities, bacterial guilds, and functional genes predominantly affected RPE under biochar application. Bacterial functional genes associated with C degradation and fixation were concurrently enhanced with biochar application. Our results indicate that interactions between plants and biochar can enhance native SOC mineralization and accumulation in a short timeframe by modulating functional traits of soil microorganisms, particularly the bacterial community involved in C degradation and fixation.Graphical

Co-application effects of rice husk biochar and different types of nitrogen fertilizers on rice yield, yield components, and nitrogen use efficiency

The Nitrogen (N) recovery index in rice paddy soils has never been exceeded by more than 30%, regardless of using proven fertilizer management strategies and amounting to significant N being out of rice plant use. Biochar application and the newly released N biofertilizers (singly or in combination) increase the N use efficiency by increasing the soil nutrient retention capacity. Therefore, a two-year pot experiment was conducted to explore the effects of the co-application of rice husk biochar and nitrogen fertilizers on rice yield, yield components, and nitrogen use efficiency indexes. The applied treatments were rice husk biochar at three levels (0, 20, and 40 tha−1) and N fertilizers at three levels (0, chemical (Urea), and N-biofertilizer). The most effective combination treatments (200 kg N ha−1 in source of urea plus 40 tha−1 rice husk biochar) significantly (p ≤ 0.01) increased the grain yield (2.38 times), straw and biological yield (2.83 and 2.62 times, respectively), N content of soil N (59%), nitrogen uptake efficiency (2.92 times), and nitrogen internal efficiency (59%), whereas, nitrogen recovery efficiency decreased by about 52%. Moreover, the maximum increase in N content of grain and harvest index was recorded at combination application of 20 tha−1 biochar plus urea by about 48 and 15%, respectively. It can be concluded that the co-application of rice husk biochar and N fertilizers has substantial advantages over their single applications in terms of N fertilizer recovery, N uptake. Thus, it might be an effective way to enhance rice yield.

Diversity and Functional Characteristics of Fungal Communities and Influencing Factors in Typical Paddy Fields of China

To investigate the structure, diversity, and function of different paddy soil fungal communities and the factors affecting them in typical paddy cropping areas in China, five typical Chinese paddy soils were selected in this study, and the composition and diversity of soil fungal communities were comparatively analyzed using high-throughput sequencing technology and functionally predicted using the FUNGuild microecological tool. The results showed that： ① The fungal community diversity of soil samples from Heilongjiang （HLJ） was significantly lower than that of the other four regions （P<0.05）； the highest fungal community richness was found in paddy soils from Yunnan （YN）, which was significantly higher than that of the other regions （P<0.05）； and the soil samples from Hainan （HN）, Jiangxi （JX）, and Shandong （SD） were relatively close to each other. The highest average relative abundance at the level of the five typical paddy phyla was Ascomycota, and the genus with the highest average relative abundance was Tausonia. ② Fungi had the largest proportion of saprophytic trophic types, and their corresponding environmental functions were stronger. ③ The species abundance of soil fungi was highly significantly correlated with soil TP, EC, and BD （P<0.01）, and redundancy analyses also showed that soil TP was the main driver of the fungal community as well as the saprophytic functional taxa. The above results showed that the soil fungal community diversity and structure varied greatly among samples, and the relative abundance of fungal genera was affected by soil physical and chemical properties and altered the fungal community structure in paddy fields. The development of this study will provide theoretical references for the sustainable management based on fungal diversity and function of paddy fields.

Isolation, Selection, and Biological Evaluation for Bacteria that Fix Nitrogen and Produce Indole-3-Acetic Acid from Paddy Soils in Vietnam

