KESHAB RAJ PANDE, SHREE CHANDRA SHAH, MATHIAS BECKER
University of Bonn, Plant Nutrition in the Tropics and Subtropics, Germany
Tribhuvan University, Department of Plant Nutrition, Nepal
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 week-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 build-up in soils during the eight weeks DWT by N immobilization in plant or microbial biomass was studied under field conditions in Rampur, Nepal in 2002. Treatments included bare soil (farmers' practice), green manure (Mucuna pruriens var utilis), grain legume (Vigna radiata L.), wheat straw (5Mgha) and combinations of straw application and transition season crops. The gradual increase in soil moisture with the onset of the rainy season resulted in a nitrate peak of about 60kgNha that rapidly declined and nearly completely disappeared with soil moisture levels exceeding 46% water-filled pore space. Incorporation of wheat straw and/or N uptake by green manure crops reduced nitrate accumulation in the soil to (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 uptake at least 37kgha and corresponding grain yield increases from 1.7 to 3.0Mgha. 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 farm level require further studies.
Keywords: Biological nitrogen fixation, denitrification, Mucuna, Oryza sativa, Triticum aestivum, Vigna radiata