Straw and no-tillage management, as important practices in conservation agriculture, have the potential to improve soil structure. However, their effects on the aggregate stability of soil and on active organic carbon pools in paddy fields are unclear. To investigate how different tillage and straw management practices affect soil properties, this study drew on a 15-year long-term experiment conducted in a double-cropped rice region in South China. It systematically compared four treatments: no-tillage (NT), conventional tillage (CT), conventional tillage with incorporated straw (CT-SR), and no-tillage with straw mulch (NT-SMR)—in terms of their effects on the distribution and stability of mechanical and water-stable aggregates, as well as the distribution of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) across various aggregate size fractions. The results showed that: (1) Relative to the CT, NT, and CT-SR treatments, NT-SMR significantly enhanced soil structure, as evidenced by a higher percentage of large aggregates (>0.25 mm) and improved aggregate stability. (2) NT-SMR consistently increased soil organic carbon pools, raising SOC, POC, and MAOC contents by 2.0–14.2%, 5.7–24.3%, and 1.0–11.9%, respectively, compared to other treatments. (3) In this study, stability of soil aggregates parameters (R>0.25, MWD and GMD) increased combined with higher levels of bulk SOC and >0.053 mm MAOC, but decreased with higher fractal dimension, indicating a direct causal link between organic carbon accumulation and the betterment of soil structure. Overall, NT-SMR promotes aggregate stability through an optimized particle-size distribution and increased SOC, particularly in the >0.053 mm MAOC fraction. This practice is a sustainable long-term strategy for enhancing SOC sequestration and structural stability in paddy.

Returning straw to the field substantially improves soil quality and is a key agronomic practice for sustainable rice production. However, the long-term effects of straw returning on soil quality in different soil layers under different tillage practices remain insufficiently understood. Here, we performed a long-term experiment in Nanning and Hezhou to examine variation in soil quality across multiple depths (0-10 cm, 10-20 cm, 20-30 cm, and 30-50 cm) under four treatments: no-tillage (NT), no-tillage with straw mulching (NT-SMR), conventional tillage (CT), and conventional tillage with straw incorporation (CT-SR). Long-term straw return significantly improved the physical, chemical, and biological properties of topsoil, regardless of the tillage method. In comparison to the straw no return treatment, the soil quality index (SQI) in the 0-10 cm soil layer at the Nanning and Hezhou experimental sites increased by 32.3% and 35.8%, respectively; for the 10-20 cm soil layer, the corresponding SQI increments were 11.6% and 47.0%, respectively. Correlation analysis and random forest modeling indicated that the soil quality index (SQI) was significantly correlated with soil chemical properties, including total nitrogen (TN), available nitrogen (AHN), total phosphorus (TP), available potassium (AK), and organic carbon (SOC). The structural equation model showed that tillage and straw returning mainly improved soil quality indirectly by influencing chemical properties. Therefore, the CT-SR treatment is an efficient soil management measure to enhance soil quality. This study provides practical guidance for sustainable soil management in paddy fields in southern China.

A high-quality rice population structure serves as the foundation for achieving high yields, with plant spacing and the number of seedlings per hill playing key roles in regulating this structural development. However, the combined effects of seedling number per hill and plant spacing on yield formation and resource allocation in rice (Oryza sativa L.) are not yet fully understood. This study aimed to assess the effects of coordinated changes in seedling number per hill and plant spacing on source-sink characteristics and yield in rice, while maintaining a constant seedling density. The study evaluated the response of four rice varieties to combined variations in seedling number per hill and plant spacing, all under a baseline seedling density of 60 × 104 ha-¹, over a two-year field experiment. Linear models, mixed-effects models, ridge regression, and structural equation modeling were used to examine the effects of these factors on rice growth, biomass allocation, nitrogen and carbon accumulation, and yield. Results showed that increasing seedlings per hill and plant spacing reduced yield and above-ground biomass (AGB) but increased the harvest index (HI). Single seedlings with narrow spacing produced the highest yield (7.27 t ha-¹), AGB (16.4 t ha-¹), and the lowest HI (0.46). The combined effects of seedling number per hill and plant spacing negatively affected tiller number, number of effective panicles, leaf area index, and nutrient accumulation, while positively influencing specific leaf area and panicle biomass allocation. Organic carbon accumulation (43.52%) and biomass accumulation (31.32%) were major contributors to yield variation. Path analysis indicated that 96% of yield variance was accounted for, highlighting the importance of panicle biomass, organic carbon accumulation, and the source-sink balance between stems and panicles. In conclusion, single seedlings with narrow spacing (12.93 cm spacing with 1 seedling per hill) optimize the source-sink balance in rice, compensating for a lower harvest index by enhancing biomass accumulation, thereby increasing yield.

