Paddy Soil Fertility Research Articles

Organic fertilizer significantly mitigates N2O emissions while increase contributed of comammox Nitrospira in paddy soils

Nitrification is the dominant process for nitrous oxide (N2O) production under aerobic conditions, but the relative contribution of the autotrophic nitrifiers (the ammonia-oxidising archaea (AOA), the ammonia-oxidising bacteria (AOB) and the comammox) to this process is still unclear in some soil types. This is particularly the case in paddy soils under different fertilization regimes. We investigated active nitrifiers and their contribution to nitrification and N2O production in a range of unfertilized and fertilized paddy soils, using 13CO2-DNA based stable isotope probing (SIP) technique combined with a series of specific nitrification inhibitors, including acetylene (C2H2), 3, 4-dimethylpyrazole phosphate (DMPP) and 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO). The soils had a long-term history of fertilizer application, including chemical fertilizer only, a mixture of chemical fertilizers (70 %) and chicken manure (30 %) or a mixture of rice straw and chemical fertilizers. 13CO2-DNA-SIP and Illumina MiSeq sequencing demonstrated that comammox clades A.1 and B were active nitrifiers in all fertilized paddy soils. Inhibitor experiment showed that AOB largely contributed to nitrification activity and N2O emission in all paddy soils, while comammox contribution was more significant than AOA. Fertilization considerably altered nitrifiers' relative contribution to nitrification activity and N2O emissions. Applying organic fertilizers significantly decreased the N2O emissions but increased the contribution of comammox to the process. These findings expand the functional ecological niche of comammox, revealing their nitrification role and N2O production in other ecosystems than oligotrophic habitats.

Assessing the formation and stability of paddy soil aggregate driven by organic carbon and Fe/Al oxides in rice straw cyclic utilization strategies: Insight from a six-year field trial

Soil organic carbon (SOC) and iron/aluminum (Fe/Al) oxides are key cementing agents in driving soil aggregate formation, yet their direct effects and interactions on aggregate under long-term rice straw cyclic utilization (LSCU) in cold regions are still unclear. We compared chemical fertilizer (CF) with LSCU strategy: rice-straw (RS), biochar (RB), and biochar-based fertilizer (BF). We showed that the increase of macroaggregate (2–0.25 mm) is associated with SOC, dissolved organic carbon (DOC), humin carbon (HUC), amorphous and organic complexed Fe/Al oxides (Feo, Fep, Alo, Alp), and in each size of the aggregate, there exists an interaction between SOC (fractions) and Fe/Al oxides. Furthermore, aggregate stability was determined by Feo, Fep, and Alo. LSCU enhances macroaggregate and aggregate stability by increasing SOC and Fe/Al oxides in the bulk soil and aggregates, but there are differences among LSCU. In all treatments, RS had more DOC, fulvic acid carbon (FAC), humic acid carbon (HAC) and Fep; while RB had more SOC, HUC, free Fe/Al oxides (Fed, Ald), Feo, Alp; and BF had more Alo in bulk soil. Over the years, RS increased the DOC, FAC and HAC, whereas RB enhanced the stable SOC fractions (HUC) and promoted high reactive Fe/Al oxides formation (Feo, Fep, Alo), and BF increased DOC, Feo, Fep and Alo. Moreover, RB increases the direct pathway of SOC and Fe/Al oxides to aggregate, promoting aggregate formation. Our study provides new perspective on the mechanisms and promising practice for improving rice straw utilization efficiently, paddy soil fertility and productivity sustainably in cold regions.

Arsenic immobilization and greenhouse gas emission depend on quantity and frequency of nitrogen fertilization in paddy soil

