Nitrogen gas (N2+N2O) flux from urea applied to lowland rice as affected by green manure

Abstract

A field study was conducted on a clay soil (Andaqueptic Haplaquoll) in the Philippines to directly measure the evolution of (N2+N2O)−15N from 98 atom %15N-labeled urea broadcast at 29 kg N ha−1 into 0.05-m-deep floodwater at 15 days after transplanting (DT) rice. The flux of (N2+N2O)−15N during the 19 days following urea application never exceeded 28 g N ha−1 day−1. The total recovery of (N2+N2O)−15N evolved from the field was only 0.51% of the applied N, whereas total gaseous15N loss estimated from unrecovered15N in the15N balance was 41% of the applied N. Floodwater (nitrate+nitrite)−N in the 5 days following urea application never exceeded 0.14 g N m−3 or 0.3% of the applied N. Prior cropping of cowpea [Vigna unguiculata (L.) Walp.] to flowering with subsequent incorporation of the green manure (dry matter=2.5 Mg ha−1, C/N=15) at 15 days before rice transplanting had no effect on fate of urea applied to rice at 15 DT. The recovery of (N2+N2O)−15N and total15N loss during the 19 days following urea application were 0.46 and 40%, respectively. Direct recovery of evolved (N2+N2O)−15N and total15N loss from 27 kg applied nitrate-N ha−1 were 20% and 53% during the same 19-day period. The failure of directly-recovered (N2+N2O)−15N to match total15N loss from added nitrate-15N might be due to entrapment of denitrification end products in soil or transport of gaseous end products to the atmosphere through rice plants. The rapid conversion of added nitrate-N to (N2+N2O)−N, the apparently sufficient water soluble soil organic C for denitrification (101 μg C g−1 in the top 0.15-m soil layer), and the low floodwater nitrate following urea application suggested that denitrification loss from urea was controlled by supply of nitrate rather than by availability of organic C.