Underlying mechanisms of high carbon budget surplus in low-stubble rice ratooning in Southeast China

The rice ratooning system has attracted increasing attention in southern China due to its low carbon emissions and high yield potential. However, the net carbon budget and underlying mechanisms remain unclear. Three rice cropping systems were established in this trial experiment conducted from 2021 to 2022 in Fuzhou (25°05xN, 119°13xE), Southeast China: ratooning rice (RR: MC+RSR) pattern for rice ratooning, single-cropping rice (LR₁), and double-cropping rice (DC: ER+LR₂). The closed static dark box gas collection, dry matter determination, life cycle assessment (LCA) etc. approaches were utilized to investigate the mechanism of “high carbon fixation–low emissions” mechanism in RR. A comprehensive assessment was conducted across multiple dimensions, including crop yield, greenhouse gas (GHG) emissions, carbon and nitrogen footprints, resource use efficiency, carbon sequestration capacity, and carbon budget balance. Results showed that the average daily yield of ratoon season rice (RSR) across RR treatments from 2021 to 2022 was 28.21– 47.40% higher than that of the main crop (MC) and LR₁, and the average daily yield of RR was 13.50–27.76% higher than DC. This yield advantage was attributed to a 32.32–39.26% increase in the allocation of ¹³C-labeled photosynthetic products (including non-structural carbohydrates, NSCs) to panicle organs, and a 21.77–43.51% reduction in allocation to underground roots and soil. Furthermore, the average daily global warming potential (GWP) was 16.44 kg CO₂-eq ha^–1 for RR, 24.99 kg CO₂-eq ha^–1 for LR₁, and 21.32 kg CO₂-eq ha^–1 for DC. Specifically, the average daily GWP of ratoon rice was 34.21% lower than that of LR₁ and 22.90% lower than double-cropping rice. Similarly, the average daily greenhouse gas intensity (GHGI) of ratoon rice was 62.28% lower than LR₁ and 28.96% lower than double-cropping rice. In terms of carbon and nitrogen footprints, the ratoon rice system exhibited average daily values of 34.54 kg CO₂-eq ha^–1 and 0.47 kg N ha^–1, respectively. In comparison, LR₁ had values of 45.63 kg CO₂-eq ha^–1 and 0.49 kg N ha^–1, while double-cropping rice showed 43.38 kg CO₂-eq ha^–1 and 0.53 kg N ha^–1. These values represent reductions of 24.30% in carbon footprint and 4.28% in nitrogen footprint relative to LR₁, and 20.38 and 11.45% relative to double-cropping rice, respectively. Moreover, the average annual carbon budget surplus across systems was 22,380.01 kg CO₂-eq ha^–1 for ratoon rice (MC+RSR), 11,228.54 kg CO₂-eq ha^–1 for LR₁, and 23,772.15 kg CO₂-eq ha^–1 for DC. Consequently, the resource utilization efficiency of the RR was 24.42 and 47.50% higher than that of single-cropping and double-cropping systems, respectively. Average daily economic returns also increased by 32.71 and 80.75%, respectively. These findings provide a robust theoretical foundation and practical guidance for advancing agricultural carbon neutrality technologies and ensuring food security.