Nitrogen (N) fertilization in paddy soils decreases arsenic mobility and methane emissions. However, it is unknown how quantity and frequency of N fertilization affects the interlinked redox reactions of iron(II)-driven denitrification, iron mineral (trans-)formation with subsequent arsenic (im-)mobilization, methane and nitrous oxide emissions, and how this links to microbiome composition. Thus, we incubated paddy soil from Vercelli, Italy, over 129 days and applied nitrate fertilizer at different concentrations (control: 0, low: ∼35, medium: ∼100, high: ∼200 mg N kg−1 soil−1) once at the beginning and after 49 days. In the high N treatment, nitrate reduction was coupled to oxidation of dissolved and solid-phase iron(II), while naturally occurring arsenic was retained on iron minerals due to suppression of reductive iron(III) mineral dissolution. In the low N treatment, 40 μg L−1 of arsenic was mobilized into solution after nitrate depletion, with 69 % being immobilized after a second nitrate application. In the non-fertilized control, concentrations of dissolved arsenic were as high as 76 μg L−1, driven by mobilization of 36 % of the initial mineral-bound arsenic. Generally, N fertilization led to 1.5-fold higher total GHG emissions (sum of CO2, CH4 and N2O as CO2 equivalents), 158-fold higher N2O, and 7.5-fold lower CH4 emissions compared to non-fertilization. On day 37, Gallionellaceae, Comamonadaceae and Rhodospirillales were more abundant in the high N treatment compared to the non-fertilized control, indicating their potential role as key players in nitrate reduction coupled to iron(II) oxidation. The findings underscore the dual effect of N fertilization, immobilizing arsenic in the short-term (low/medium N) or long-term (high N), while simultaneously increasing N2O and lowering CH4 emissions. This highlights the significance of both the quantity and frequency of N fertilizer application in paddy soils.

Response of comammox Nitrospira clades A and B communities to long-term fertilization and rhizosphere effects and their relative contribution to nitrification in a subtropical paddy field of China

The recently discovered complete ammonia oxidation (comammox Nitrospira) containing clade A and clade B has further complemented our understanding of nitrification process. Nevertheless, understanding the community feature of comammox Nitrospira clades A and B and their relative contribution to nitrification in paddy rhizosphere are still in its infancy. In this study, we assessed the community diversity and structure of comammox Nitrospira clades A and B in paddy rhizosphere and bulk soils under thirty years of different fertilization strategies, i.e., non-fertilization control (CK), chemical fertilizers application (NPK), and NPK plus swine manure (NPKM), respectively. NPKM significantly increased the a-diversity (Chao1 and Shannon indices) of comammox Nitrospira clade A and altered the community structure (P < 0.05) but had little effect on clade B. A two-way analysis of variance (ANOVA) showed that the effect of long-term fertilization on soil comammox Nitrospira community and nitrification potential rate (PNR) was much greater than that of rhizosphere. Compared with NPK, soil PNR was greatly increased by 51.0% under the NPKM treatment in the rhizosphere (P < 0.05). Phylogenetic analysis showed that NPKM improved the relative abundances of sub-clade A.2.1 and sub-clade A.3.2 of the comammox clade A community, with an average increase of 212.2 and 210.4% in both rhizosphere and bulk soils relative to the NPK treatment. Soil organic matter, NH4+-N, and pH were significant soil drivers of comammox Nitrospira clades A and B community. Furthermore, linear regression and structural equation modeling clearly showed that comammox Nitrospira clade A a-diversity were significantly associated with soil PNR (P < 0.05). Our results suggest (i) that comammox Nitrospira clade A are sensitive to the organic fertilization; and (ii) that comammox Nitrospira clade A contribute more to nitrification than clade B under the long-term organic fertilized paddy soil.

Biochar Amendment was Less Effective for Rice Yield Improvement but More Effective for Soil Quality Relative to Inorganic Fertilization in a Low Fertility Paddy Soil

Biochar Amendment was Less Effective for Rice Yield Improvement but More Effective for Soil Quality Relative to Inorganic Fertilization in a Low Fertility Paddy Soil

Impact of soil moisture regimes on greenhouse gas emissions, soil microbial biomass, and enzymatic activity in long-term fertilized paddy soil

