Rice Paddy Soil Research Articles

Increased Soil Soluble Nitrogen Stocks and Decreased Nitrogen Leaching Loss in Rice Paddy Soil by Replacing Nitrogen Fertilizer with Chinese Milk Vetch

Reducing soil nitrogen leaching losses and improving nitrogen-use efficiency with effective fertilization management strategies are extremely important for sustainable agricultural development. A 2-year field study was executed with the same nitrogen input in a subtropical rice production system in Southeast China, using chemical fertilizers as a control (CK), to study the influences of different application amounts of Chinese milk vetch (CMV), i.e., 15,000 kg hm−2 (CL), 30,000 kg hm−2 (CM), and 45,000 kg hm−2 (CH), on soil soluble nitrogen stock and leaching risks in a clay paddy field. The results showed that the soil stocks of soluble inorganic nitrogen (SIN) and soluble organic nitrogen (SON) in a 0–60 cm soil profile under different application amounts of CMV significantly increased by 12.43–36.03% and 19.43–71.75% compared with CK, respectively, which was more favorable to soil SON accumulation. In the 2-year experiment, the total dissolved nitrogen leaching loss was 23.51–61.88 kg hm−2 under different application rates of CMV, of which 50.08–62.69% was leached by dissolved inorganic nitrogen (DIN), and 37.31–49.92% was leached by dissolved organic nitrogen (DON). CMV application improved soil properties (pH, SOM, and urease/protease), increased SIN and SON stocks, and decreased surface water DIN and DON concentrations, thereby reducing DIN and DON leaching. The leachings of DIN and DON in different application rates of CMV were reduced by 11.37–66.23% and 13.39–52.07% compared with the CK treatment, respectively. Conclusively, nitrogen leaching loss in paddy fields was severe, and the DIN and DON leaching loss in CMV treatments were lower than those in the control under the same nitrogen input. Thus, replacing nitrogen fertilizer with CMV under the same nitrogen input could reduce the risk of nitrogen nonpoint pollution in clay paddy fields.

Tartrate fermentation with H2 production by a new member of Sporomusaceae enriched from rice paddy soil.

In rice paddies, soil and plant-derived organic matter are degraded anaerobically to methane (CH4), a powerful greenhouse gas. The highest rate of methane emission occurs during the reproductive stage of the plant when mostly dicarboxylic acids are exudated by the roots. The emission of methane at this stage depends largely on the cooperative interaction between dicarboxylic acid-fermenting bacteria and methanogenic archaea in the rhizosphere. The fermentation of tartrate, one of the major acids exudated, has been scarcely explored in rice paddy soils. In this work, we characterized an anaerobic consortium from rice paddy soil composed of four bacterial strains, whose principal member (LT8) can ferment tartrate, producing H2 and acetate. Tartrate fermentation was accelerated by co-inoculation with a hydrogenotrophic methanogen. The assembled genome of LT8 possesses a Na+-dependent oxaloacetate decarboxylase and shows that this bacterium likely invests part of the H2 produced to reduce NAD(P)+ to assimilate C from tartrate. The phylogenetic analysis of the 16S rRNA gene, the genome-based classification as well as the average amino acid identity (AAI) indicated that LT8 belongs to a new genus within the Sporomusaceae family. LT8 shares a few common features with its closest relatives, for which tartrate degradation has not been described. LT8 is limited to a few environments but is more common in rice paddy soils, where it might contribute to methane emissions from root exudates.IMPORTANCEThis is the first report of the metabolic characterization of a new anaerobic bacterium able to degrade tartrate, a compound frequently associated with plants, but rare as a microbial metabolite. Tartrate fermentation by this bacterium can be coupled to methanogenesis in the rice rhizosphere where tartrate is mainly produced at the reproductive stage of the plant, when the maximum methane rate emission occurs. The interaction between secondary fermentative bacteria, such as LT8, and methanogens could represent a fundamental step in exploring mitigation strategies for methane emissions from rice fields. Possible strategies could include controlling the activity of these secondary fermentative bacteria or selecting plants whose exudates are more difficult to ferment.

