Greenhouse Study on the Effects of the Urease Inhibitors Phenyl Phosphorodiamidate and N-(n-butyl) Thiophosphoric Triamide on the Efficiency of Urea Applied to Flooded Rice

Urea was split applied to transplanted rice in a greenhouse experiment with two-thirds applied as labeled ¹⁵N urea at 15 days after transplanting (DAT) and one-third (not labeled) at 42 DAT to determine the effect of the urease inhibitors phenyl phosphorodiamidate (PPDA) and N-(n-butyl) thiophosphoric triamide (NBPT) on urea hydrolysis, plant uptake, yield, and loss of fertilizer N. An acidifying agent Al₂(SO₄)₃] and an algicide were used to reduce the floodwater pH and thus slow the degradation of PPDA, keeping it effective for a longer period. Algicide addition extended the effectiveness of PPDA inhibition by about 2 days and increased plant uptake and grain yield significantly over that with urea use alone. Al₂(SO₄)₃ addition extended the effectiveness of PPDA only about 1 day, increased N uptake slightly, but failed to increase grain yield. NBPT effectively slowed urea hydrolysis, more than doubled plant uptake over that with urea alone, and increased grain yield by 38%. Percolation at 0.5 cm per day caused plant N uptake to increase by about 6% in all treatments but it was not essential for the inhibitors to have a beneficial effect. For the first split application, fertilizer losses of 50% from urea were decreased to about 10% by use of NBPT and to 28% with PPDA alone, and by combination of PPDA with the algicide losses were 22%.