Effect of a Urease Inhibitor Phenyl Phosphorodiamidate on the Efficiency of Urea Applied to Rice

Abstract

Urea, the major source of nitrogen for rice, is subject to extensive ammonia volatilization losses when broadcast on flooded soils. As part of a program to modify and improve urea, the effect of the urease inhibitor phenyl phosphorodiamidate (PPD) on the efficiency of broadcast urea for wetland rice (Oryza sativa L.) was studied in the greenhouse using 15N. Addition of PPD to urea (5% wt/wt) increased plant recovery of 15N applied as basal broadcast and incorporated urea by 10 to 15% and reduced losses by 20% in absolute terms. Addition of PPD to urea topdressed at maximum tillering (30 d after transplanting) increased 15N plant recovery from 41 (without PPD) to 67% (with PPD) and decreased losses from 23 to 0% of the 15N applied. Despite these improvements in N efficiency, the addition of PPD reduced dry matter production under the particular conditions of the experiment. The effect of PPD on NH3 volatilization was then studied using 15N-labeled urea added to flooded soils placed in forced-draft systems in the absence of plants. Phenyl phosphorodiamidate addition reduced NH3 volatilization losses from 31% (without PPD) to < 5% (with PPD) of the 15N added. A rate of 1% PPD was as effective as 5% in delaying urea hydrolysis, maintaining a lower pH in the floodwater, reducing NH3 concentrations in the floodwater, and decreasing NH3 volatilization losses. In short-term (16 h) incubation studies, as little as 0.5% PPD in urea or 4 ppm PPD dissolved in the floodwater effectively blocked urea hydrolysis. These results indicate that the concept of using urease inhibitors to reduce losses and improve urea efficiency may have potential for wetland rice.