[This corrects the article DOI: 10.3389/fpls.2022.895230.].

Incorporating straw return into tillage systems is a potential strategy for sustaining rice production, while achieving multiple environmental benefits. The effect of different tillage management on methane emissions has been well documented; however, the combined effects with straw return management require further exploration. To investigate this, an experiment was initiated in 2008 using five management practices: conventional tillage, no tillage, conventional tillage with straw mulching, conventional tillage with straw incorporation, and no tillage with straw mulching. Changes in soil carbon pool properties, hydrolytic and oxidative enzyme activities, phospholipid fatty acids, and the abundance of methanogenic and methanotrophic genes were measured during the early and late rice-growing seasons in 2022 and 2023. The results indicated the following: (1) Straw return significantly increased cumulative methane emissions by 75.5 % compared with straw removal. However, conventional tillage with straw incorporation reduced cumulative methane emissions by 37.2 % and 20.3 % compared to conventional tillage with straw mulching and no tillage with straw mulching, respectively. (2) Conventional tillage with straw incorporation enhanced beta-acetylglucosaminidase and cellobiohydrolase activities and increased mineral-associated organic carbon content compared to conventional tillage with straw mulching and no tillage with straw mulching. (3) Under conventional tillage with straw incorporation, the content of phospholipid fatty acids in bacteria, fungi, and actinomycetes increased by 7.6 %-19.3 %, 7.2 %-18.3 %, and 6.0 %-19.8 % compared with conventional tillage, no tillage, and conventional tillage with straw mulching, respectively. (4) Conventional tillage with straw incorporation reduced methanogens/methanotrophs by 14.1 % and 4.0 % compared with conventional tillage with straw mulching and no tillage with straw mulching, respectively. Structural equation modeling revealed that tillage and straw management promoted the conversion of particulate organic carbon to mineral-associated organic carbon by increasing the soil microbial populations and beta-acetylglucosaminidase and cellobiohydrolase activities, which regulated methane production by methanerelated functional communities. Thus, regulating the conversion of activated carbon to inert carbon through a rational combination of tillage and straw return methods can effectively reduce methane emissions from double rice paddies.

In recent years, rice (Oryza sativa L.) yield in China has significantly increased, resulting in a substantial amount of straw residue and a low utilization rate. In situ straw returning is the most convenient and efficient method for utilizing straw. However, the long-term effects of straw returning on rice yield and the optimal straw returning method remain unclear in the double-cropping rice regions of South China. This study investigated the effects of straw mulching (SMR) and straw incorporation (SIR) on rice yield, nutrient absorption, and the source/sink, using straw removal as a control (CK). The findings indicated that, compared to the CK, rice yield under SIR was 6.6%-24.8% higher. Furthermore, SIR significantly enhanced nitrogen (N) accumulation in rice plants. At maturity, the total N accumulation and distribution ratio were highest under SIR, with total N accumulation increased by 5.6%-26.8% compared to SMR, and by 13.0%-57.8% compared to CK. Compared with CK, the grain-leaf ratio (GLR) and harvest index (HI) under SIR increased by 5.4%-20.7% and 11.6%-15.6%, respectively. Similarly, compared with SMR, GLR and HI under SIR increased by 2.2%-15% and 6.5%-17.8%, respectively. Overall, SIR not only enhances nitrogen accumulation in rice plants but also boosts the net assimilation rate, ultimately maximizing the GLR. Therefore, the SIR strategy represents the most effective approach for sustainable crop production in the double-cropping rice regions of South China. Plain Language SummaryIncreased rice yields in China have generated substantial straw residues with lowutilization efficiency. In situ straw return is widely adopted, yet its long-term agro-nomic impacts in South China's double-cropping systems require clarification.This study compared straw mulching (SMR), incorporation (SIR), and removal(CK) to assess effects on yield formation, nutrient dynamics, and source-sink relationships. SIR increased yield by 6.6%-24.8% versus CK enhanced N accumu-lation: 5.6%-26.8% (vs. SMR) and 13.0%-57.8% (vs. CK) improved source-sinkbalance: higher grain-leaf ratio (5.4%-20.7%) and harvest index (11.6%-15.6%). SIRoptimizes nutrient use efficiency and yield potential, representing the superior strawmanagement strategy for sustainable rice production in subtropical double-croppingsystems.