Fractionation of stable carbon isotopes during formate consumption in anoxic rice paddy soils and lake sediments

Abstract. Formate is energetically equivalent to hydrogen and thus is an important intermediate during the breakdown of organic matter in anoxic rice paddy soils and lake sediments. Formate is a common substrate for methanogenesis, homoacetogenesis and sulfate reduction. However, how much these processes contribute to formate degradation and fractionate carbon stable isotopes is largely unknown. Therefore, we measured the conversion of formate to acetate, CH4 and CO2 and the δ13C of these compounds in samples of paddy soils from Vercelli, Italy, and the International Rice Research Institute (IRRI) in the Philippines and of sediments from the NE and SW basins of Lake Fuchskuhle, Germany. The samples were suspended in a phosphate buffer (pH 7.0) in both the absence and presence of sulfate (gypsum) and of methyl fluoride (CH3F), an inhibitor of aceticlastic methanogenesis. In the paddy soils, formate was mainly converted to acetate under both methanogenic and sulfidogenic conditions. Methane was only a minor product and was mainly formed from the acetate. In the lake sediments, the product spectrum was similar but only under methanogenic conditions. In the presence of sulfate, however, acetate and CH4 were only minor products. The isotopic enrichment factors (εform) of formate consumption, determined by Mariotti plots, were in the low range of −8 ‰ to −2.5 ‰ when sulfate was absent, and formate was mainly converted to acetate and CH4. However, no enrichment factor was detectable when formate was degraded with sulfate to mainly CO2. The δ13C of acetate was by about 25 ‰–50 ‰ more negative than that of formate, indicating acetate production by chemolithotrophic homoacetogenesis. Hence, formate seems to be an excellent substrate for homoacetogenesis in anoxic soils and sediments, so that this process is competing well with methanogenesis and sulfate reduction.

Combined Effects of Rice Husk Biochar and Organic Manures on Soil Chemical Properties and Greenhouse Gas Emissions from Two Different Paddy Soils

The application of biochar is considered an alternative amendment strategy for improving soil fertility. In this study, we performed pot experiments using soils of low and medium fertility to assess the effects of different combinations of biochar and organic manure on the chemical properties of paddy rice soils and determined the best combination to improve the grain yield without increasing N2O and CH4 emissions. The applied treatments were without biochar (control), the application of rice husk biochar alone (5 and 10 t ha−1), and biochar combined with chicken or cow manure. The results indicated that for both soils, the application of 5 t ha−1 biochar combined with 5 t ha−1 chicken manure increased grain yield by improving soil total nitrogen and soil NH4+-N without increasing cumulative N2O and CH4 emissions. Multiple regression analysis showed that when combined with biochar, chicken manure significantly contributed to a higher grain yield and was negatively associated with cumulative CH4, N2O emissions, and total GWP. Furthermore, regardless of soil type, combined applications of biochar and cow manure promoted significant increases in soil available P. Our findings indicate that the C/N ratio of organic manure influences CH4 fluxes, and soil type was identified as a factor driving greenhouse gas emissions.

Erythrobacter oryzae sp. nov., isolated from rice paddy soil.

