Potential of Inhibitors for Increasing Response of Lowland Rice to Urea Fertilization

Abstract

Abstract The full potential of biological inhibitors to increase the yield response of lowland rice ( Oryza sativa L.) to urea fertilization has remained uncertain, because no chemical has been available to completely eliminate all gaseous loss of applied urea‐N. Two field 15 N balance studies at multiple rates of urea with and without a urease inhibitor, phenyl phosphorodiamidate, were conducted in the Philippines. The urea was split broadcast into floodwater at 16 or 18 d after transplanting and at 5 to 10 d after panicle initiation. The applied urea‐ 15 N not recovered in the rice and soil at crop maturity was assumed to represent total gaseous N loss and hence the potential savings in urea‐N that could be obtained with the use of an inhibitor that completely eliminated gaseous N loss. The potential savings in urea‐N consistently increased with increasing N rate. In 1985. the potential urea‐N savings were 17, 23, and 29% at 30, 60, and 120 kg N ha −1 , respectively. In 1986, they were 13, 34, and 37% at 40,80, and 120 kg N ha −1 , respectively. In each experiment, there was a significant quadratic response in grain yield to applied N. Models were developed for observed responses to applied urea and for potential yield responses to urea assuming complete elimination of gaseous N loss with no effect on rice other than increasing the availability of fertilizer‐N. Elimination of gaseous N loss could have increased grain yield by a maximum of 6 and 8% in 1985 and 1986. respectively. These percentages corresponded to 0.4 and 0.5 Mg ha −1 increases in grain yield, respectively.