Rice (Oryza sativa L.) cultivation using direct seeding is susceptible to chilling stress, particularly during seed germination and early seedling growth in the early season of a double cropping system. Alternatively, seed priming with various plant growth-promoting hormones is an effective technique to promote rapid and uniform emergence under chilling stress. Therefore, we evaluated the impact of gibberellin A3 (GA3) and brassinolide (BR) priming on rice seed emergence, examining their proteomic responses under low-temperature conditions. Results indicated that GA3 and BR increased the seed germination rate by 22.67% and 7.33% at 72 h and 35% and 15% at 96 h compared to the control (CK), respectively. Furthermore, proteomic analysis identified 2551, 2614, and 2592 differentially expressed proteins (DEPs) in GA, BR, and CK, respectively. Among them, GA exhibited 84 upregulated and 260 downregulated DEPs, while BR showed 112 upregulated and 102 downregulated DEPs, and CK had 123 upregulated and 81 downregulated DEPs. Notably, under chilling stress, both GA3 and BR are involved in peroxide metabolism, phenylpropanoid biosynthesis, and inositol phosphate metabolism, enhancing antioxidant capacity and providing energy substances for germination. In addition, GA3 triggers the specific regulation of stress responsive protein activation, GTP activation, and ascorbic acid biosynthesis and promotes the stability and integrity of cell membranes, as well as the synthesis of cell walls, providing physical defense for seeds to resist low temperatures. At the same time, BR triggers specific involvement in ribosome synthesis and amino acid synthesis, promoting biosynthetic ability and metabolic regulation to maintain plant life activities under low-temperature stress. Furthermore, the various genes’ expression (OsJ_16716, OsPAL1, RINO1) confirmed GA3 and BR involved in peroxide metabolism, phenylpropanoid biosynthesis, and inositol phosphate metabolism, enhancing antioxidant capacity and providing energy substances for germination. This study provides valuable insights into how rice seed embryo responds to and tolerates chilling stress with GA3 seed priming.

