Maintaining rice yield and reducing N pollution by substituting winter legume for wheat in a heavily-fertilized rice-based cropping system of southeast China

• We compared yields and N losses for rice/wheat and rice/legume rotations over 3 years.
• Growing legumes with no N fertilizer instead of wheat can maintain high rice yields.
• Substituting a winter legume for wheat results in a ∼50% savings in chemical N inputs.
• Rice/legume rotation reduces NH3 emissions in the rice season and N runoff in winter.
• Mixed grain/forage legume rotations can ensure returns and reduce environmental costs.

Substituting a N-fixing legume for winter wheat has recently been suggested as a feasible method for mitigating N pollution from heavily-fertilized rice/wheat cropping system in the Taihu Lake Plain of southeast China. To understand the agronomic and environmental value of planting legumes instead of wheat, a 3-year consecutive field observation encompassing six crop seasons was conducted to compare crop yields, chemical N inputs, and N losses for three crop rotation systems; rice/wheat (the control), rice/fava bean, and rice/milk vetch. Our data showed that 52.6–59.5% of the annual N input could be saved in the two rice/legume rotations as a result of no N fertilization in the legume growing season and the replacement of 13.2–25.7% of chemical N by leguminous N via crop residue incorporation in the rice season. This reduction in N fertilizer not only produced equivalent or slightly more rice yields compared to the control, but also reduced N loss by half. In terms of N loss, ammonia (NH3) volatilization during the rice season and N runoff in the winter season for the two rice/legume rotations were greatly reduced by 31.3–38.0% and 82.1–86.0%, respectively compared to the control. A decreasing trend was also found in N leaching and nitrous oxide (N2O) in both seasons, NH3 volatilization in the winter season, and N runoff in the rice season for the two rice/legume rotations. The preliminary economic evaluation of yield benefits, fertilizer costs, and environmental costs related to N losses suggested that a mixed rice/fava bean and rice/milk vetch crop rotation (50% of each type) could ensure farmers’ returns and achieve a half-maximum reduction in environmental risk. These results demonstrate that substituting winter grain/forage legumes for wheat may be a technically feasible, low-input solution to the N pollution problems in the intensive rice-based cropping systems in the Taihu Lake Plain.