Introduction/Background Acidic soils are limiting the production of crops and indirectly harming the environment due to the use of nitrogen (N) chemical fertilizer. Therefore, the current study aims to isolate a promising N2-fixing candidate to solve this issue in some communes of Phu Luong District, Thai Nguyen Province, Vietnam. Materials and Methods The bacteria were isolated using the Ashby medium. The IAA generation and nitrogen fixation were assessed using spectroscopy. The selected bacteria were tested at temperatures ranging from 28°C to 42°C and pH levels of 4.0 to 9.0. As a result, the Box-Behnken model yielded a growth equation. The API kit test was used to measure the biochemical properties of the selected bacteria. Results The result demonstrated that there were ten nitrogen-fixing bacteria (NFB) isolates with capacities of N2 fixation and IAA production at 2.35–24.61 and 0.00–119.13 µg/ml. Among them, the NL3 strain was the best strain. The optimum condition to grow the NL3 strain was calculated as 5.44 days at pH 7.10 and temperature of 32.18°C to reach a bacterial density of 9.77929×108 CFU/ml. In addition, the NL3 strain was identified as an Azotobacter species. The genomic analysis revealed that the genome of Azotobacter sp. NL3 was nearly 5.4 Mb long and contained 45 N2 fixation-relating genes and 7 IAA production-relating genes. Conclusion Not only can this study provide insight into the features of Azotobacter spp., but it also introduces a potent candidate that can improve soil health and crop yield by fixing N2 and producing IAA in order to limit the use of N chemical fertilizer for a sustainable agriculture. Therefore, the selected bacterial strain should be further tested under the local field conditions and subsequently commercialized as a biofertilizer.

Phosphogypsum with Rice Cultivation Driven Saline-Alkali Soil Remediation Alters the Microbial Community Structure.

The improvement of saline-alkali land plays a key role in ensuring food security and promoting agricultural development. Saline soils modifies the response of the soil microbial community, but research is still limited. The effects of applying phosphogypsum with rice cultivation (PRC) on soil physicochemical properties and bacterial community in soda saline-alkali paddy fields in Songnen Plain, China were studied. The results showed that the PRC significantly improved the physicochemical properties of soil, significantly reduced the salinity, increased the utilization efficiency of carbon, nitrogen, and phosphorus, and significantly increased the activities of urease and phosphatase. The activities of urease and phosphatase were significantly correlated with the contents of total organic carbon and total carbon. A redundancy analysis showed that pH, AP, ESP, HCO3-, and Na+ were dominant factors in determining the bacterial community structure. The results showed that PRC could improve soil quality and enhance the ecosystem functionality of soda saline-alkali paddy fields by increasing nutrient content, stimulating soil enzyme activity, and regulating bacterial community improvement. After many years of PRC, the soda-alkali soil paddy field still develops continuously and healthily, which will provide a new idea for sustainable land use management and agricultural development.

Simultaneous Reduction and Methylation of Nanoparticulate Mercury: The Critical Role of Extracellular Electron Transfer.

Mercury nanoparticles are abundant in natural environments. Yet, understanding their contribution to global biogeochemical cycling of mercury remains elusive. Here, we show that microbial transformation of nanoparticulate divalent mercury can be an important source of elemental and methylmercury.Geobacter sulfurreducensPCA, a model bacterium predominant in anoxic environments (e.g., paddy soils), simultaneously reduces and methylates nanoparticulate Hg(II). Moreover, the relative prevalence of these two competing processes and the dominant transformation pathways differ markedly between nanoparticulate Hg(II) and its dissolved and bulk-sized counterparts. Notably, even when intracellular reduction of Hg(II) nanoparticles is constrained by cross-membrane transport (a rate-limiting step that also regulates methylation), the overall Hg(0) formation remains substantial due to extracellular electron transfer. With multiple lines of evidence based on microscopic and electrochemical analyses, gene knockout experiments, and theoretical calculations, we show that nanoparticulate Hg(II) is preferentially associated with c-type cytochromes on cell membranes and has a higher propensity for accepting electrons from the heme groups than adsorbed ionic Hg(II), which explains the surprisingly larger extent of reduction of nanoparticles than dissolved Hg(II) at relatively high mercury loadings. These findings have important implications for the assessment of global mercury budgets as well as the bioavailability of nanominerals and mineral nanoparticles.

Impact of Viruses on Prokaryotic Communities and Greenhouse Gas Emissions in Agricultural Soils.