Cadmium (Cd) contamination in agricultural soil is a global concern for soil health and food sustainability because it can cause Cd accumulation in cereal grains. An in-situ stabilizing technology (using organic amendments) has been widely used for Cd remediation in arable lands. Therefore, the current study examined the influence of vermicompost (VC) on soil biochemical traits, bacterial community diversity and composition, Cd uptake and accumulation in rice plants and grain yield in a Cd-contaminated soil during the late growing season in 2022. Different doses of VC (i.e., V1 = 0 t ha(-1), V2 = 3 t ha(-1) and V3 = 6 t ha(-1)) and two concentrations of Cd (i.e., Cd1 = 0 and Cd2 = 50 mg Cd Kg(-1) were used. We performed high-throughput sequencing of 16S ribosomal RNA gene amplicons to characterize soil bacterial communities. The addition of VC considerably affected the diversity and composition of the soil bacterial community; and increased the relative abundance of phyla Chloroflexi, Proteobacteria, Acidobacteriota, Plantomycetota, Gemmatimonadota, Patescibacteria and Firmicute. In addition, VC application, particularly High VC treatment, exhibited the highest bacterial diversity and richness (i.e., Simpson, Shannon, ACE, and Chao 1 indexes) of all treatments. Similarly, the VC application increased the soil chemical traits, including soil pH, soil organic carbon (SOC), available nitrogen (AN), total nitrogen (TN), total potassium (TK), total phosphorous (TP) and enzyme activities (i.e., acid phosphatase, catalase, urease and invertase) compared to non-VC treated soil under Cd stress. The average increase in SOC, TN, AN, TK and TP were 5.75%, 41.15%, 18.51%, 12.31%, 25.45% and 29.67%, respectively, in the High VC treatment (Pos-Cd + VC3) compared with Cd stressed soil. Redundancy analysis revealed that the leading bacterial phyla were associated with SOC, AN, TN, TP and pH, although the relative abundance of Firmicutes, Proteobacteria, Bacteroidata, and Acidobacteria on a phylum basis and Actinobacteria, Gammaproteobacteria and Myxococcia on a class basis, were highly correlated with soil environmental factors. Moreover, the VC application counteracted the adverse effects of Cd on plants and significantly reduced the Cd uptake and accumulation in rice organs, such as roots, stem + leaves and grain under Cd stress conditions. Similarly, applying VC significantly increased the fragrant rice grain yield and yield traits under Cd toxicity. The correlation analysis showed that the increased soil quantities traits were crucial in obtaining high rice grain yield. Generally, the findings of this research demonstrate that the application of VC in paddy fields could be useful for growers in Southern China by sustainably enhancing soil functionality and crop production.

In order to explore the effects of different silicon fertilizers and their application rates on grain quality, Zhuang-xiangyou Baijin5,the main indica hybrid rice variety in Guangxi, was used as the test material in 2021,and the two-factor split-plot experiment was conducted.The main area of silicon fertilizer application rate was 4 levels: Si0（0 kg/hm~2）,Si1（90 kg/hm~2）,Si2（180 kg/hm~2） and Si3（270 kg/hm~2）,the silicon fertilizer varieties were the secondary area, including two silicon fertilizer varieties: Xianyinong（K1,SiO 2 ＞25%）and Guozhenggui（K2,SiO 2 ＞29%）.The results showed that the application of silicon fertilizer significantly improved the appearance and eating quality of Zhuangxiangyou Baijin5 grain, and its chalkiness rate and chalkiness degree decreased.The content of protein increased slightly; with the decrease of amylose content, the peak viscosity, trough viscosity, final viscosity, breakdown viscosity and setback viscosity in RVA spectrum of grain decreased, and the pasting temperature increased, but no significant influence on the processing quality.The effect was better when the amount of silicon fertilizer was low, and the suitable amount of silicon fertilizer for Zhuangxiangyou Baijin5 was 90 kg/hm~2（Guozhenggui）.

Plant productivity, soil quality, and nitrogen uptake can be increased via the combined application of biochar and arbuscular mycorrhizal fungi (AMF). Here, we evaluate the effects of the combination of four different rates of biochar (B) (B0: 0 t ha−1, B1: 20 t ha−1, B2: 40 t ha−1, and B3: 60 t ha−1) and four rates of AMF (M) (M0: 0 g polybag−1, M1: 15 g polybag−1, M2: 30 g polybag−1, and M3: 45 g polybag−1) on the rice (Oryza sativa L.) cultivar Trisakti, grown in polybags using a completely randomized design with three replications. Our results show that the combination of 60 t Biochar ha−1 and 45 g AMF polybag−1 (B3M3) was the best treatment for improving some parameters, such as soil porosity (with the highest values of 68.25 and 68.45%), BD (0.88 and 0.88 g cm−3), pH (6.77 and 6.76), SOM (3.05 and 3.02%), TN (0.48 and 0.47%), AP (31.04 and 31.15 ppm), AK (235.11 and 235.20 ppm), plant height (116.78 and 117 cm), SPAD chlorophyll at maturity stage (43.59 and 43.88), flag leaf area (15.12 and 15.33 cm2), root length (42.10 and 42.17 cm), root volume (53.79 and 53.08 cm3), and shoot dry matter (59.29 and 59.66 g), in the early and late season, respectively. However, the combination of 20 t Biochar ha−1 and 45 g AMF polybag−1 (B1M3) was the best treatment for enhancing the tiller number with the maximum values (52.67 and 53.22), flowering day (67 and 66 day), root dry matter (32.37 and 32.51 g), panicle number (34.67 and 35.21), panicle length (21.44 and 21.67 cm), 1000 grain weight (41.26 and 41.37 g), and nitrogen uptake (32.37 and 32.51 g polybag−1), in the early and late season, respectively. These findings indicate that rice growth and productivity, the physical and chemical soil characteristics, and nitrogen uptake were better with the combined application of biochar and AMF treatments than sole biochar, sole AMF, or the control treatments.

