Inhibition of urease activity in flooded soils: Effect of thiophosphorictriamides and phosphorictriamides

Abstract

The effectiveness of two thiophosphorictriamides, [N-(n-butyl)thiophosphorictriamide (NBPT) and thiophosphoryltriamide (TPT)], and three phosphorictriamides, [N-(n-butyl)phosphorictriamide (NBPTO), phosphoryltriamide (PT) and cyclohexylphosphorictriamide (CHPT)], to inhibit urease activity in flooded soils in the presence or absence of photosynthetic algae was evaluated in laboratory experiments. Both TPT and its oxygen analogue PT were poor inhibitors of urease activity in these flooded soils in the presence or absence of algae. In the absence of photosynthetic algae, the rate of urea hydrolysis was retarded slightly by the addition of NBPT. However, in the presence of algae the capacity of NBPT to inhibit urease activity was enhanced considerably. It appears that the oxygen produced by the photosynthetic algae resulted in more rapid conversion of the pro inhibitor NBPT to NBPTO, which is the effective urease inhibitor. The phosphorictriamides CHPT and NBPTO were significantly better than the other compounds for retarding urease activity, and CHPT ranked above NBPTO as a urease inhibitor. In the presence of photosynthetic algae the effectiveness of these inhibitors appears to be reduced, and this seems to be related to algal uptake of nitrogen and more rapid volatilization of ammonia and nitrification. The results suggest that the phosphorictriamides CHPT and NBPTO in conjunction with an algicide, to control the growth of floodwater algae, have the potential to considerably reduce ammonia loss from flooded rice soils.