Viruses are abundant and ubiquitous in soil, but their importance in modulating greenhouse gas (GHG) emissions in terrestrial ecosystems remains largely unknown. Here, various loads of viral communities are introduced into paddy soils with different fertilization histories via a reciprocal transplant approach to study the role of viruses in regulating greenhouse gas emissions and prokaryotic communities. The results showed that the addition of viruses has a strong impact on methane (CH4) and nitrous oxide (N2O) emissions and, to a minor extent, carbon dioxide (CO2) emissions, along with dissolved carbon and nitrogen pools, depending on soil fertilization history. The addition of a high viral load resulted in a decrease in microbial biomass carbon (MBC) by 31.4%, with changes in the relative abundance of 16.6% of dominant amplicon sequence variants (ASVs) in comparison to control treatments. More specifically, large effects of viral pressure are observed on some specific microbial communities with decreased relative abundance of prokaryotes that dissimilate sulfur compounds and increased relative abundance of Nanoarchaea. Structural equation modeling further highlighted the differential direct and indirect effects of viruses on CO2, N2O, and CH4 emissions. These findings underpin the understanding of the complex microbe-virus interactions and advance current knowledge on soil virusecology.

Residual Effect of Microbial-Inoculated Biochar with Nitrogen on Rice Growth and Salinity Reduction in Paddy Soil.

Increasing soil and water salinity threatens global agriculture, particularly affecting rice. This study investigated the residual effects of microbial biochar and nitrogen fertilizer in mitigating salt stress in paddy soil and regulating the biochemical characteristics of rice plants. Two rice varieties, Shuang Liang You 138 (SLY138), a salt-tolerant, and Jing Liang You 534 (JLY534), a salt-sensitive, were grown under 0.4 ds/m EC (S0) and 6.84 ds/m EC (S1) in a glass house under controlled conditions. Three types of biochar-rice straw biochar (BC), fungal biochar (BF), and bacterial biochar (BB)-were applied alongside two nitrogen (N) fertilizer rates (60 kg ha-1 and 120 kg ha-1) in a previous study. The required salinity levels were maintained in respective pots through the application of saline irrigation water. Results showed that residual effects of microbial biochars (BF and BB) had higher salt mitigation efficiency than sole BC. The combination of BB and N fertilizer (BB + N120) significantly decreased soil pH by 23.45% and Na+ levels by 46.85%, creating a more conducive environment for rice growth by enhancing beneficial microbial abundance and decreasing pathogenic fungi in saline soil. Microbial biochars (BF and BB) positively improved soil properties (physicochemical) and biochemical and physiological properties of plants, ultimately rice growth. SLY138 significantly had a less severe response to salt stress compared to JLY534. The mitigation effects of BB + N120 kg ha-1 were particularly favorable for SLY138. In summary, the combined residual effect of BF and BB with N120 kg ha-1, especially bacterial biochar (BB), played a positive role in alleviating salt stress on rice growth, suggesting its potential utility for enhancing rice yield in paddy fields.

Long-term rice-crayfish farming alters soil dissolved organic carbon quality and biodegradability by regulating microbial metabolism and iron oxidation

The biodegradability of dissolved organic carbon (DOC) is a crucial process in the migration and transformation of soil organic carbon (SOC), and play a vital role in the global soil carbon (C) cycle. Although the significance of DOC in SOC transportation and microbial utilization is widely acknowledged, the impact of long-term rice-crayfish (RC) farming on the content, quality, and biodegradability of DOC in paddy soils, as well as regulatory mechanisms involved, remains unclear. To address this gap, a space-for-time method was employed to investigate the effects of different RC farming durations (1-, 5-, 10-, 15-, and 20- years) on the quality and biodegradability of DOC, as well as their relationship with soil microbial metabolism and minerals in this study. The results revealed that continuous RC farming increased the soil DOC content, but reduced DOC biodegradability. Specifically, after 20 years of continuous RC farming, the DOC content increased by 52.7% compared to the initial year, whereas the DOC biodegradability decreased by 63.4%. Analysis using three-dimensional fluorescence and ultraviolet spectroscopy demonstrated that continuous RC farming resulted in a decrease in the relative abundance of humus-like fractions, humification, and aromaticity indexes in DOC, but increased the relative abundance of protein-like fractions, biological, and fluorescence index, indicating that long-term RC farming promoted the simple depolymerization of the molecular structure of DOC. Continuous RC farming increased the activity of hydrolase involved in soil nitrogen (N) and phosphorus (P) cycles and oxidase, but decreasing the hydrolase C/N and C/P acquisition ratios; moreover, it also stimulated an increase in soil iron oxides and exchangeable calcium content. Structural equation modeling suggests that soil hydrolases and iron oxides are the primary drivers of DOC quality change, with DOC biodegradability being driven solely by soil iron oxides and not regulated by DOC quality. In conclusion, long-term RC farming promotes the catalytic decomposition of DOC aromatic substances and the production of DOC protein-like components by increasing soil oxidase activity and decreasing the hydrolase C/N acquisition ratio; these processes collectively contribute to the simple depolymerization of DOC molecular structure. Additionally, long-term RC farming induced legacy effects of soil iron oxides and enhanced chemical protection role leading to reduced DOC biodegradability. These findings suggested that long-term RC farming may reduce the rapid turnover and loss of DOC, providing a negative feedback on climate warming.

