Legume-Based Rotations Enhance Ecosystem Sustainability in the North China Plain: Trade-Offs Between Greenhouse Gas Mitigation, Soil Carbon Sequestration, and Economic Viability

Reconciling agricultural productivity with greenhouse gas (GHG) mitigation remains a pivotal challenge for achieving climate-smart food systems. This study evaluates the capacity of legume-based crop rotations to balance economic viability, yield stability, and GHG reduction in the North China Plain. A two-year randomized complete block field experiment compared six cropping systems: conventional wheat–maize (WM) rotations and legume-integrated systems (wheat–soybean, WS; wheat–soybean–maize, WSM), under fertilized and unfertilized regimes. Results revealed that nitrogen fertilization increased cumulative N2O emissions and global warming potential (GWP), with seasonal peaks occurring post-fertilization. Legume systems enhanced CH4 uptake but showed no significant effect on N2O emissions compared to conventional systems. N2O fluxes correlated positively with soil moisture and soil temperature, while CH4 uptake increased with soil moisture alone. Soybean phases reduced short-term yields by 32–52% relative to the maize yield of conventional systems, but boosted subsequent wheat/maize productivity by 2–47% through hydraulic redistribution and N priming. The wheat–soybean rotation with 200 kg N ha−1 (WS200) achieved optimal sustainability, delivering the highest net profit (8061.56 USD ha−1) alongside a 9% reduction in global warming potential (3980.21 kg CO2-eq ha−1) versus conventional systems. These findings provide actionable insights for sustainable intensification in global cereal systems, demonstrating that strategic legume integration can advance both food security and climate goals.