Two potent greenhouse gases that are mostly found in agricultural soils are methane and nitrous oxide. Therefore, we investigated the effect of different moisture regimes on microbial stoichiometry, enzymatic activity, and greenhouse gas emissions in long-term paddy soils. The treatments included a control (CK; no addition), chemical fertilizer (NPK), and NPK + cattle manure (NPKM) and two moisture regimes such as 60% water-filled pore spaces (WFPS) and flooding. The results revealed that 60% water-filled pore spaces (WFPS) emit higher amounts of N2O than flooded soil, while in the case of CH4 the flooded soil emits more CH4 emission compared to 60% WFPS. At 60% WFPS higher N2O flux values were recorded for control, NPK, and NPKM which are 2.3, 3.1, and 3.5 µg kg−1, respectively. In flooded soil, the CH4 flux emission was higher, and the NPKM treatment recorded the maximum CH4 emissions (3.8 µg kg−1) followed by NPK (3.2 µg kg−1) and CK (1.7 µg kg−1). The dissolved organic carbon (DOC) was increased by 15–27% under all flooded treatments as compared to 60% WPFS treatments. The microbial biomass carbon, nitrogen, and phosphorus (MBC, MBN, and MBP) significantly increased in the flooded treatments by 8–12%, 14–21%, and 4–22%, respectively when compared to 60% WFPS. The urease enzyme was influenced by moisture conditions, and significantly increased by 42–54% in flooded soil compared with 60% WFPS while having little effect on the β-glucosidase (BG) and acid phosphatase (AcP) enzymes. Moreover DOC, MBC, and pH showed a significant positive relationship with cumulative CH4, while DOC showed a significant relationship with cumulative N2O. In the random forest model, soil moisture, MBC, DOC, pH, and enzymatic activities were the most important factors for GHG emissions. The PLS-PM analysis showed that soil properties and enzymes possessed significantly directly impacted on CH4 and N2O emissions, while SMB had indirect positive effect on CH4 and N2O emissions.

Effects of Biochar and Straw Amendment on Soil Fertility and Microbial Communities in Paddy Soils.

Straw and biochar, two commonly used soil amendments, have been shown to enhance soil fertility and the composition of microbial communities. To compare the effects of straw and biochar on soil fertility, particularly focusing on soil dissolved organic matter (DOM) components, and the physiochemical properties of soil and microbial communities, a combination of high-throughput sequencing and three-dimensional fluorescence mapping technology was employed. In our study, we set up four treatments, i.e., without biochar and straw (B0S0); biochar only (B1S0); straw returning only (B0S1); and biochar and straw (B1S1). Our results demonstrate that soil organic matter (SOM), available nitrogen (AN), and available potassium (AK) were increased by 34.71%, 22.96%, and 61.68%, respectively, under the B1S1 treatment compared to the B0S0 treatment. In addition, microbial carbon (MBC), dissolved organic carbon (DOC), and particulate organic carbon (POC) were significantly increased with the B1S1 treatment, by 55.13%, 15.59%, and 125.46%, respectively. The results also show an enhancement in microbial diversity, the composition of microbial communities, and the degree of soil humification with the application of biochar and straw. Moreover, by comparing the differences in soil fertility, DOM components, and other indicators under different treatments, the combined treatments of biochar and straw had a more significant positive impact on paddy soil fertility compared to biochar. In conclusion, our study revealed the combination of straw incorporation and biochar application has significant impacts and is considered an effective approach to improving soil fertility.

A Study on the Impacts of Fertilizer Crisis on Paddy Cultivation in Five Selected Divisional Secretariat Divisions of Trincomalee District During Maha 2021/22

Rice is one of the most important cereal crops in the world. With population growth, farmers have to increase paddy production to meet existing as well as future demand. Fertilizer is the major input in rice production. Growth and yield traits of rice are affected by improper and imbalanced application of plant nutrients. Hence, suitable practices of adequate rate and timing of fertilizer application are needed to increase the rice yield. In May 2021, Imports &amp; Exports (Control) Regulation No 07 of 2021 was issued, banning the importation of chemical fertilizers, pesticides, &amp; herbicides in Sri Lanka. Low fertility of paddy soil created the need for adequate and constant supply of chemical fertilizers in paddy cultivation, which is a major threat to production and endangers the national food security and economy of Sri Lanka, especially in the major cropping season of Maha 2021/22. Therefore, a questionnaire survey was conducted to investigate the impact of the fertilizer crisis in major paddy-cultivating Divisional Secretariat divisions of the Trincomalee district. The random sampling method was used to select respondents for the survey, and the collected data were statistically analyzed by SPSS version 26.0 software. The results revealed that the majority of the farmers experienced a 50% yield reduction compared to the last cropping season of Maha 2020/21 in the entire five DS divisions. The seedling stage was highly affected during the fertilizer crisis in paddy cultivation in all five DS divisions. Further, the cost of production in paddy cultivation increased by more than 50% during the fertilizer crisis compared to the cost involved in Maha 2020/21. The potential for organic manure production is very poor, while the majority of farmers are interested in adopting the sustainable farming system of the Integrated Plant Nutrient System through the judicious application of chemical and organic fertilizers received on a subsidy basis, according to the present government’s fertilizer policy, which emphasizes 70% of inorganic fertilizers and 30% of organic manure application to improve sustainability in paddy cultivation in the country.

