Direct and Residual Effects of Green Manure and Fertilizer Nitrogen in a Rice-Rapeseed Production System in the Semiarid Subtropics

Abstract

ABSTRACT Integrating fertilizer nitrogen (FN) with legume green manure (GM) could lead to sustainable and environmentally sound agricultural systems in subtropical-semiarid soils low in organic matter. A six-year replicated field experiment was conducted on a semiarid irrigated Tolewal sandy loam soil (Typic Ustochrepts) in northwest India. The purpose of the experiment was to evaluate the direct and residual effects and cost effectiveness of cowpea (Vigna unguiculata L.) grown as a GM-crop in situ for 45–50 days during either the hot-dry period (May-June) prior to transplanting of rice (Oryza sativa L.) or the mild-rainy season (September-October) prior to seeding of rapeseed (Brassica napus L.) in rice-rapeseed double cropping annual rotations. Rice grain yields with incorporation of 20 t GM ha−1 (GM20) ranged from 3.93 to 5.28t ha−1, which were 48 to 78% greater than the no-N control. Rice yield with 60 kg FN ha−1 (FN60) and GM20 was more than double the control yield and 24% greater than the otherwise optimum rate of 120 kg FN ha−1. Incorporation of 20t GM ha−1 to rapeseed produced 28 to 50% greater yields than the control. Combined application of GM20 with 100 kg FN ha−1 (recommended N rate in this region) further improved the yield potential of rapeseed by 16%. While FN had no residual effects, residual GM produced 9 and 35% greater yields of a succeeding crop due to the supply of N equivalent to 17 and 44 kg FN ha−1 in rice-applied and rapeseed-applied GM, respectively. Nitrogen utilization by the rice-rapeseed system was greater for GM20 than FN, as indicated by greater apparent N recovery by crops of 67% versus 51%. These results suggest that (a) green manuring in rice-rapeseed production systems is cost effective, and (b) under constrained water resources, GM produced during the mild-rainy season and applied to rapeseed is more beneficial than rice-applied GM.