Elevated O3 has stronger effects than CO2 on soil nematode abundances but jointly inhibits their diversity in paddy soils

Anthropogenic activities have resulted in rising atmospheric concentrations of carbon dioxide (CO2) and ozone (O3), exerting substantial direct and indirect impacts on soil biodiversity within agroecosystems. Despite the considerable attention given to the individual impacts of elevated CO2 and O3 levels, the combined effects on soil nematode communities have not been extensively explored. In this study, we investigated the interactive effects of elevated CO2 (+200 ppm, eCO2) and O3 (+40 ppb, eO3) levels on the abundance, diversity, and trophic composition of soil nematode communities associated with two rice cultivars (Nanjing 5055, NJ5055 and Wuyujing 3, WYJ3). Our findings revealed that soil nematodes had greater abundances under eO3, whereas eCO2 had no significant impacts. Conversely, both eCO2 and eO3, and their combination led to significant reductions in nematode generic richness, accompanied by a decline in the diversity particularly associated with the WYJ3 cultivar. Moreover, eCO2 and eO3 influenced nematode community composition and environmental factors, particularly for the WYJ3 cultivar. Both eCO2 and eO3 significantly increased soil nitrate levels. The changes in nematode community composition were related to soil nitrate levels, as well as nitrogen and carbon concentrations in rice plant roots. Furthermore, interactions between eCO2 and eO3 significantly impacted soil nematode abundance and trophic composition, revealing intricate consequences for soil nematode communities that transcend predictions based on single-factor experiments. This study unveils the potential impacts posed by eCO2 and eO3 on soil biodiversity mediated by rice cultivars, plant functional characteristics and soil feedback mechanisms, thereby underscoring the complex and interactive outcomes arising from concurrent drivers of climate change within the soil food web.

Does higher ratio of wheat straw addition decrease PAHs degradation in PAHs-contaminated paddy soils and PAHs concentrations in rice?

There are considerable studies focusing on impacts of straw returning on PAHs degradation and bioavailability in PAHs-contaminated upland soils, while similar research in paddy soils is limited. Incubation experiments and pot trials were conducted to study effects of straw returning on PAHs degradation in paddy soils and PAHs accumulation in rice, respectively. There are threshold effects of straw returning on PAHs degradation in PAHs-contaminated paddy soils. The inflection point of PAHs degrading was recorded under 0.8 % wheat straw treatment (conventional (CS) and pretreated wheat straw (PS)), which increased PAHs degradation by 18.13–32.36 %. The lowest PAHs concentrations in rice were recorded under 1 % straw (CS and PS) treatment, which was attributed to the highest PAHs degradation in rhizosphere soils. Compared to CS treatment, PS treatment significantly (p < 0.05) increased PAHs degradation by 7.93–10.28 % and PAHs concentrations in rice by 12.38–45.87 % due to that increasing dissolved organic carbon (DOC) enhanced PAHs concentrations in porewater of rhizosphere soils. Higher diversity enhanced the metabolic pathways and function genes to degrade PAHs by improving bacterial phenotypes and biochemical processes under 1 % wheat straw and PS treatment. The present study firstly demonstrated that the effects of straw returning on PAHs degradation in PAHs-contaminated paddy soils and PAHs concentrations in rice depended on amount and methods of straw returning.

Chromium(VI) Adsorption and Reduction in Soils under Anoxic Conditions: The Relative Roles of Iron (oxyhr)oxides, Iron(II), Organic Matters, and Microbes.