Two novel bacterial strains, designated as COR-2T and CR-8, were isolated from paddy soil. These isolates were aerobic, Gram-stain-negative, non-spore-forming, non-motile, rod-shaped, and formed orange-coloured colonies. Phylogenetic analysis based on 16S rRNA gene sequences showed that two strains formed a clear phylogenetic lineage with the genus Erythrobacter. Strains COR-2T and CR-8 showed 99.9 % 16S rRNA gene sequence similarity. Both strains had the highest 16S rRNA gene similarity of 99.1-99.7 % to Erythrobacter colymbi TPW-24T, Erythrobacter donghaensis SW-132T and Erythrobacter tepidarius DSM 10594T, respectively. The genome of strain COR-2T comprised 3 559 918 bp and the genomic DNA G + C content was 67.7 mol%. The average nucleotide identity and digital DNA-DNA hybridization values between strain COR-2T and its closely related species of the genus Erythrobacter were 79.3-85.5% and 24.1-29.1 %, respectively. The major respiratory quinone was Q-10, while the major fatty acids were C18 : 1 ω7c and C17 : 1 ω6c. The major polar lipids were phosphatidylethanolamine, phosphatidylglycerol, phosphatidylcholine, two unidentified phospholipids and eight unidentified lipids. Based on phylogenetic and phenotypic considerations, the two strains [COR-2T (type strain; = KACC 22941T=JCM 35529T) and CR-8 (= KACC 22945=JCM 35530)] are considered to represent novel species of the genus Erythrobacter, for which the name Erythrobacter oryzae sp. nov. is proposed.

Manure derived hydrochar reduced phosphorus loss risk via an alteration of phosphorus fractions and diversified microbial community in rice paddy soil

Phosphorus (P) loss caused by the irrational use of manure organic fertilizer has become a worldwide environmental problem, which has caused a potential threat to water safety and intensified agricultural non-point source pollution. Hydrothermal carbonization is method with a low-energy consumption and high efficiency to deal with environmental problems. Application of pig manure-derived hydrochar (PMH) to soil exhibited potential of sustainable development compared with the pristine pig manure (PM). However, the effects of PMH on the distribution of P among the fractions/forms and the interaction between microorganisms and P forms and its relevance to the potential loss of P in paddy fields has not been clarified. Therefore, in this study, a soil column experiment was conducted using the untreated soil (control), and the PM, PMH1 (PMH derived at 180 °C), and PMH2 (PMH derived at 260 °C) treated soils (at the dose of 0.05 %) and rice was cultivated to investigate the effects of PM and PMH on the P fractions, mobilization, ad potential loss via the induced changes on soil microbial community after a complete growing season of rice. The trend of P utilization was evaluated by P speciation via continuous extraction and 31P NMR. The addition of PMH reduced the proportion of residual P in soil by 23.8–26.3 %, and increased the proportion of HCl-P and orthophosphate by 116.2–158.6 % and 6.1–6.8 % compared to PM. The abundance of gcd gene developed after the application of PMH2, which enhanced the mobile forms of soil P utilization via secreting gluconic acid. The network diagram analysis concluded that the changes in various P forms were mainly related to Proteobacteria, Bacteroides, Firmicutes and Acidobacteria. The results illustrated that PMH mitigate the potential risk of P loss more than PM by altering P fractions and affecting soil microbial community.

Nitrous Oxide Emissions during Cultivation and Fallow Periods from Rice Paddy Soil under Urea Fertilization