The cooperative interaction of AOB and comammox clade A drives nitrification and N2O emissions in a long-term organic fertilized paddy soil

The newly discovered complete ammonia oxidizer (comammox Nitrospira) is able to single-step nitrification capability, and increased our understanding of soil nitrogen cycling. However, the response of comammox and ammonia-oxidizing bacteria (AOB) and archaea (AOA) to long-term fertilization and rhizosphere effects in paddy soils and their relative contribution to the nitrification-derived N2O emissions is still unclear. Here, we collected rhizosphere and bulk soils with thirty years of different fertilization strategies, i.e., non-fertilization (CK), chemical N, P, and K application (NPK), and NPK plus pig manure application (NPKM), respectively, to test changes in nitrification potential rate (PNR), N2O emission fluxes, abundance of ammonia oxidizers and their significant drivers. The result showed that NPKM significantly increased soil C and N levels, the proportion of soil middle-size particles (40.35–148.00 μm class), and soil PNR, but decreased soil N2O emissions, especially in the drying time of paddy (P < 0.05), compared to NPK fertilization. NPKM had the highest values of AOA, AOB, and comammox clade A amoA gene copy numbers (P < 0.05), but clade B was increased by the NPK in the rhizosphere soil. Furthermore, fertilization showed greater effects on ammonia oxidizers (except for clade B) than the rhizosphere effect. Mantel test showed that SOM, TP, pH, NH4+, and NO3− were main abiotic factors causing the niche separation among ammonia oxidizers. Linear regression analysis and structural equation model (SEM) showed that both PNR and N2O emission fluxes were significantly associated with the abundance of AOB and comammox clade A (P < 0.05). Therefore, our results underline the importance of AOB together with comammox clade A in nitrification and N2O production in long-term organic fertilized paddy fields, which could provide new ideas for the mitigation of N2O emission by adopting organic fertilization scenarios in Chinese paddy fields.

Modeling and simulation of sulfur availability in paddy soils under reducing environment considering heavy metal content

ABSTRACT Insufficient sulfur (S) can reduce rice yield under certain soil conditions. The availability of S in paddy soils is determined by the concentrations of inorganic S and some heavy metals that react with sulfide ions to form sparingly soluble sulfides. In order to evaluate the S fertility of paddy soils, a predictive model was formulated to simulate the formation of sulfides and the effect of heavy metal content on the concentration of sulfate ions (SO4 2-) in the soil solution under reducing conditions. The basic computational algorithm of the model was an iterative method incorporating the equilibria of major aqueous species, sparingly soluble salts, and soil adsorption. A field survey was also conducted to evaluate S availability in actual rice paddy soils. SO4 2- and heavy metal contents were measured in 250 paddy fields located in central Japan. The model calculation using the field survey results showed that as the redox potential (Eh) decreases from −80 to −170 mV, sulfide-forming metals (Ni, Cu, Zn, Cd, and Pb) take precedence over iron (Fe) in sulfide formation. The major sulfides present were chalcocite (Cu2S), greenockite (CdS), sphalerite (ZnS), galena (PbS), chalcopyrite (CuFeS) and α-NiS, Due to their predominance in paddy soil, zinc (Zn) and copper (Cu) have a strong influence on the reduction of SO4 2- concentration. The formation of metal sulfides increases the Eh value corresponding to the critical SO4 2- concentration and decreases the S availability under the individual Eh condition. The model calculation also showed that an increase in sulfide-forming metal content leads to S deficiency, even if the SO4 2- content is not exceeded. Model calculations for each geographical division against the field survey results suggest that S deficiency is more likely to occur in lowland paddy fields formed by river sediments than in Andisols on terraces.