Chromium (Cr) transformation in soils mediated by iron (Fe) (oxyhr)oxides, Fe(II), organic matter (OM), and microbes is largely unexplored. Here, their coupling processes and mechanisms were investigated during anoxic incubation experiments of four Cr(VI) spiked soil samples with distinct physicochemical properties from the tropical and subtropical regions of China. It demonstrates that easily oxidizable organic carbon (EOC, 55-84%) and microbes (16-48%) drive Cr(VI) reduction in soils enriched with goethite and/or hematite, among which in dryland soils microbial sulfate reduction may also be involved. In contrast, EOC (38 ± 1%), microbes (33 ± 1%), and exchangeable and poorly crystalline Fe (oxyhr)oxide-associated Fe(II) (29 ± 3%) contribute to Cr(VI) reduction in paddy soils enriched with ferrihydrite. Additionally, exogenous Fe(II) and microbes significantly enhance Cr(VI) reduction in ferrihydrite- and goethite-rich soils, and Fe(II) greatly promotes but microbes slightly inhibit Cr passivation. Both Fe(II) and microbes, especially the latter, promote OM mineralization and result in the most substantial OM loss in ferrihydrite-rich paddy soils. During the incubation, part of the ferrihydrite converts to goethite but microbes may hinder the transformation. These results provide deep insights into the geochemical fates of redox-sensitive heavy metals mediated by the complicated effects of Fe, OM, and microbes in natural and engineered environments.

Source apportionment of Cd in karst soil based on the delayed geochemical hazard model.

Soil Cd contamination has become increasingly prominent in karst regions. Studies have generally elucidated the natural sources of Cd in high-background areas and analyzed their migration and enrichment mechanisms. This study comprehensively analyzed the total content and speciation of Cd in high-background areas using the delayed geochemical hazard (DGH) model to identify the sources of Cd in the region. The results indicated that Cd in the research area followed a pattern of gradual geochemical disasters. In Quaternary soil, brick-red soil, and submergenic paddy soil with hydromorphic characteristics, 32%, 7.69%, and 30% of soil Cd samples exceeded the critical threshold of the releasable total amount, respectively. Based on the DGH model, it was concluded that Cd in this region was mainly influenced by human activities. Field investigations corroborated this conclusion and aligned with the findings. Compared with the traditional source apportionment receptor models (mainly PCA and PMF), the DGH model not only saved considerable time and cost, but also avoided uncertainty associated with the results and complex and varied data processing and computational analysis processes. Moreover, the DGH model was able to identify the factors having the greatest impact on the ecological risk of Cd in the research area, thus facilitating targeted prevention and management planning based on the characteristics or chemical properties of their elements.

Lack of inhibitory effects of 1-Octyne and PTIO on ammonia oxidizers, nitrite oxidizers, and nitrate formation in acidic paddy soils

The feasibility of employing double inhibitors consisting of PTIO and 1-octyne with the 15N pool dilution technique to distinguish comammox process from archaeal and bacterial nitrification remains to be confirmed. As such, we carried out a nitrification inhibition experiment on acidic paddy soils at a pH of 5.0, 5.3, and 5.4 to investigate the effects of 1-octyne and PTIO on nitrate (NO3−) formation, ammonia (NH3)-oxidizing archaea (AOA) and bacteria (AOB), comammox Nitrospira, and nitrite (NO2−)-oxidizing bacteria (NOB) under urea fertilized- and unfertilized- conditions. We observed that 1-Octyne stimulated the growth of comammox Nitrospira clades A and B, and Nitrobacter-like NOB and Nitrospira-like NOB without inhibiting AOB growth. This observation indicates that 1-Octyne has the potential to promote comammox process and NO2− oxidization, and its usage may lead to an underestimation of AOB activity. Unexpectedly, we found that PTIO was incapable of causing a rapid inhibition of AOA growth and NO3− formation. In the short term, PTIO was usually not detrimental to the growth of comammox Nitrospira clades A and B, AOB, and Nitrobacter-like NOB. Instead, increased growth of AOB and Nitrospira-like NOB were observed following its application. Likely due to increased bacterial nitrification and NO2− oxidation and the reaction of PTIO with nitric oxide (NO) produced by non-nitrification processes, increased NO3− formation with PTIO was observable in acidic soils with varying N statuses. These findings highlight the need to develop novel and more efficient inhibition strategies to actualize rate distinction among AOA, AOB, comammox Nitrospira in acidic soils.