Biochar has been widely applied as a soil modifier to improve the physical properties and increase the water content of the soil. Although the response of soil hydrological properties to biochar has been extensively studied, the effect of biochar on soil hydrological properties in different soil textures has not been comprehensively quantified. Here, we performed a meta-analysis of 939 observations to quantify the effects of biochar on the water retention capacity of soil with different textures. The effect of biochar on field capacity (FC) and available water capacity (AWC) of the soil was greater in coarse-textured soils (23.8%, 25.6%) than those in medium(5.0%, 20.9%) and fine-textured soils (7.2%, 11.5%). The permanent wilting point increased in coarse-textured soils (19.9%), decreased in medium-textured soils (-4.2%), and did not change considerably in fine-textured soils as affected by biochar. We demonstrated that FC is not always the main driver of increased AWC. In coarse-textured soils, there is a high correlation between hydrological properties. In addition, the specific surface area, surface functional groups, and cation exchange capacity of biochar are important factors affecting the water retention of soils with different textures. Our study establishes that the differences in soil texture cause heterogeneity in the mechanisms by which biochar improves soil water retention.

Current soil management practices are highly dependent on chemical fertilizers, which threaten human health, the environment, and crop yield. Eco-friendly strategies have influenced various applications of plant growth-promoting rhizobacteria (PGPR), endo-mycorrhizal fungi, cyanobacteria, and numerous other beneficial microorganisms, which have enhanced plant growth yield by enhancing nutrient uptake and plant tolerance to abiotic stress. Thus, using beneficial microorganisms as bio-fertilizers has become vital in agriculture due to their potential to improve food safety and agricultural production sustainability. Microbial bio-fertilizers may be a viable solution for maintaining productivity while implementing eco-friendlier and integrative nutrient management techniques. They have been developed to use naturally occurring nutrient transport mechanisms, which boost soil health and crop productivity. Plant nutrients are critical for crop productivity and the production of nutritious food for the world's ever-growing population. The application of bio-fertilizers promotes plant water and nutrients uptake, growth, and tolerance to abiotic and biotic stresses. These possible biological fertilizers can perform an imperative and significant part in soil production on a sustainable basis with cost-effective inputs for agriculture production. This chapter focuses on how bio-fertilizers control crop functional attributes such as growth and yield of plants, nutrient characteristics, and plant defensive performance and protection, with a particular emphasis on their function to activate numerous growth- and defense-related genes in the signalling network of cellular pathways, causing cellular responses and thus crop improvement. Several biotic and abiotic factors affect crop production globally. The published information will help us understand the physiological basis of bio-fertilizers for sustainable farming in reducing issues related to using chemical fertilizers.

