Synergistic effects of biochar and organic fertilizer on the regulation of microbial carbon and nitrogen metabolism in paddy soil to reduce net greenhouse gas emissions.

Biochar and organic fertilizer have been widely applied to improve soil carbon sequestration and mitigate greenhouse gas (GHG) emissions in rice production systems. However, the synergistic mechanisms through which these amendments regulate microbial carbon/nitrogen (C/N) metabolism, net GHG emissions, and crop productivity in dry direct-seeded rice (DDSR) systems remain insufficiently understood. In this study, a two-year field experiment was conducted with combined treatments of organic fertilizer substitution (0% and 20%) and biochar application (0, 5, and 10 t/hm2) to evaluate their integrated effects on GHG emissions, soil properties, rice yield, and microbial functional genes involved in C/N cycling. Biochar and organic fertilizer significantly reduced CH4 and N2O emissions, leading to lower global warming potential (GWP), greenhouse gas emission intensity (GHGI), and net greenhouse gas emissions (NGHGE), while simultaneously increasing rice yield. The combined application of 20% organic fertilizer substitution with 5 t/hm2 biochar achieved the optimal performance, increasing yield by 11.64%-15.44% and reducing GWP and GHGI by 7.58%-16.56% and 17.21%-27.72%, respectively. Metagenomic analysis revealed that the combined application modified microbial community structure while regulated microbial C/N metabolism by reshaping key functional pathways. The enhancement of Calvin-Benson-Basham (CBB) pathway was related to the increased abundance of PRK and prkB under biochar conditions; meanwhile, reduced abundances of aclA, aclB, and ccsA were associated with suppressed reductive tricarboxylic acid (rTCA) cycle. Concurrently, denitrification and nitrification pathways showed positive correlations with increased abundances of nirK, norB, and nosZ. Redundancy analysis (RDA) and mantel analyses indicated that soil organic carbon (SOC), pH, ammonium nitrogen (NH4+-N), and nitrate nitrogen (NO3--N) were the primary drivers shaping microbial functional gene structure. Overall, the synergistic application of biochar and organic fertilizer effectively coordinated carbon sequestration, emission reduction, and yield enhancement, providing a feasible strategy for low-carbon and sustainable management of DDSR.