Biochar and Manure Co-Application Increases Rice Yield in Low Productive Acid Soil by Increasing Soil pH, Organic Carbon, and Nutrient Retention and Availability.

In recent years, overuse of chemical fertilization has led to soil acidification and decreased rice yield productivity in southern China. Biochar and manure co-application remediation may have positive effects on rice yield and improve acid paddy soil fertility. This study was conducted to understand the effects of co-application of wood biochar and pig manure on rice yield and acid paddy soil quality (0-40 cm soil layers) in a 5-year field experiment. The experiment consisted of six treatments: no biochar and no fertilizer (CK); biochar only (BC); mineral fertilizer (N); mineral fertilizer combined with biochar (N + BC); manure (25% manure N replacing fertilizer N) combined with mineral fertilizer (MN); and manure combined with mineral fertilizer and biochar (MN + BC). Total nitrogen application for each treatment was the same at 270 kg nitrogen ha-1y-1, and 30 t ha-1 biochar was added to the soil only in the first year. After five years, compared with N treatments, N + BC, MN, and MN + BC treatments increased the rice yield rate to 2.8%, 4.3%, and 6.3%, respectively, by improving soil organic matter, total nitrogen, and available phosphate under a 0-40 cm soil layer. MN + BC had the strongest resistance to soil acidification among all the treatments. The interaction between fertilizers and biochar application was significant (p < 0.05) in rice yield, soil electrical conductivity (10-20 cm), and soil available phosphate (20-40 cm). Principal component analysis indicated that the effect of manure on soil property was stronger than that of biochar in the 0-40 cm soil layer. The overall rice yield and soil fertility decreased in the order of biochar + mineral fertilizer + manure > mineral fertilizer + manure > biochar + mineral fertilizer > mineral fertilizer > biochar > control. These results suggest that biochar and manure co-application is a long-term viable strategy for improving acid soil productivity due to its improvements in soil pH, organic carbon, nutrient retention, and availability.

Impacts of Organic and Non-Organic Cultivation Practices on Soil Fertility Parameters of Selected Paddy Lands Based on the Duration of Cultivation in Wet and Intermediate Climate Zones of Sri Lanka

Sri Lanka has been practicing the cultivation of rice as its main staple food in two major agricultural techniques; organic and non-organic. The major aim of the present study was to analyze and compare the time duration-based impacts of the above two agricultural practices on paddy soil fertility indicators as the preliminary step of a long-term research project. Organic and non-organic paddy fields that have been cultivated for 2 seasons in Thorapitiya in the Matale district were selected as short-term cultivated lands. Land cultivated for 10 years organically in Homagama and land with a non-organic cultivation history of 20 years in Kottawa was selected as long-term cultivated lands. A total number of 35 samples were collected including the control samples from adjacent uncultivated land during the first phase of the Maha season 2021/2022. The samples were dried, sieved and then analyzed in their physical and chemical properties; color, texture, particle size distribution, pH, electrical conductivity (EC), organic carbon content (OCC), total nitrogen content (TNC), available phosphorus content (APC), exchangeable potassium content (EPC), and exchangeable magnesium content (EMC). The soil samples from Thorapitiya had a reddish-brown nature while most of the samples from Kottawa and Homagama had a grayish-brown color. Higher clay percentages were found in the soil samples from long-term organically cultivated lands in Homagama. The uniformity coefficient (Cu) values for soils from study areas except Homagama lie in the range for well-graded soil while the coefficient of gradation (Cc) values of all three areas were not in the above range. Short-term cultivation using organic and non-organic fertilizers did not show a significant difference in soil texture, pH, EC, APC, EPC, and EMC values. Most of their cultivated lands had a lower pH (~5.5–6.0) than the corresponding control plots (~5.8–6.4). The TNC levels in non-organically cultivated lands were significantly higher than in organic lands. Practicing agricultural activities for a prolonged period has resulted in significant differences in soil texture, pH, EC, OCC, TNC and APC between organic and non-organic lands. Higher OCC and lower pH, EC and APC values have been obtained for long-term organically cultivated lands than their non-organic counterparts. The TNC, APC and EPC levels were higher in most of the long- term organic and non-organic lands than in their control samples. Several amendments have to be made to bring the fertility parameters to an optimum level in long-term cultivated lands. &#x0D; Keywords: Soil fertility, Paddy soil, Cultivation history, Organic cultivation, Non-organic cultivation

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.

