N-15 BALANCES AND NITROGEN-FERTILIZER USE EFFICIENCY IN UPLAND RICE

Field experiments were conducted in the 1984 and 1985 wet seasons to determine the effect of N fertilizer application method on N-15 balances and yield for upland rice (Oryza sativa L.) on an Udic Arguistoll in the Philippines. The test cultivars were 'IR43' and 'UPLRi-5' in 1984 and 'IR43' in 1985. Unrecovered N-15 in N-15 balances for 70 kg applied urea-N ha-1, which represented N fertilizer losses as gases and movement below 0.5 m soil depth, ranged from 11-58% of the applied N. It was lowest (11-13%) for urea split applied at 30 days after seeding (DS) and at panicle initiation (PI), and highest (27-58%) for treatments receiving basal urea in the seed furrows. In all treatments with basal-applied urea, most N losses occurred before 50 DS. Heavy rainfall in 1985 before rice emergence resulted in large losses of native soil N and fertilizer N by leaching and possibly by denitrification. During the week of seeding, when rainfall was 492 mm, 91 kg nitrate-N ha-1 disappeared from the 0.3-m soil layer in unfertilized plots. Although rainfall following the basal N application was less in 1984 than in 1985, the losses from basal applied urea-N were comparable in the two years. Daily rainfall of 20-25 mm on 3 of the 6 days following basal N application in 1984 may have created a moist soil environment favorable for ammonia volatilization. In both years, highest grain yield was obtained for urea split-applied at 30 DS and at PI. Delayed rather than basal application of N reduced losses of fertilizer N and minimized uptake of fertilizer N by weeds.