Straw Biochar Incorporation Reduces CH4 Emissions in Double-Cropping Rice Paddies and Partially Alleviates Warming-Induced Yield Losses

Straw biochar has the potential to improve rice productivity and mitigate greenhouse gas emissions. However, its synergistic regulatory effects on yield performance and methane (CH4) emissions in double-cropping rice systems, especially under the intermediate climate warming scenario, remain poorly understood. A two-year field experiment was carried out from 2022 to 2023 with four treatments: ambient temperature with direct straw incorporation (CKS), ambient temperature with straw biochar incorporation (CKB), warming with direct straw incorporation (WS), and warming with straw biochar incorporation (WB). Rice yield and yield components, CH4 emission characteristics, soil physicochemical properties, and functional microbial abundance were systematically determined. The results showed that for early rice, compared with CKS, CKB maintained a stable yield, while WS had a significantly reduced yield of 7.01% (p < 0.05); compared with WS, WB had an increased yield of 4.65% (p > 0.05), which was mainly attributed to the increases in grain weight, dry matter accumulation from heading to maturity, and leaf SPAD value after heading. For late rice, compared with CKS, CKB had no significant difference in yield, while WS had a decreased yield of 8.47% (p < 0.05); compared with WS, WB had a significantly increased yield of 6.47% (p < 0.05), mainly due to the increases in spikelets per panicle, dry matter accumulation at tillering, as well as the number of secondary branches and spikelets on secondary branches. For CH4 emissions, compared with CKS, CKB significantly reduced CH4 emissions and yield-scaled CH4 emissions by 48.67% (p < 0.05) and 47.75% (p < 0.05) in early rice, and by 35.22% (p < 0.05) and 37.22% (p < 0.05) in late rice, respectively. Compared with WS, WB significantly reduced CH4 emissions and yield-scaled CH4 emissions by 57.81% (p < 0.05) and 59.61% (p < 0.05) in early rice, and by 42.55% (p < 0.05) and 46.09% (p < 0.05) in late rice, respectively. Correlation analysis revealed that CKB and WB reduced CH4 emissions by decreasing soil NH4+-N, DOC, and methanogen abundance in early rice, and by lowering DOC and methanogen abundance in late rice. In summary, straw biochar incorporation can significantly reduce CH4 emissions from double-cropping rice paddies under ambient temperature. Under the intermediate climate warming scenario, this practice can partially mitigate yield losses (significant in the late-rice season) and simultaneously lower CH4 emissions. It is an efficient agricultural management strategy adapted to climate warming, providing a key scientific basis and feasible technical pathways for sustainable production in double-cropping rice regions of South China.