Improving net carbon balance and irrigation water productivity of subtropical organic double-rice cropping through a near-saturation water-saving management

Organic rice farming sustains soil productivity but risks elevated greenhouse gas (GHG) emissions from organic inputs, which can be mitigated by optimizing water-saving irrigation and implementing regenerative practices like cover cropping. Compared to the most common water-saving irrigation (-15 cm water level threshold), a more conservative near-saturation approach (reflooding at 0 cm) offers a potential paradigm for stabilizing yields; however, its integrated impact on the net carbon (C) outcomes remains poorly understood. This study evaluated a near-saturation water-saving strategy and winter cover cropping [Tartary buckwheat ( Fagopyrum tataricum )] to improve net C balance characteristic of subtropical organic double-rice systems in Taiwan (2023–2024). Compared to continuous flooding (CF), the near-saturation threshold significantly reduced cumulative CH 4 emissions and net GHG intensity by approximately 29% (1.37 vs. 0.97 Mg CO 2 -eq Mg −1 grain) while maintaining stable grain yield (∼5.24 Mg ha −1 ). Irrigation water productivity improved by 16.5% (0.92 vs. 0.79 kg grain m −3 irrigated water) under this near-saturation management. Furthermore, the near-saturation strategy resulted in a 17.3% reduction (CF: 23.32 Mg CO 2 -eq ha −1 vs. water-saving: 19.28 Mg CO 2 -eq ha −1 ) in the net ecosystem C source strength (net GHG emissions minus soil organic C sequestration) through the substantial CH 4 mitigation over the two-year period. Winter cover crop incorporation did not significantly influence GHG dynamics or crop productivity. These results demonstrate that a precision near-saturation threshold optimizes the environmental footprint of organic rice farming by significantly reducing net C source strength and improving water performance without compromising yield stability.