Introduction/Background Direct-seeded rice is exceptionally vulnerable to chilling stress, especially at the seed germination and seedling growth stages in the early season of the double cropping system. Methods Therefore, we conducted two experiments to evaluate the role of various seed primings and their different concentrations of plant growth regulators [experiment 1—abscisic acid (ABA), gibberellin (GA 3 ), salicylic acid (SA), brassinolide (BR), paclobutrazol, uniconazole (UN), melatonin (MT), and jasmonic acid (JA)] and osmopriming substances (chitosan, polyethylene glycol 6000 (PEG6000), and CaCl 2 ) and experiment 2—GA, BR (two best), CaCl 2 (worst), and control (CK)] on rice seedlings under low temperature condition. Results Results showed that the maximum germination rate of 98% was recorded in GA 3 (10 mgL −1 ) and BR (0.3 mgL −1 ) among treatments. Compared to CK, root and shoot length were improved in ABA (0.5 mgL −1 ) and GA 3 (100 mgL −1 ) by 64% and 68%, respectively. At the same time, root and shoot weights (fresh and dry) were enhanced in Paclobutrazol (300 mgL −1 ) and GA3 among treatments. Furthermore, the average root volume, average root diameter, and total root surface area were increased by 27%, 38%, and 33% in Paclobutrazol (300 mgL −1 ), Paclobutrazol (200 mgL −1 ) and JA (1 mgL −1 ) treatments, respectively compared to CK. In the second experiment, a respective increase of 26%, 19%, 38%, and 59% was noted in SOD, POD, CAT, and APX enzyme activities in GA treatment compared to CK. Similarly, proline, soluble sugar, soluble protein, and GA content were also improved by 42%, 25.74%, 27%, and 19%, respectively, in GA treatment compared to CK. However, a respective reduction of 21% and 18% was noted in MDA and ABA content in GA treatment compared to CK. Our finding highlighted that better germination of primed-rice seedlings was associated with fresh and dry weights of the roots and shoots and the average root volume of the seedlings. Discussion Our results suggested that GA 3 (10 mg L −1 ) and BR (0.3 mg L −1 ) seed priming prevent rice seedlings from chilling-induced oxidative stress by regulating antioxidant enzyme activities and maintaining ABA, GA, MDA, soluble sugar, and protein content. However, further studies (transcriptome and proteome) are needed to explore the molecular mechanisms involved in seed priming-induced chilling tolerance under field conditions.

Excessive application of nitrogen fertilizer during rice cultivation leads to progressive soil contamination in the long term and increases production costs. An alternative to reduce over fertilization is to partially replace the fertilizer with microbes that promote nutrition and growth, such as arbuscular mycorrhizal fungi (AMF). We investigated the combination of four different rates of AMF (M): (M0: 0 g polybag−1, M1: 15 g polybag−1, M2: 30 g polybag−1, and M3: 45 g polybag−1) and three rates of nitrogen (N) fertilizer: (N0: 0 kg N ha−1, N1: 90 kg N ha−1, N2: 180 kg N ha−1) on Trisakti rice cultivar cultivated in polybag. Our findings indicate that the combination of 45 g AMF polybag−1 and 180 kg N ha−1 decreased soil bulk density by 38.02% and 37.24%, increased soil pH by 14.81% and 14.95%, soil porosity by 60.68% and 61.09%, soil organic matter by 28.62% and 30.46%, total N by 92.59% and 89.66%, available phosphorus by 30.12% and 29.85%, available potassium by 3.75% and 4.01%, rice plant height by 19.19% and 19.79%, tiller number by 25.27% and 26.08%, SPAD by 20.71% and 20.62%, flag leaf area by 107.76% and 108.02%, panicle length by 49.72% and 52.31%, panicle number by 67.44% and 72.35%, 1000-grain weight by 30.70% and 32.44%, root dry matter by 54.34% and 53.69%, shoot dry matter by 26.08% and 28.26%, root length by 54.68% and 56.44%, root volume by 42.73% and 43.37%, and N uptake by 107.93% and 108.06% compared to control during the early and late seasons, respectively. Conclusively, the combined application of AMF and N fertilizer increased the physiochemical properties, rice growth, rice productivity, and N uptake compared to AMF alone, N fertilizer alone, and the control treatment.

