NITRATE DYNAMICS DURING THE AEROBIC SOIL PHASE IN LOWLAND RICE-BASED CROPPING SYSTEMS

In tropical rice (Oryza sativa L.) lowlands, soil NO3 is lost during the transition from the dry to the wet season. To understand how soil and crop management influences NO3 loss,we examined NO3 dynamics during a 2-yr period in an Alfisol in the Philippines: weedy, weed free, and frequently tilled main plots during the February to May dry season (DS), and Sesbania rostrata (Brem. and Oberm), mungbean [Vigna radiata (L.) R. Wilczek var. radiata], weedy, and weed free subplots during the May to July dry-to-wet transition (DWT). Weed-free plots were maintained by removing weeds as they emerged. Soil NH4, (0-60 cm), which was not affected by management, averaged only 9 kg N ha(-1). While soil NO3 increased under frequent tillage and weed-free following, it decreased rapidly under weedy fallowing. On most sampling dates, NO3, was the highest in DS tilled main plots. The widest range of NO3 during the DS or DWT was 14 to 110 kg N ha(-1) in the first year, and 12 to 155 kg N ha(-1) in the second. During the second half of the DWT, NO3 declined in all plots, but more markedly when plants were present than when not, indicating plant N uptake. Aboveground plant N prior to permanent flooding ranged widely from 31 kg N ha(-1) in weeds to 222 kg N ha(-1) in N-2-fixing S. rostrata plants in the first year, and 37 to 193 kg N ha(-1) in the second. The data also indicate NO3 leaching following heavy rains. Further, the high water-filled pore space, exceeding 0.7 L L(-1) in the second half of the DWT and approaching 1 L L(-1) with permanent flooding, is presumed to have favored denitrification. Regardless of DS management or DWT plant N accumulation, the soil was virtually depleted of NO3 soon after permanent flooding; NO3 rarely exceeded 10 kg N ha(-1) when measured after 9 d (first year) and 11 d (second year) of permanent flooding. Our data indicate the immense capacity of this lowland soil to accumulate NO3 and the marked effect of DS and DWT management on the amount of NO3 that actually accumulates. In tropical rice lowlands, soil and crop management during the DS should be designed to limit NO3 buildup so as to reduce NO3 that is prone to lass during the DWT.