Organic fertilizers incorporation increased microbial necromass accumulation more than mineral fertilization in paddy soil via altering microbial traits

Long-term fertilization is considered as the most effective practice in organic C accrual in intensively managed paddy soils. However, the contribution and mechanism of microbial necromass C for soil organic C accrual, and the effective incremental component in paddy soils under the different fertilization remain unclear. To address the knowledge gap, we collected soils from a 31-year long-term fertilization paddy field including no fertilization (control), mineral fertilizers alone (NPK), mineral fertilizers plus rice straw incorporation (NPK + S), and mineral fertilizers plus manure incorporation (NPK + M). Living microbial biomass and their necromass were quantified by the biomarker analysis of phospholipid fatty acids and amino sugars, respectively. Results showed that the three fertilization treatments significantly increased the bacterial and fungal biomass in both 0–10 and 10–20 cm layers, which were universally greatest in NPK + M. The microbial necromass C was little affected by 31-year mineral fertilization; however, mineral combined organic fertilizers incorporation greatly stimulated microbial necromass C accumulation. Besides, soil treated with NPK + M showed 3.55 ± 0.12 g kg−1 higher accumulation of microbial necromass C than NPK + S, because manure application increased the abundance of Gram-positive r-strategy bacteria (mainly Firmicutes and Actinobacteria), and enriched filamentous fungi (Ascomycota). Compared to the control, the increase in the microbial necromass C contribution to SOC was lowest in NPK and highest in NPK + M, mainly derived from the variation in the contribution of bacterial necromass. Consequently, this study suggested long-term mineral combined manure application in subtropical paddy fields could effectively increase soil C sequestration by strengthening microbial necromass accumulation. Bacterial and fungal necromass C accrual was closely associated with living microbial biomass and their community structure. Given the C sequestration potential of manure in paddy soil, as the crucial resource of livestock and poultry waste in this region, it should be incorporated into subtropical agricultural ecosystems.

Comammox plays a functionally important role in the nitrification of rice paddy soil with different nitrogen fertilization levels

Chemoautotrophic ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), and the recently discovered complete ammonia oxidizers (comammox) catalyze ammonia oxidation; however, quantifying their individual gross ammonia oxidation activities in complex soil ecosystems remains elusive. In this study, we investigated the nitrifying population and community dynamics in response to fertilization in paddy soil under long-term treatment with different urea concentrations (0, 100, 200, and 300 kg N ha−1; designated N0, N100, N200, and N300, respectively). The community structure of nitrifiers, including comammox, was changed by long-term N fertilizers input, which also enhanced AOB abundance and comammox clade A/B abundance ratio, whereas it did not affect AOA abundance. Nevertheless, the microbial population sizes and community compositions remained largely similar following short-term fertilization throughout one rice-growing season. Furthermore, by employing a 15NO3− pool dilution technique in combination with chlorate and 1-octyne inhibition, we conducted a soil microcosm incubation without substrate supplementation to distinguish the relative contribution of comammox to gross ammonia oxidation from those of AOA and AOB. Comammox exhibited higher nitrification activity in soils treated with more N fertilizers, with the contribution increasing from 34 % (N0) to 75 % (N300), although its abundance was not dominant. This could be attributed to the repeated N-based fertilization, which reduced the soil pH, thereby decreasing the soil-free ammonia concentration. In contrast, the contribution of AOA declined from 57 % (N0) to 19 % (N300), while that of AOB only accounted for a minor fraction (<25 %) despite being in high abundance. Altogether, our findings provide evidence that the activity of nitrifying guilds is more valuable than their abundance and provide insights into the potential functional significance of comammox in the nitrification process in rice paddy systems.