Modelling pesticide concentrations in Japanese paddy fields using the RICEWQ model

Increasing concerns about plant protection products in surface waters have highlighted the importance of pesticide monitoring and modelling, both nowadays integral components of the pesticide registration process. The Rice Water Quality (RICEWQ) model predicts the fate and transport of pesticides under various paddy environmental conditions. The model has been used widely for regulatory purpose in the U.S., while its use in Europe has been limited to regulatory submissions, despite multiple recommendations to adopt RICEWQ as a high tier assessment model for regulatory purposes. The applicability of the RICEWQ model under Japanese agricultural conditions remains unexplored. This study leverages field experimental data to evaluate the RICEWQ model's capability to simulate daily concentrations of two herbicides, mefenacet and pretilachlor, in paddy water and paddy soil in Japan.The RICEWQ model provided a reasonable level of performance for predicting the fate and transport of the two herbicides by ensuring that the simulated daily water balance corresponded with field observations and with minor pesticide-specific calibration. To further improve the performance of the simulations, we implemented a straightforward calibration framework. Calibrating the RICEWQ model improved the accuracy of all simulations; for both herbicides and in both paddy water and paddy soil compartments, the lowest Nash-Sutcliffe efficiency achieved was 0.725, demonstrating excellent performance. However, the RICEWQ model consistently underestimated the peak concentrations of herbicides in paddy water within the first three days of simulation. Simulation results suggested that approximately 50 % of the total applied herbicide was lost from the paddy system. Increasing the duration of the water holding period after pesticide application or increasing the storage capacity of the rice field via excess water storage depth management has the potential to reduce greatly herbicide loss to below 10 % of the total applied herbicide.

Aggravation of Cd availability in the plastisphere of paddy soil

Soil plastisphere has attracted many concerns, however, its influence on cadmium (Cd) availability in paddy soil was still unclear. This study carried out batch microcosmic and bagging experiments to explore the influence of microplastic (MPs) on Cd availability in paddy soil under flooding conditions in the view of plastisphere. Results showed that the presence of MPs could act as plastisphere micro-environment. The bacterial community composition changed dramatically around the plastisphere compared with MPs-contaminated bulk soil and control soil. The relative abundance of Symbiobacteraceae, Rhodocyclaceae and Bryobacteraceae was improved in the plastisphere which contributed to the enhanced the reduction of Fe(III) and sulfate in flooding paddy soil. The higher content of Fe(II) and S content contributed to the enrichment of Cd in the plastisphere which aggravated Cd availability in paddy soil under flooding conditions. The partial least squares structure equation modeling results confirmed the presence of MPs in paddy soil could act as plastisphere which could change the bacterial community composition and improve the content Fe and S that was conductive to gather Cd in plastisphere. This study shed lights on the understanding of the role of plastisphere on Cd availability in paddy field ecosystem under flooding conditions.

Efficiency-enhancing methods for predicting nitrogen mineralization characteristics in paddy soils using soil properties and rapid soil extraction

Soil mineralized nitrogen (N) is a vital component of soil N supply capacity and an important N source for rice growth. Unveiling N mineralization (Nm) process characteristics and developing a simple and effective approach to evaluate soil Nm are imperative to guide N fertilizer application and enhance its efficiency in various paddy soils with different physicochemical properties. Soil properties are important driving factors contributing to soil Nm differences and must be considered to achieve effective N management. Nevertheless, discrepancies in Nm capacity and other key influencing factors remain uncertain. To address this knowledge gap, this study collected 52 paddy soil samples from Taihu Lake Basin, south China, which possesses vastly different physicochemical properties. The samples were subjected to a submerged anaerobic incubation experiment at a constant temperature to obtain the soil Nm characteristics. In addition to other analyses, reaction kinetics models were employed to compare characteristic differences between Nm potential (Nmp) and short-term accumulated mineralized N (Amn) processes in relation to soil physicochemical properties. Based on these relationships, simplified Nmp prediction methods for paddy soils were established. The results revealed that the Nmp values in pH < 6.50, pH range from 6.50 to 7.50, and pH > 7.50 were 145.18 mg kg-1, 88.64 mg kg-1 and 21.03 mg kg-1, respectively. Significantly, after comparing different incubation days, short-term Amn at 14 days showed a good correlation (P < 0.01) with Nmp (R2 = 0.94), indicating that the prevailing short-term incubation experiment is an acceptable marker for Nmp. Moreover, Nmp correlated well with the ultraviolet absorbance obtained at 260 nm using NaHCO3 extraction (Na260), streamlining the Nmp estimation method further. The incorporating easily obtainable soil properties, including pH, total nitrogen (TN), and the ratio of total organic carbon to TN (C/N) alongside Na260 for Nmp evaluation allowed the multiple regression model, Nmp = 58.62 × TN - 23.18 × pH + 13.08 × C/N +86.96 × Na260, to achieve high predictive accuracy (R2 = 0.95). The reliability of this prediction was further validated with published data in paddy soil from the same region and the other rice regions, demonstrating the regional applicability and prospects of this model. This study underscored the roles of soil properties in Nm characteristics and mechanisms and established a site-specific prediction model based on rapid extraction and edaphic properties from the paddy soil, paving the way for developing rapid, precise Nm prediction models.