Rice cultivation serves as a significant anthropogenic source of methane (CH4) and nitrous oxide (N2O). Although N2O emissions remain relatively small compared to CH4 emissions, they are remarkably affected by nitrogen-fertilized soil conditions during rice cultivation. While numerous studies have investigated nitrous oxide emissions in response to nitrogen fertilization, existing research assessing nitrous oxide emissions based on nitrogen fertilizer levels has often been limited to cultivation periods. Therefore, there is a need for comprehensive analyses covering the entire year, including the dry periods, to address nitrous oxide emissions as an important source throughout the entire agricultural cycle. In this case study, we investigated the characteristics of N2O emissions in a central region of South Korea, where a single rice-cropping cycle occurs annually over a span of three whole years, from May 2020 to May 2023. We investigated the impact of variations in temperature and soil moisture on N2O emissions during rice cultivation and fallow periods. In this context, we attempted to discover the complex dynamics of N2O emissions by comparing longer fallow periods with the rice cultivation periods and extended non-dry periods with irrigated periods. We discovered that the greater contribution of cumulative N2O emissions during the fallow period made a much greater contribution (up to approximately 90%) to the whole-year N2O emissions than those during the rice cultivation period. During the fallow period from rice harvest to rice planting in the following year, variations in N2O emissions were associated with high-flux events after rainy periods on dry soils. This highlights the considerable influence of soil moisture content and weather conditions on N2O emissions during the fallow period. This affects high emission events, which in turn significantly impact the cumulative emissions over the entire period. We underscore that assessing N2O emissions solely based on the rice cultivation period would underestimate annual emissions. To prevent underestimation of N2O emissions, periodic gas collection throughout a year covering both rice cultivation and fallow phases is required in alignment with the monitoring of different temperature and soil moisture conditions. We captured statistical differences in cumulative N2O emissions due to nitrogen fertilization treatments across the three years. However, no significant difference was observed in the three-year average emissions among the different (one, one-and-a-half, and double) nitrogen fertilization treatments, with the exception of the control treatment (no fertilization). Based on the findings, we recommend at least three whole-year evaluations to ensure the estimation accuracy of N2O emissions under different nitrogen fertilization conditions. The findings from this study could help prepare the further revision or refinement of N2O emission factors from rice cultivation in the national greenhouse gas inventories defined by the inter-governmental panel on climate change (IPCC).

Enhancement of hydrogenotrophic methanogenesis for methane production by nano zero-valent iron in soils

Nanoscale zero-valent iron (nZVI) is attracting increasing attention as the most commonly used environmental remediation material. However, given the high surface area and strong reducing capabilities of nZVI, there is a lack of understanding regarding its effects on the complex anaerobic methane production process in flooded soils. To elucidate the mechanism of CH4 production in soil exposed to nZVI, paddy soil was collected and subjected to anaerobic culture under continuous flooding conditions, with various dosages of nZVI applied. The results showed that the introduction of nZVI into anaerobic flooded rice paddy systems promoted microbial utilization of acetate and carbon dioxide as carbon sources for methane production, ultimately leading to increased methane production. Following the introduction of nZVI into the soil, there was a rapid increase in hydrogen levels in the headspace, surpassing that of the control group. The hydrogen levels in both the experimental and control groups were depleted by the 29th day of culture. These findings suggest that nZVI exposure facilitates the enrichment of hydrogenotrophic methanogens, providing them with a favorable environment for growth. Additionally, it affected soil physicochemical properties by increasing pH and electrical conductivity. The metagenomic analysis further indicates that under exposure to nZVI, hydrogenotrophic methanogens, particularly Methanobacteriaceae and Methanocellaceae, were enriched. The relative abundance of genes such as mcrA and mcrB associated with methane production was increased. This study provides important theoretical insights into the response of key microbes, functional genes, and methane production pathways to nZVI during anaerobic methane production in rice paddy soils, offering fundamental insights into the long-term fate and risks associated with the introduction of nZVI into soils.

Soil Fertility Evaluation in Long-term Paddy Field on Small Island: A Preliminary Research in Savana Jaya Village, Buru Island, Maluku Province, Indonesia

Efforts to increase rice production continue to face severe challenges, primarily attributed to the declining quality of paddy field soil resources. This study aims to investigate the management of fertilizer utilization and chemical properties of six rice paddy soils in Savana Jaya Village, Waeapo District, Buru Regency. Additionally, it seeks to assess the soil fertility status after 10 to 40 years of continuous rice planting and recommend an approach for fertilizer application. These fields were selected using a purposive sampling method. The soil samples were analyzed total nitrogen (N), potential phosphorus (P), and potential potassium (K) content, as well as cation exchange capacity (CEC). Rice production data was collected by interviewing farmers. The results showed that the soil organic C content and the total N were very low to low, the potential P status was moderate to very high, the potential K status was low to moderate, and the CEC-s of all fields were very low. The soil fertility status of all fields was very low. Organic matter is essential for maintaining the health and fertility of paddy soil, particularly in small islands where the soil is often subjected to intensive use of inorganic fertilizers, posing potential environmental contamination concerns.

