LEGUMES AS NITRATE CATCH CROPS DURING THE DRY-TO-WET TRANSITION IN LOWLAND RICE CROPPING SYSTEMS

In tropical rice (Oryza sativa L.) lowlands, NO3- assimilation by plants during the transition from the dry to the wet season can preclude NO3- loss upon soil flooding and permit recycling of this N. In a 2-yr field study in an Alfisol in the Philippines, we examined the role of legume crops and weeds during the dry-to-wet (DTW) transition in conserving and/or recycling soil NO3- that accumulated under varied dry-season fallow management. During the May-to-July DTW transition, Sesbania rostrata (Bremek. & Oberm.), mungbean [Vigna radiata (L.) Wilczek], weedy, and weed-free treatments were subplots in the February-to-May dry-season mainplots of weedy, weed-free, and frequently tilled fallows. Legume biological N2 fixation (BNF) was measured by N-15 dilution. Depending on dry-season management, the maximum extracted N (top 60-cm layer) in the DTW transition ranged from 38 to 164 kg ha-1; this N was 62 to %% NO3-. Soil N uptake by weeds ranged from 31 to 46 kg N ha-1, that by mungbean from 29 to 80, and by S. rostrata from 46 to 125. The minimum estimates of DTW transition NO3- loss varied from none when plants were present to 107 kg N ha-1 in weed-free fallows. Legume BNF partially offset NO3- loss, although increased soil NO3- decreased BNF-N. Nitrogen fixed ranged from 37 to 63 kg N ha-1 by mungbean and 68 to 154 by S. rostrata. Harvest of mungbean caused negative BNF-N contribution to the succeeding rice crop in a few cases. In lowland rice-based cropping systems, weeds are effective in conserving soil N during the DTW transition, but legumes are more suitable nitrate catch crops because they allow the harvest of an economic product or the recycling of more N to a subsequent flooded rice crop.