Effects of long-term substitution of chemical fertilizer with Chinese milk vetch on soil phosphorus availability and leaching risk in the double rice systems of Eastern China

The cultivation of Chinese milk vetch (MV) as green manure is an effective practice for improving paddy soil fertility and rice yields in China. However, few studies have investigated the effects of MV on phosphorus (P) availability and the risk of P loss. An 11-year field experiment was conducted to analyze the effects of returning MV to paddy soil on soil P forms, late rice yield and soil fertility, as well as the related mechanisms. The experiment included seven treatments: (1) F0M0 (no-fertilizer and no-MV); (2) F1M0 (chemical fertilizer alone), (3) F2M0 (70% of chemical fertilizer), and F2MV1–4 (70% of chemical fertilizer and 7500 kg ha-1 MV, 15000 kg ha-1 MV, 22500 kg ha-1 MV, 30000 kg ha-1 MV). The Olsen-P of F2M1–4 were significantly higher (38.3–54.5%) than that of F2M0 and the CaCl2-P levels of F2M1–4 were significantly lower (47.7–61.6%) than that of F1M0. The Olsen-P was mainly influenced by soil total nitrogen (TN), total phosphorus (TP), pH, free Fe/Al (FeDCB/AlDCB), organic bound Fe (Feo), noncrystalline amorphous Al (Alox) and exchange Mg2+, while the CaCl2-P was mainly influenced by TP, pH Alox, FeDCB, AlDCB, and exchange Mg2+/Ca2+. Partial least squares path model analysis indicated that chemical fertilizer played more important role on the Olsen-P and the CaCl2-P. MV application had the negative effect (the total effect value −0.48) on the CaCl2-P and decreased the risk of P loss yet. Two-segment regression analyses indicated the critical Olsen-P levels were 19.39 mg kg-1 for late rice yield and 31.94 mg kg-1 for soluble P, respectively. In conclusion, the continuous application of MV not only improved P availability to enhance late rice yield, but also reduced the risk of P loss in paddy soil in double rice systems.

Chemical Fertilization Alters Soil Carbon in Paddy Soil through the Interaction of Labile Organic Carbon and Phosphorus Fractions

The influence of long-term chemical fertilization in paddy soils is based on the interaction between labile carbon and phosphorus fractions and the manner in which this influences soil organic carbon (SOC). Four soil depths (0–30 cm) were analyzed in this study. Easily oxidized organic carbon components, such as permanganate oxidized carbon (POXC) and dissolved organic carbon (DOC), and other physicochemical soil factors were evaluated. The correlation and principal component analyses were used to examine the relationship between soil depth and the parameter dataset. The results showed that Fe-P concentrations were greater in the 0–5 cm soil layer. DOC, inorganic phosphate fraction, and other soil physiochemical characteristics interacted more strongly with SOC in the 0–5 cm soil layer, compared to interactions in the 10–15 cm layer, influencing soil acidity. An increase in DOC in the 0–5 cm soil layer had a considerable effect on lowering SOC, consistent with P being positively correlated with POXC, but negatively with SOC and water-soluble carbon (WSC). The changes in SOC could be attributed to the relationship between DOC and inorganic phosphate fractions (such as Fe-P) under specific soil pH conditions. An increase in soil DOC could be caused by changes in the P fraction and pH. The DOC:Avai. P ratio could serve as a compromise for the C and P dynamic indicators. The soil depth interval is a critical element that influences these interactions. Agricultural policy and decision-making may be influenced by the P from chemical fertilization practices, considering the yields and environmental effects.