Isolation and Characterization of Novel Methanogens and sMMO Producing Methanotrophs from Rice Paddy Soil

Isolation and Characterization of Novel Methanogens and sMMO Producing Methanotrophs from Rice Paddy Soil

Integrated Effects of Phosphate Rock and Chemical Fertilizers on the Dynamics of Soil Bacterial in Acidic Rice Paddy Soils of Man (Ivory Coast)

Integrated Effects of Phosphate Rock and Chemical Fertilizers on the Dynamics of Soil Bacterial in Acidic Rice Paddy Soils of Man (Ivory Coast)

A mechanistic approach to arsenic adsorption and immobilization in aqueous solution, groundwater, and contaminated paddy soil using pine-cone magnetic biochar

Arsenic (As) poisoning in groundwater and rice paddy soil has increased globally, impacting human health and food security. There is an urgent need to deal with As-contaminated groundwater and soil. Biochar can be a useful remedy for toxic contaminants. This study explains the synthesis of pinecone-magnetic biochar (PC-MBC) by engineering the pinecone-pristine biochar with iron salts (FeCl3.6H2O and FeSO4.7H2O) to investigate its effects on As(V) adsorption and immobilization in water and soil, respectively. The results indicated that PC-MBC can remediate As(V)-contaminated water, with an adsorption capacity of 12.14 mg g−1 in water. Isotherm and kinetic modeling showed that the adsorption mechanism involved multilayer, monolayer, and diffusional processes, with chemisorption operating as the primary interface between As(V) and biochar. Post-adsorption analysis of PC-MBC, using FTIR and XRD, further revealed chemical fixing and outer-sphere complexation between As(V) and Fe, O, NH, and OH as the main reasons for As(V) adsorption onto PC-MBC. Recycling of PC-MBC also had excellent adsorption even after several regeneration cycles. Similarly, PC-MBC successfully immobilized As in paddy soil. Single and sequential extraction results showed the transformation of mobile forms of As to a more stable form, confirmed by non-destructive analysis using SEM, EDX, and elemental dot mapping. Thus, Fe-modified pine-cone biochar could be a suitable and cheap adsorbent for As-contaminated water and soil.

High-resolution chemical imaging to understand Cd activation in rice rhizosphere of karstic soils

Cadmium (Cd) activation, especially at a high spatial resolution, in paddy soils with a high geogenic Cd background is yet to be understood. To investigate the temporal and spatial patterns of Cd activation in rice rhizosphere, pot and rhizotron experiments were conducted using four paddy soils with high geogenic Cd (0.11–3.70 mg kg−1) from Guangxi, southwestern China. The pot experiment results showed that porewater Cd concentrations initially decreased and then increased over the complete rice growth period, reaching its lowest value during the late-tillering and early-filling stages. Besides, correlation analysis identified organic matter and root manganese (Mn) content as the main factors affecting rice Cd uptake, with Mn having a negative effect and organic matter having a positive effect. Sub-millimeter two-dimensional chemical imaging revealed that the distribution of labile Cd in the rhizosphere (by diffusive gradients in thin-films, or DGT) was influenced by the root system and soil properties, such as pH (by planar optode) and acid phosphatase activity (by soil zymography). Soil acid phosphatase activity increased under Cd stress. The overall pH at rice rhizosphere decreased. Moreover, a close relationship was found between the spatial distributions of soil labile Mn and Cd at the rhizosphere, with higher Mn being associated with lower Cd lability. This study highlights Mn as a key element in regulating rice Cd uptake and enlightens future Mn-based strategies for addressing Cd pollution in rice paddy soils, especially in karst areas with high geochemical background.

