SHORT-TERM EFFECTS OF UREA AMENDED WITH THE UREASE INHIBITOR N-(N-BUTYL) THIOPHOSPHORIC TRIAMIDE ON PERENNIAL RYEGRASS

The current study investigated the short-term physiological implicatio ns of plant nitrogen uptake of urea amended with the urease inhibitor N-(n-butyl) thiophosphoric triamide (nBTPT) under both greenhouse and held conditions. N-15 labelled urea amended with 0.0, 0.01, 0.1 and 0. 5% nBTPT (w/w) was surface applied at a rate equivalent to 100 kg N ha (-1) to perennial ryegrass in a greenhouse pot experiment. Root, shoot and soil fractions were destructively harvested 0.75, 1.75, 4, 7 and 10 days after fertilizer application. Urease activity was determined i n each fraction together with N-15 recovery and a range of chemical an alyses. The effect of nBTPT amended urea on leaf tip scorch was evalua ted together with the effect of the inhibitor applied on its own on pl ant urease activity. nBTPT-amended urea dramatically reduced shoot ure ase activity for the first few days after application compared to unam ended urea. The higher the nBTPT concentration the longer the time req uired for shoot activity to return to that in the unamended treatment. At the highest inhibitor concentration of 0.5% shoot urease activity had returned to that of unamended urea by 10 days. Root urease activit y was unaffected by nBTPT in the presence of urea but was affected by nBTPT in the absence of urea. Transient leaf tip scorch was observed a pproximately 7-15 days after nBTPT + urea application and was greatest with high concentrations of nBTPT and high urea-N application rates. New developing leaves showed no visual sign of tip necrosis. Urea hydr olysis of unamended urea was rapid with only 1.3% urea-N remaining in the soil after 1.75 days. N uptake and metabolism by ryegrass was rapi d with N-15 recovery from unamended urea, in the plant (shoot + root) being 33% after 1.75 days. Most of the N-15 in the soil following the urea + 0.5% nBTPT application was still as urea after 1.75 days, yet N -15 plant recovery at this time was 25% (root+shoot). This together wi th other evidence, suggests that if urea hydrolysis in soil is delayed by nBTPT then urea can be taken up by ryegrass as the intact molecule , albeit at a significantly slower initial rate of uptake than NH4+-N. Protein and water soluble carbohydrate content of the plant were not significantly affected by amending urea with nBTPT however, there was a significant effect on the composition of amino acids in the roots an d shoots, suggesting a difference in metabolism. Although nBTPT-amende d urea affected plant urease activity and caused some leaf-tip scorch the effects were transient and short-lived. The previously reported be nefit of nBTPT in reducing NH3 volatilization of urea would appear to far outweigh any of the observed short-term effects, as dry-matter pro duction of ryegrass is increased.