Straw is more effective than biochar in mobilizing soil organic phosphorus mineralization in saline-alkali paddy soil

Fertilizer management can alter the availability and biological cycle of soil phosphorus (P). The objective of this study was to explore the relationship among soil P availability, organic phosphorus (Po) mineralization and soil bacteria characteristics under organic and inorganic fertilization in saline-alkali paddy soil. Four treatments were designed, namely no fertilizer (Control), mineral fertilizer (NPK), mineral fertilizer with rice straw (NPKS) or rice straw-derived biochar (NPKB). Compared with NPK treatment, straw and biochar addition greatly increased P uptake by rice while decreased soil organic P/total P ratio. The contents of soil available P (AP) and alkaline phosphatase (ALP) in NPKS treatment were higher than those in NPKB treatment. This was most likely due to the higher abundance of Streptomyces in NPKS treatment than that in NPKB treatment, which was known as a potential Po mineralizer. NPKS and NPKB treatments significantly increased diversity and abundance of phoD-harboring bacteria but have no significant effect on the total bacteria. Based on SEM analysis, the composition of phoD-harboring bacteria community was more important in ALP activity than the increased phoD-harboring bacterial abundance and diversity. Soil dissolved organic carbon (DOC) content and DOC: AP ratio were the key factors affecting the formation of total bacterial and phoD-harboring bacterial community, which were significantly increased in NPKS treatment. Compared with biochar application, straw was more efficient in increasing the abundance of some phoD-harboring bacteria, which preferred carbohydrate-rich environments, thereby improving the availability of P and its potential biological transformation in saline-alkali paddy soil.

Fertilization regimes and the nitrification process in paddy soils: Lessons for agricultural sustainability from a meta-analysis

Increasing evidence shows that fertilization plays a key role in improving nitrogen (N) storage and increasing N supply capacity in paddy soils. How fertilizer regimes (N application rate, substitution rate, and fertilizer type) and environmental variables (soil pH and SOM) affect N pools, nitrification processes (abundance of nitrifying microorganisms, nitrification rate), and N losses (NH3 volatilization and N2O emission) in paddy soils is still unclear. Here, we conducted a meta-analysis by compiling a total of 1307 individual experimental observations from 64 peer-reviewed publications to evaluate the impacts of N fertilization with other amendments (manure, biochar, or nitrification inhibitors) on the abovementioned paddy soil N sequestration and turnover. Overall, fertilization increased N pools (total N, NH4+-N, and NO3−-N content) and the abundance of main nitrifying microorganisms (AOA and AOB) by 7.3 % to 51.4 % and 70.3 % to 146.9 % in paddy soil, respectively, while stimulating NH3 volatilization and N2O emissions by 277.3 % and 149.7 %, respectively. The increased N losses following fertilization were positively correlated with the expansion of paddy soil N pools. In particular, the substitution of organic fertilizer for chemical fertilizer at 30 % to 70 % had a better effect on elevating soil N sequestration, while a substitution rate of >70 % reduced N losses. Moreover, the application of biochar and nitrification inhibitors was conducive to increasing total N, NH4+-N, and NH3 volatilization and reducing N2O emissions in fertilized paddy soils. Our findings indicated that fertilization regimes substantially stimulated N pools and N losses in paddy ecosystems. Thus, to avoid environmental and economic consequences, the optimization of N management and adjusting soil properties should be considered to increase N use efficiency and suppress N losses.

Rice–fish coculture system enhances paddy soil fertility, bacterial network stability and keystone taxa diversity

High-input, modern rice farming (MRF) has caused severe soil degradation worldwide, necessitating a transition towards more sustainable practices. The traditional rice–fish coculture (RFC) and manure fertilization (Manure) may provide valuable insights to this transition. However, it remains elusive how long-term agricultural conversion influences microbial community structure, soil fertility, and food supply. Here, we performed six-year conversions of MRF to either RFC or Manure in a hilly area of Sichuan (China). We found that agricultural conversions exerted a greater impact than rice growing stages on bacterial community structure. The RFC bacterial network exhibited the highest modularity and robustness, but also harbored the most diverse keystone taxa, followed by Manure. In contrast, MRF displayed network properties that are characteristic of unstable communities. Importantly, RFC also exhibited the greatest capability in improving the preservation of soil organic carbon, nitrogen, and phosphorus, and has significantly increased soil pH (> 1.5 units). Yet, adopting traditional practices, particularly Manure, decreased rice yields, but fish harvested in RFC could offset the decrease in rice yield. Our field study highlights bacterial network structure and keystone taxa diversity as possible indicators for agriculture sustainability, but also incentivizes the integration of traditional RFC in developing novel sustainable agricultural practices.