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 15N 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‐15N 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.
Dry seeding has been identified as an option for increasing cropping intensity and productivity in rainfed ricelands. Managing drought and nutrients are important for increasing yield, but the interactive effects of drought and nutrients on dry-seeded rice (Oryza sativa L.) growth have not been systematically investigated. Two experiments were carried out in 1994 and 1995 to analyze the effects of N fertilizer rate and the use of controlled-release fertilizers (CRFs) on the growth and yield of dry-seeded rice grown on a silty clay loam (Typic Tropaquept) subjected to water stress at different crop stages. In both years, in the main plots, rice was subjected to water stress at four different stages of development. The subplots were designed to compare the effect of the application of prilled urea and CRFs Osmocote (1994) and Polyon 12 (1995). Four N rates (0, 60, 120 and 180 kg ha −1 ) were imposed on rice in the sub-subplots (1994 only). The N fertilizer source did not affect any of the measured parameters. Irrespective of the N the fertilizer rates, grain yield and total dry matter accumulation of rice plants stressed at the flowering stage (WS FL , 1994) and panicle initiation stage (WS PI , 1995) were significantly lower than those of well-watered plants and plants stressed at the vegetative stage. Water stress during the grain-filling stage reduced the grain yield in 1995 when the stress was severe. Application of N fertilizer increased the yield compared with zero N in all water treatments, except for the WS FL plants whose yield did not change. The WS FL treatment also significantly reduced agronomic N-use efficiency.
One proposed strategy for overcoming N loss by ammonia volatilization from urea applied to flooded rice (Oryza sativa L.) fields is to amend urea with a urease inhibitor. The inhibitor, in theory, would delay urea hydrolysis and thereby prevent the buildup of ammonia in the floodwater. Field studies were conducted at two locations in the Philippines to determine the effect of two urease inhibitors, phenyl phosphorodiamidate (PPD) and N‐(n‐butyl) thiophosphoric triamide (NBPT), on the vapor pressure of ammonia (pNH3) following application of urea to floodwater. Three rates of urea were applied two‐thirds dosage at 18 d after transplanting (DT) and one‐third dosage at 5 to 10 d after panicle initiation (DAPI). The amendment of urea with either PPD or NBPT retarded the disappearance of urea from floodwater and reduced the buildup of floodwater ammoniacal‐N. NBPT generally matched or exceeded PPD in ability to reduce pNH3 at each N rate and timing for both locations. Whereas PPD only delayed the buildup in pNH3, particularly at higher N rates, NBPT effectively prevented a buildup in pNH3. For the 10 d following the first N application, NBPT reduced the mean pNH3 at 1400 h by 95, 96, and 98% for 23, 47, and 93 kg N ha−1, respectively, at one location. The corresponding reductions with PPD were 82, 54, and 25%, respectively. At the other location, NBPT reduced the mean pNH3 following the first N application by 97, 77, and 79% for 23, 47, and 93 kg N ha−1, respectively. The corresponding reductions with PPD were 79, 77, and 28%, respectively. The results suggested that urease inhibitors can effectively reduce ammonia loss from urea broadcast into the floodwater of rice fields.
The amendment of urea with a urease inhibitor has been proposed as a strategy to increase yield of lowland rice (Oryza sativa L.) through the reduction of N loss by ammonia volatilization. Field studies were conducted at two locations in the Philippines to determine the effects of two urease inhibitors, phenyl phosphorodiamidate (PPD) and N‐(n‐butyl) thiophosphoric triamide (NBPT), on N uptake and yield of rice. Urea was split applied at 16 to 18 d after transplanting (DT) and at 5 to 10 d after panicle initiation (DAPI). On a silty clay soil (Vertic Tropaquept) at Muñoz, the amendment of urea with PPD or NBFT significantly increased grain yield relative to conventional urea (P = 0.05). Agronomic efficiency for 35 kg N ha‐' increased from 43 kg grain kg−1 fertilizer‐N for urea to 51 and 52 kg grain kg−1 fertilizer‐N for urea + 20 g PPD kg−1 and urea + 9 g NBPT kg−1, respectively. Both PPD and NBPT increased grain N, averaged for four non‐zero N rates, by 5.7 and 7.8 kg ha−1, respectively. The increased uptake of N with the inhibitors occurred prior to 35 DT; subsequent N uptake was not affected. On a clay soil (Andaqueptic Haplaquoll) at Pila, the addition of inhibitors failed to give significantly greater grain yields than unamended urea in each of the three experiments. However, the quadratic response models for urea and urea + PPD obtained from combined results for the three Pila experiments were significantly different (P = 0.01). Amendment of urea with PPD increased agronomic efficiency by 4 kg grain kg−1 urea‐N. Physiological efficiency (kg grain kg−1 N uptake) was not significantly affected by the inhibitors, which suggests that an increase in N uptake with the inhibitors resulted in a corresponding yield increase. This study indicates that urease inhibitors can increase grain yield of lowland rice, but the yield increases with inhibitors might be small.
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