DINITROGEN AND NITROUS-OXIDE FLUX FROM UREA BASALLY APPLIED TO PUDDLED RICE SOILS

Nitrification-denitrification is often regarded as an important N loss process in puddled rice soils, but few direct field measurements of denitrification loss have been reported. Field studies were conducted on a Typic Tropaquept in Thailand, an Aeric Tropaqualf in Indonesia, and an Andaqueptic Haplaquoll in the Philippines to measure (N2 + N2O)-N-15 flux, (N2 + N2O)-N-15 trapped in soil, and total gaseous N-15 loss following basal application of N-15-labeled urea (87 kg N ha-1) to irrigated rice (Oryza sativa L.) grown on puddled soils. Urea treatments included broadcast applications with and without incorporation to 0.07-m soil depth. Directly measured (N2 + N2O)-N-15 flux during the 10 d following urea application by each method was less than 0.1% of the applied N at the Thailand and Indonesia sites. The (N2 + N2O)-N-15 collected from the soil after 10 d did not exceed 0.02% of the applied N. Total gaseous N loss determined from a N-15 balance at 10 d after N application without incorporation was 34% of the applied N in Thailand and 31% in Indonesia. Incorporation reduced total N loss to 22% in Thailand and 10% in Indonesia. In the Philippines, the measured (N2 + N2O)-N-15 flux during the 15 d immediately following urea application with incorporation was 2.2% of the applied N with chambers placed between rice plants and 3.0% with chambers over the plants. Total N loss at the Philippine site was 40% of the applied N. Ammonia volatilization appeared to be much more important than nitrification-denitrification as a mechanism of urea-N loss at the three irrigated lowland study sites.