Yields and nitrogen dynamics in a rice-wheat system using green manure and inorganic fertilizer

Integrating fertilizer N (FN) with legume green manures (GR I) 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 NO3 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) (FN120). Rice yield with 60 kg FN ha(-1) (FN60) and GM(20) was double the control yield and 6% greater than the FN120 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 NO3-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 FN120 for both crops resulted in 35 kg of residual NO3-N ha(-1) in the 150 cm soil profile (74% below 90 cm), whereas only 19 kg NO3-N ha(-1) remained with GM(20) plus FN60. Use of GM(20) with FN60 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 NO3 contamination.