Rj. Buresh et al., NITROGEN LOSSES IN PUDDLED SOILS AS AFFECTED BY TIMING OF WATER-DEFICIT AND NITROGEN-FERTILIZATION, Plant and soil, 157(2), 1993, pp. 197-206
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.