Green manure substitution reduces carbon and nitrogen footprints and improves net ecosystem economic benefits in double rice systems

Sustainable rice production requires not only high yields but also reduced environmental impacts and improved economic outcomes. However, sustainability assessments often overlook key indicators such as the carbon footprint (CF), nitrogen footprint (NF), and net ecosystem economic benefit (NEEB). This study, therefore, aims to comprehensively evaluate the long-term effectiveness of substituting green manure (GM) for chemical nitrogen fertilizer (CNF) in improving productivity, nitrogen use efficiency, and NEEB, while simultaneously reducing CF and NF in a double rice cropping system. The experimental treatments included: no CNF (N0), conventional CNF (N100), N100 plus GM (GN100), 80 %N100 plus GM (GN80), 60 %N100 plus GM (GN60), and N0 plus GM (GN0). Annually, substituting GM for 20 % of N100 (GN80) increased grain yield by 12.2 %–99.1 %, harvest index by 3.4 %–20.8 %, nitrogen recovery efficiency by 38.5 %–148.0 % and NEEB by 19.0 %–129.0 % compared with N0, N100, GN100, GN60, and GN0 treatments. The results also revealed a strong positive correlation between CF and NF under GN80 and GN60 but showed trade-off relationships under N100 and GN100, indicating that reduced CNF substitution by GM can simultaneously mitigate CF and NF. In this study, methane emissions were the primary contributors to CF, while reactive nitrogen losses were the main drivers of NF. Compared with GN100 and GN0, GN80 significantly reduced annual CF by 24.1 % and 28.2 %, respectively. It also substantially lowered annual NF and yield-scaled NF by 17.3 % and 37.5 % relative to N100, and by 27.7 % and 31.3 % relative to GN100. Furthermore, GN80 notably decreased annual yield-scaled CF by 22 % compared to N0 and by up to 50 % compared to GN0. Therefore, we recommend the GM substitution for 20 % of N100 to enhance environmental sustainability without sacrificing yield or NEEB. These findings broaden our understanding of how substituting GM for CNF influences CF and NF, offering new insights for promoting sustainable double rice production.