Although organic fertilizer is usually recommended for improving soil quality and crop yields, it inevitably enhances methane (CH4) emissions. Effective measures to mitigate CH4 emissions while ensuring crop yields are urgently needed. A two-year experiment was performed to examine the effects of organic-inorganic fertilizer combinations on CH4 emissions, the CH4-related microbial community, soil biochemical characteristics, and rice yields during dual rice growing seasons in 2020-2021 at two locations (i.e., Nanning and Yulin City, Southern, China). The treatments were as follows: no N fertilizer (Neg-CF), 100 % chemical fertilizer (CF) (Pos-CF), 60 % cattle manure (CM) + 40 % CF (High-CM), 30 % CM + 70 % CF (Low-CM), 60 % poultry manure (PM) + 40 % CF (High-PM), and 30 % PM + 70 % CF (Low-PM). CH4 fluxes and related functional microorganism abundances were investigated using the closed chamber method and molecular procedures, respectively. The addition of organic manure significantly improved rice grain yields and soil properties, including total nitrogen (TN), soil organic carbon (SOC), and pH compared to the Pos-CF. Similarly, integrating organic and inorganic fertilizers led to significant increases in seasonal CH4 emissions, global warming potential (GWP), and the abundance of CH4related soil microbes. Average increases in SOC, TN, CH4 emissions, and GWP in the High-PM treatment relative to the Pos-CF were 44.7, 36.4, 20.6, and 24.4 %, respectively, in Nanning; and 28.9, 33.3, 23.4, and 20.8 % in Yulin City, averaged across the years. Moreover, co-fertilization resulted in the highest increase in methanogenic diversity and abundance relative to Pos-CF only. Applying high proportions of manure improved the abundance of methanogenic soil archaea related to CH4 production compared with the plots with low proportions of manure. Interestingly, lower manure amendments produced the highest rice grain yields and lowest CH4 emissions while maintaining high soil qualities. In addition, regression analysis exhibited that SOC was highly positively associated with CH4 emissions. Above all, controlling the proportions of organic manure and CF may be a feasible approach for enhancing rice yields and soil quality while mitigating CH4 emissions. Of the treatments tested, the combination of 30 % organic manure (i.e., CM or PM) with 70 % CF (urea) performed the best. This study showed that combining manure and CF at a 30:70 ratio was the most effective approach for improving soil health and rice yields while ensuring environmental sustainability.

Biochar is an important soil amendment that can enhance the biological properties of soil, as well as nitrogen (N) uptake and utilization in N-fertilized crops. However, few studies have characterized the effects of urea and biochar application on soil biochemical traits and its effect on paddy rice. Therefore, a field trial was conducted in the early and late seasons of 2020 in a randomized complete block design with two N levels (135 and 180 kg ha−1) and four levels of biochar (0, 10, 20, and 30 t ha−1). The treatment combinations were as follows: 135 kg N ha−1 + 0 t B ha−1 (T1), 135 kg N ha−1 + 10 t B ha−1 (T2), 135 kg N ha−1 + 20 t B ha−1 (T3), 135 kg N ha−1 + 30 t B ha−1 (T4), 180 kg N ha−1 + 0 t B ha−1 (T5), 180 kg N ha−1 + 10 t B ha−1 (T6), 180 kg N ha−1 + 20 t B ha−1 (T7) and 180 kg N ha−1 + 30 t B ha−1 (T8). The results showed that soil amended with biochar had higher soil pH, soil organic carbon content, total nitrogen content, and mineral nitrogen (NH4+-N and NO3−-N) than soil that had not been amended with biochar. In both seasons, the 20 t ha−1 and 30 t ha−1 biochar treatments had the highest an average concentrations of NO3–-N (10.54 mg kg−1 and 10.25 mg kg−1, respectively). In comparison to soil that had not been treated with biochar, the average activity of the enzymes urease, polyphenol oxidase, dehydrogenase, and chitinase was, respectively, 25.28%, 14.13%, 67.76%, and 22.26% greater; however, the activity of the enzyme catalase was 15.06% lower in both seasons. Application of biochar considerably increased the abundance of ammonia-oxidizing bacteria (AOB), which was 48% greater on average in biochar-amended soil than in unamended soil. However, there were no significant variations in the abundances of ammonia-oxidizing archaea (AOA) or nitrite-oxidizing bacteria (NOB) across treatments. In comparison to soil that had not been treated with biochar, the average N content was 24.46%, 20.47%, and 19.08% higher in the stem, leaves, and panicles, respectively. In general, adding biochar at a rate of 20 to 30 t ha−1 with low-dose urea (135 kg N ha−1) is a beneficial technique for improving the nutrient balance and biological processes of soil, as well as the N uptake and grain yield of rice plants.