Yields and Nitrogen Dynamics in a Rice–Wheat System Using Green Manure and Inorganic Fertilizer

Integrating fertilizer N (FN) with legume green manures (GM) can foster sustainable and environmentally sound agricultural systems in subtropical, semiarid soils low in organic matter. A 4‐yr replicated field experiment with irrigated rice ( Oryza sativa L.) and wheat ( Triticum aestivum L.) rotations was conducted on a sandy loam soil in the Punjab of India to evaluate the effects of incorporating cowpea ( Vigna unguiculata L.) or sesbania ( Sesbania aculeata L.) GM on crop productivity, N‐use efficiency, and NO 3 leaching. Rice grain yields with pretransplant incorporation of 20 and 40 Mg GM ha −1 (GM 20 and GM 40 ) ranged from 5.18 to 5.81 Mg ha −1 , which were equivalent to the 5.40 Mg ha −1 obtained with 120 kg FN ha −1 (FN 120 ). Rice yield with 60 kg FN ha −1 (FN 60 ) and GM 20 was double the control yield and 6% greater than the FN 120 treatment. Nitrogen utilization by rice was greater for GM 20 than FN, as indicated by greater fertilizer N equivalents and an apparent N recovery by rice of 79 vs. 63%. While FN had no residual effects, residual GM produced greater wheat yields, indicating that FN applied to wheat could be reduced by 25%. Soil NO 3 –N leaching to 60 cm in the rice crop was apparently used by the subsequent wheat crop, which has a deeper and more extensive rooting system. After 4 yr, use of FN 120 for both crops resulted in 35 kg of residual NO 3 –N ha −1 in the 150‐cm soil profile (74% below 90 cm), whereas only 19 kg NO 3 –N ha −1 remained with GM 20 plus FN 60 . Use of GM 20 with FN 60 for a rice–wheat rotation demonstrated greater yields of rice and wheat, 25 to 50% reduction in fertilizer N use, and decreased potential for groundwater NO 3 contamination.