NITROGEN LOSSES IN PUDDLED SOILS AS AFFECTED BY TIMING OF WATER-DEFICIT AND NITROGEN-FERTILIZATION

Erratic rainfall in rainfed lowlands and inadequate water supply in ir rigated lowlands can result in alternate soil drying and flooding duri ng a rice (Oryza sativa L.) cropping period. Effects of alternate soil drying and flooding on N loss by nitrification-denitrification have b een inconsistent in previous field research. To determine the effects of water deficit and urea timing on soil NO3 and NH4, floodwater NO3, and N loss from added N-15-labeled urea, a field experiment was conduc ted for 2 yr on an Andaqueptic Haplaquoll in the Philippines. Water re gimes were continuously flooded, not irrigated from 15 to 35 d after t ransplanting (DT), or not irrigated from 41 to 63 DT. The nitrogen tre atments in factorial combination with water regimes were no applied N and 80 kg urea-N ha-1, either applied half basally and half at 37 DT o r half at 11 DT and half at 65 DT. Water deficit at 15 to 35 DT and 41 to 63 DT, compared with continuous soil flooding, significantly reduc ed extractable NH4 in the top 30-cm soil layer and resulted in signifi cant but small (< 1.0 kg N ha-1) soil NO3 accumulations. Soil NO3, whi ch accumulated during the water deficit, rapidly disappeared after ref looding. Water deficit at 15 to 35 DT, unlike that at 41 to 63 DT, inc reased the gaseous loss of added urea N as determined from unrecovered N-15 in N-15 balances. The results indicate that application of urea to young rice in saturated or flooded soil results in large, rapid los ses of N (mean = 35% of applied N), presumably by NH3 volatilization. Subsequent soil drying and flooding during the vegetative growth phase can result in additional N loss (mean = 14% of applied N), presumably by nitrification-denitrification. This additional N loss due to soil drying and flooding decreases with increasing crop age, apparently bec ause of increased competition by rice with soil microorganisms for NH4 and NO3.