Patterns and determinants of nitrification and denitrification potentials across 24 rice paddy soils in subtropical China

Nitrification and denitrification processes are major pathways for nitrogen (N) losses in agricultural soils, and highly depend on soil properties. Improved understanding of soil nitrification and denitrification processes is essential for better N management in agroecosystems. However, the effects of geochemical properties on nitrification and denitrification potentials remain largely uncertain, especially in paddy soils. Here, we investigated the patterns and controlling factors of nitrification and denitrification potentials across 24 paddy soil sampling sites (varied in soil texture, pH, and contents of soil organic carbon (SOC), nutrients, and exchangeable metals) in major rice-producing areas of Hubei Province, China. The results showed substantial variations in the potential rates of nitrification (0.46–17.65 mg N kg−1 soil d−1) and denitrification (6.31–18.12 mg N kg−1 soil d−1) across the paddy soil sampling sites. Soil pH and SOC were the predominant factors regulating nitrification and denitrification potentials, respectively. Nitrification potential strongly increased with soil pH at value above 6.2, while showed an insignificant correlation with pH at value below 6.2. Increased soil pH stimulated nitrification potential via enhanced NH4+ availability, likely due to increased soil active iron (Fe) and manganese (Mn) oxides facilitating chemisorption. Denitrification potential strongly positively correlated with SOC at value below 15 g kg−1, dissolved organic carbon and nitrate contents, but had little relationship with active Fe and Mn properties and clay contents. The potentials of nitrification uncoupled with denitrification, possibly resulting from their distinct controlling factors across the paddy soils. Overall, these findings could help improve the understanding of the key factors controlling potential nitrification and denitrification across paddy soils, and provide a theoretical basis for optimizing agricultural management to mitigate N losses.

The divergent accumulation mechanisms of microbial necromass C in paddy soil under different long-term fertilization regimes

To investigate the stabilization mechanism of microbial necromass carbon (C) in rice paddy soil, the accumulation of microbial necromass and its contribution to SOC in bulk soil and in various fractions were addressed under a 40-year fertilization trial. The fertilization regimes included no fertilizer (Control), nitrogen, phosphorus and potassium inorganic fertilizer (NPK), and inorganic fertilizer plus manure (NPKM). Coarse particulate organic matter (cPOM), fine inter-microaggregate POM (fPOM), intra-microaggregate POM (iPOM), non-occluded silt plus clay fraction (s + c_f), and silt plus clay occluded within microaggregates (s + c_m) fractions were isolated. The results indicated that fungal necromass were the main component of microbial necromass across the fractions; however, the bacterial role was enhanced in mineral fractions (s + c_f and s + c_m) compared to POM fractions (cPOM, fPOM, and iPOM). The iPOM and mineral fractions stored more than 90 % of microbial necromass C due to the physical protection (organo-mineral associations and occlusion within microaggregates). Compared to Control, the significantly increased content of microbial necromass C and its contribution to SOC in bulk soil were attributed to the enhanced content of fungal necromass in iPOM fraction under NPK treatment. The highest content of microbial necromass C was found under NPKM treatment; moreover, the increased content of microbial necromass C in iPOM and s + c_m fractions made up to 82.80 % of that in bulk soil compared to Control, while a saturation behavior was found in s + c_f fraction. The contribution of microbial necromass to SOC was not altered under NPKM treatment compared to Control. However, a reduced fungal/bacterial necromass C ratio was found in bulk soil and iPOM fraction under NPKM treatment compared to Control, mainly due to the supply of higher quality substrates and the increased soil pH. As a whole, this study revealed the diverse responses of microbial necromass C accumulation and its contribution to SOC under different fertilization regimes in paddy soil.

