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

Zhao, Xu, Wang, Shenqiang, Xing, Guangxi
Agriculture, ecosystems & environment 2015 v.202 pp. 79-89
Astragalus, Vigna umbellata, ammonia, crop residues, crop rotation, ecological value, economic analysis, faba beans, farmers, forage legumes, grain yield, growing season, lakes, leaching, nitrogen fertilizers, nitrous oxide, planting, pollution, rice, risk, runoff, volatilization, winter, winter wheat, China
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.