Direct Measurement of Ammonia and Denitrification Fluxes from Urea Applied to Rice

Abstract Ammonia volatilization and denitrification are widely reported to be mechanisms of fertilizer‐N loss from lowland rice fields, but few researchers have simultaneously measured NH 3 and denitrification losses from applied N in order to ascertain the relative importance of these two loss mechanisms. Ammonia volatilization and denitrification losses from urea broadcast into 0.05‐m‐deep floodwater at 11 d after transplanting were directly measured at a lowland irrigated site at Calauan, Laguna, Philippines. Ammonia loss was measured by the mass‐balance micrometeorological technique in two dry seasons (1986 and 1988), and denitrification loss was measured from evolution of (N 2 + N 2 O)‐ 15 N into a confined chamber during the 20 d following application of 15 N‐labeled urea in two dry seasons (1987 and 1988). Ammonia loss was 54 and 46% of the applied urea‐N during the 8 d following N application in 1986 and 1988, respectively. Recovery of evolved (N 2 + N 2 O)‐ 15 N was 0.1% of the applied urea‐N in both 1987 and 1988. Total N loss, determined from unrecovered 15 N in 15 N balances at 20 d after urea application, was 64, 43, and 52% of the applied N in 1986, 1987, and 1988, respectively. Denitrification loss, determined by the difference between total N loss and directly measured NH 3 loss, was 10 and 6% of the applied urea‐N in 1986 and 1988, respectively. Estimates of denitrification tended to be greater with the difference method (10 and 6%) than with the direct recovery of (N 2 + N 2 O)‐ 15 N (<1%). Nonetheless, NH 3 volatilization was consistently the dominant mechanism of gaseous N loss from broadcast urea.