Comparative study of efficiencies of purification of cadmium contaminated irrigation water by different purification systems

Cadmium (Cd) contamination in rice threats food safety and human health. Control of Cd pollution has become an urgent need. Most existing studies on heavy metal pollution control have focused on industrial wastewater and few on irrigation water. Some researchers have found ecological ditches, plant ponds and constructed wetlands have the potential of treating heavy metal contaminated irrigation water, but they examined only one of the methods and the validity needs to be verified by field studies. Our study has filled the gap by combining the methods and using field experiments. We examined efficiencies of removal of Cadmium from irrigation water by 14 different combinations of ecological ditches, plant ponds, and constructed wetlands using field experiments. The effects of the purification on Cd concentration in paddy soil and rice grains were also examined. Results showed that there were significant differences among efficiencies of purification of Cd contaminated irrigation water using different systems and that pH, chemical form of Cd in irrigation water, vegetation coverage and biomass of aquatic plants significantly affect the efficiency. Of the 14 purification systems, seven resulted in the concentration of Cd in the effluent water meeting the National Standard for Irrigation Water Quality (GB5084–2021) for all days of the experiment period. The highest amount and rate of Cd removal were achieved by the combination of two-stage ecological ditch, two-stage plant pond, and one-stage constructed wetland, while the highest removal amount and rate per 100 m2 was achieved by the combination of one-stage plant pond and one-stage constructed wetland. Considering purification efficiency, area of coverage, and cost of construction and maintenance, we suggest that combination of plant pond and constructed wetland be a priority choice for purification of Cd pollution in irrigation water. Compared to the control data collected from rice grain and paddy soil irrigated by unpurified water, Cd concentration in rice grain and paddy soil irrigated by purified water declined by 5.08–19.42 % and 30.93–77.15 % respectively. All results showed that removal of Cd contamination from irrigation water effectively controlled cadmium pollution in rice grain and paddy soil. Our study not only contributes to pollution control practice, but also warrants further investigation of the mechanisms of how the treatment systems work. The most efficient method we identified could be applied locally, regionally and in areas of similar topography, climate, soil, vegetation, agriculture, and heavy metal pollution.

Hydrothermal carbonization of biogas slurry and cattle manure into soil conditioner mitigates ammonia volatilization from paddy soil

Hydrothermal carbonization of biogas slurry and animal manure into hydrochar could enhance waste recycling waste and minimize ammonia (NH3) volatilization from paddy fields. In this study, cattle manure-derived hydrochar prepared in the presence of Milli-Q water (CMWH) and biogas slurry (CMBSH), and biogas slurry-based hydrochar embedded with zeolite (ZHC) were applied to rice-paddy soil. The results demonstrated that CMBSH and ZHC treatments could significantly mitigate the cumulative NH3 volatilization and yield-scale NH3 volatilization by 27.9–45.2% and 28.5−45.4%, respectively, compared to the control group (without hydrochar addition), and significantly correlated with pH and ammonium–nitrogen (NH4+–N) concentration in floodwater. Nitrogen (N) loss via NH3 volatilization in the control group accounted for 24.9% of the applied N fertilizer, whereas CMBSH- and ZHC-amended treatments accounted for 13.6−17.9% of N in applied fertilizer. The reduced N loss improved soil N retention and availability for rice; consequently, grain N content significantly increased by 6.5–14.9% and N-use efficiency increased by 6.4–16.0% (P < 0.05), respectively. Based on linear fitting results, NH3 volatilization mitigation resulted from lower pH and NH4+–N concentration in floodwater that resulted from the acidic property and specific surface area of hydrochar treatments. Moreover, NH3-oxidizing archaea abundance in hydrochar-treated soil decreased by 40.9–46.9% in response to CMBSH and ZHC treatments, potentially suppressing NH4+–N transformation into nitrate and improving soil NH4+–N retention capacity. To date, this study applied biogas slurry-based hydrochar into paddy soil for the first time and demonstrated that ZHC significantly mitigated NH3 and increased N content. Overall, this study proposes an environmental-friendly strategy to recycle the wastes, biogas slurry, to the paddy fields to mitigate NH3 volatilization and increase grain yield of rice.

Heavy Metal Contamination Characteristics in Acid Soil-rice Systems and the Corresponding Human Health Risk in a Mining and Smelting Area in Jiangxi Province.