Coupled reduction in arsenic methylation and methanogenesis with silicate amendment in arsenic-enriched paddy soils

Methanogens play an important role in the demethylation of arsenic. Soil amendments that inhibit methanogens can increase dimethylarsinic acid (DMA), which is responsible for straighthead disease in rice. A decrease in methanogenesis caused by silicate fertilizer may increase DMA concentration in paddy soils and rice grains; the relationship between these two factors and their impacts on DMA concentration remains unclear. We applied silicate fertilizer (2 Mg ha−1) to japonica and indica rice grown on arsenic-spiked soils and found a simultaneous reduction in methane emissions and pore-water DMA concentration, compared to no-silicate fertilization. Gene and transcript copies of mcrA and arsM, as well as dominant methanogens and arsenic-methylating microbes decreased significantly with silicate fertilization. However, the sulfate-reducing bacteria and the gene and transcript copies of dsrB did not change significantly in response to the application of silicate fertilizer to paddy soils. The abundance of arsenic methylating microbes was significantly and positively correlated with the abundance of methanogens, but not with the abundance of sulfate-reducing bacteria. Methylomonas and Methylobacter, which harbor the arsM gene, were suppressed under silicate fertilization, suggesting that they have the potential to methylate As and play a crucial role in reducing pore-water DMA in As-enriched flooded paddy soils. Increasing Fe concentration, soil pH, and Eh value decreased pore-water DMA concentration, while decreasing arsenite concentration, arsM and mcrA gene abundance decreased it. While silicate fertilization decreased arsenite and DMA concentrations in pore-water, it had no significant effect on rice DMA content, but significantly decreased arsenite content. Results reveal that methanogens and arsenic-methylating microbes have a synergistic relationship under silicate fertilization that facilitates a significant reduction in methane emissions and DMA concentration in As-enriched paddy soils.

Quantify the effects of water-soluble salts and available fractions of nutrient elements on mobility of cadmium in paddy soil: A case study of the Pearl River Delta

As one of the most toxic heavy metal, soil Cd mobility is crucial to understand Cd transfer in soil-crop system and ensure crop safety production. Water-soluble salts and available fractions of nutrient elements have important influence on soil Cd mobility. However, effects of multi-factor interactions in water-soluble salts and available fractions of nutrient elements on Cd mobility and their quantified contributions haven’t been enough emphasized. In this study, conditional inference tree and random forest were adopted to quantify effects of water-soluble salts, available fractions of nutrient elements and their interactions on Cd mobility in paddy soil from Pearl River Delta. The results suggested that cations (K+, Mg2+ and Na+) in water-soluble salts exerted important influence on Cd mobility, especially K+. Cd mobility is enhanced with the increase of K+ concentration and cation exchange capacity and their interaction. Available Si is the primary factor affecting Cd mobility, followed by available K and easily reducible Mn. With increases of their concentrations, interaction of available K and available Si boosts Cd mobility. When available K concentration is higher than 122 mg·kg-1, Cd mobility is primarily restricted by available Si. In general, cations improve Cd mobility through competitive adsorption with Cd in soil, and available Si affect Cd mobility by changing Cd fractionation. Therefore, measures can be taken to reduce Cd mobility in paddy soil and promote sustainable development of agriculture. Such as increasing application of organic fertilizer, reducing application of chemical fertilizer, and promoting straw carbonization to return to field and so on.