Winter legume-rice rotations can reduce nitrogen pollution and carbon footprint while maintaining net ecosystem economic benefits

Abstract

Abstract Achieving reductions in nitrogen (N) losses and carbon (C) emissions without enduring a yield penalty is an environmental and economic challenge in sustainable rice production. The use of legumes as a winter crop in rice rotations may provide environmental benefits by reducing synthetic N inputs, yet few studies have integrated long-term field measurements of cropping system N and C dynamics with life cycle assessment (LCA) and net ecosystem economic benefits (NEEB) to determine whether legumes can improve environmental performance while minimizing tradeoffs related to yields and economic returns. We evaluated four contrasting rice cropping rotations (Rice-wheat (R-W); rice-rape (R-Ra); rice-fava bean (R-F); and, rice-milk vetch (R-M)) over six years to determine N input and output balances, methane (CH4) emissions, and soil C changes. These field observations were then incorporated into LCA and NEEB to estimate C footprint, economic and environmental benefits. Results showed that R-F and R-M maintained rice yield but reduced annual synthetic N inputs by 50–63% compared with the conventional R-W and R-Ra rotations, leading to consistent reductions in reactive N losses (ammonia (NH3) volatilization: 39–48%; N runoff: 66–82%; N leaching: 14–34%; and nitrous oxide (N2O) emissions: 40–64%). The estimated C footprint was 37–50% lower in R-F and R-M than R-W and R-Ra, largely owing to reduced fertilizer use which decreased direct soil N2O emissions as well as indirect emissions relating to reactive N losses. In contrast to N losses, there were no significant differences in CH4 emissions or soil C changes among rotations. When changes in N pollution and C footprint were accounted for in the economic assessment, R-F resulted in NEEB values similar to R-W, while substituting milk vetch as a winter crop reduced NEEB by 6–37%. In the first two study years before grain legume yields declined, NEEB for R-F was 38% greater than R-W, highlighting the potential for simultaneous environmental and economic benefits. This study demonstrated the potential of mixed winter grain/forage legumes-rice crop rotations to consistently reduce N pollution and C footprint while maintaining NEEB based on economic and environmental benefits.