To understand the effects of mining activities on soil cadmium and rice, a typical mining area was selected. The Cd content in a considerable number of soils exceeded the standard limitation GB/T 36,783 - 2018, with a rate of 42.03%. Further analysis revealed soil total Cd content was strongly correlated with soil bioavailability of Cd (R2 0.721**), pH (R2 0.386**) and soil total content of lead(R2 0.678**). It suggests that soil total Cd content and soil pH significantly affect rice Cd levels, and that acid soil increases soil Cd bioavailability [Soil Cd (B)] and accumulation in rice grain. Furthermore, a mathematical dynamic fitting is developed to describe the relationship between rice Cd content, soil pH, and soil Cd bioavailability in acidic soil (pH 5-5.5). Rice Cd content (mg/kg) = - 179.2 + 67.24 × (Soil pH) - 12.81× [Soil Cd (B)] - 6.28 × 153(Soil pH) 2 + 65.79 × [Soil Cd (B)]2. This study identifies the main types of pollutants emitted by industrial activities and recommends Cd as the most concerning pollutant for rice planting and paddy soil.

Potential of Methanotroph bacteria as non-symbiotic nitrogen fixation in rice paddy fields in Morowali regency

Nitrogen makes up about 78% of all the gases in the atmosphere. Although it is very abundant in the atmosphere, nitrogen cannot be used directly by plants. The aims of this study were to isolate, characterize and identify the morphology and physiology of methanotrophic bacteria, calculate the total population of methanotrophic bacteria and analyze the chemical properties of paddy rice soil in Morowali Regency. Results Based on the gram reaction test, 11 isolates were gram negative, 11 isolates were catalase positive, 11 isolates were capable of nitrogen fixation, five isolates were yellowish white, five isolates were white, one isolate was yellow, six isolates had a convex elevation, and five isolates had a flat elevation. Based on the identification results, the Witaponda sub-district produced five isolates, three isolates from Bumiraya, two isolates from Bungku Barat and one isolate from Bungku Tengah District. Based on the results of the calculation of the number of bacteria found in Witaponda subdistrict as much as 54.05x106 Cfu/mL Bumiraya as much as 40.21x106 Cfu/mL, Bungku Barat as much as 12.06x106 Cfu/mL, Bungku Tengah as much as 11.65x106 Cfu/mL. The results of the analysis of soil chemical properties showed that pH, CEC, organic C, P205, K were classified as low in both irrigated and rainfed rice fields.

Effect of pH and temperature on the biodegradation of oxytetracycline, streptomycin, and validamycin A in soil

Residual antibiotics in agricultural soils can be of concern due to the development of antibiotic resistant microorganisms. Among various antibiotics, oxytetracycline (OTC), streptomycin (ST), and validamycin A (VA) have been used for agricultural purposes in South Korea; however, studies on the biodegradation of these antibiotics in soil are limited. Therefore, this study investigated the effects of pH (5.5, 6.8, and 7.4) and temperature (1.8, 23.0, and 31.2 °C) conditions on the biodegradation of these antibiotics in soil. The biodegradation tests were carried out in the field soil (FS) and rice paddy soil (RS) for 30 d with OTC and ST and 10 d with VA, and the residual antibiotics concentrations were monitored over the degradation period. Under various conditions, the degradation rates of ST was lower (11–69%) than that of OTC (60–90%) and VA (15–96%). The degradation half-lives of OTC and VA tend to decrease with increasing pH value, while the degradation half-life of ST tend to increase with increasing pH value. But, the effect of soil pH on the antibiotics degradation was not statistically significant, except for ST in the FS and RS and VA in the FS. The degradation of three antibiotics was greater at higher temperatures (23.0 °C and 31.2 °C) than at lower temperature (1.8 °C), and the degradation half-lives decreased with increasing temperature. The different degradation characteristics of different antibiotics in soil can be explained by the different characteristics of the antibiotics (e.g., sorption affinity, chemical forms) and soil (e.g., organic matter content). The results suggest that the degradation characteristics of antibiotics need to be considered in order to properly manage the residual antibiotics in soil.