Seasonal soil nitrogen dynamics in rice-wheat cropping systems of Nepal

Abstract

The rice-wheat annual double cropping system occupies some 0.5 million ha in the Himalayan foothills of Nepal. Alternating soil drying and wetting cycles characterize the 6-10 weeks long dry-to-wet season transition period (DWT) after wheat harvesting and before wetland rice transplanting. Mineral fertilizer use in the predominant smallholder agriculture is low and crops rely largely on native soil N for their nutrition. Changes in soil aeration status during DWT are likely to stimulate soil N losses. The effect of management options that avoid the nitrate buildup in soils during DWT by N immobilization in plant or microbial biomass was studied under controlled conditions in a greenhouse (2001/2002) and validated under field conditions in Nepal in 2002. In potted soil in the greenhouse, the gradual increase in soil moisture resulted in a nitrate N peak of 20 mg (kg soil)-1 that rapidly declined as soil moisture levels exceeded 40% water-filled pore space (equiv. 75% field capacity). Similarly, the maximum soil nitrate build-up of 40 kg N ha-1 under field conditions was followed by its near complete disappearance with soil moisture levels exceeding 46% water-filled pore space at the onset of the monsoon rains. Incorporation of wheat straw and/or N uptake by green manure crops reduced nitrate accumulation in the soil to < 5 mg N kg-1 in pots and < 30 kg N ha-1 in the field (temporary N immobilization), thus reducing the risk for N losses to occur. This "saved" N benefited the subsequent crop of lowland rice with increases in N accumulation from 130 mg pot-1 (bare soil) to 185 mg pot-1 (green manure plus wheat straw) and corresponding grain yield increases from 1.7 Mg ha-1 to 3.6 Mg ha-1 in the field. While benefits from improved soil N management on lowland rice are obvious, possible carry-over effects on wheat and the feasibility of proposed options at the farm level require further studies.