Inhibition of urease activity in flooded soils by phenylphosphorodiamidate and n-(n-butyl) thiophosphorictriamide

Rapid urea hydrolysis and loss of nitrogen from flooded rice soils by ammonia volatilization could be reduced by the use of effective urease inhibitors. The compounds phenylphosphorodiamidate (PPD), N-(n-butyl)thiophosphorictriamide (NBPT), N-(diaminophosphinyl)benzamide (DAPB) and acetohydroxamic acid (AHA) were tested in two flooded soils in laboratory experiments for their ability to prevent urea hydrolysis. In a vertisol with high urease activity, DAPB and AHA had little effect on the rate of the urea hydrolysis, irrespective of the rate of addition. The rate of urea hydrolysis was slowed by addition of PPD and NBPT at very high rates of application (10% of the weight of urea); after 8 days, 42 and 25%, respectively, of the added urea remained in the floodwater and soil. The effectiveness of the NBPT was increased by bubbling oxygen into the floodwater and by adding hydrogen peroxide; these treatments caused more rapid conversion of part of the NBPT to its oxygen analogue (which is the active urease inhibitor). Mixtures of PPD and NBPT were more effective than either inhibitor alone. Initially PPD inhibited soil urease, allowing sufficient time for NBPT to be converted to the oxygen analogue. Then as the effectiveness of the PPD declined, the oxygen analogue of NBPT inhibited urea hydrolysis. The results suggest that this mixture has the potential to improve the efficiency of urea utilization in flooded rice fields.