INHIBITION OF UREASE ACTIVITY IN FLOODED SOILS - EFFECT OF THIOPHOSPHORICTRIAMIDES AND PHOSPHORICTRIAMIDES

The effectiveness of two thiophosphorictriamides, [N-(n-butyl)thiophos phorictriamide (NBPT) and thiophosphoryltriamide (TPT)], and three pho sphorictriamides, [N-(n-butyl)phosphorictriamide (NBPTO), phosphoryltr iamide (PT) and cyclohexylphosphorictriamide (CHPT)], to inhibit ureas e activity in flooded soils in the presence or absence of photosynthet ic algae was evaluated in laboratory experiments. Both TPT and its oxy gen analogue PT were poor inhibitors of urease activity in these flood ed soils in the presence or absence of algae. In the absence of photos ynthetic algae, the rate of urea hydrolysis was retarded slightly by t he 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 eff ective urease inhibitor. The phosphorictriamides CHPT and NBPTO were s ignificantly better than the other compounds for retarding urease acti vity, and CHPT ranked above NBPTO as a urease inhibitor. In the presen ce of photosynthetic algae the effectiveness of these inhibitors appea rs to be reduced, and this seems to be related to algal uptake of nitr ogen and more rapid volatilization of ammonia and nitrification. The r esults suggest that the phosphorictriamides CHPT and NBPTO in conjunct ion with an algicide, to control the growth of floodwater algae, have the potential to considerably reduce ammonia loss from flooded rice so ils.