Apparent Persistence of N‐(n‐butyl) Thiophosphoric Triamide Is Greater in Alkaline Soils

Abstract

Recently completed NH3 volatilization field studies with urea suggest the urease inhibitor N‐(n‐butyl) thiophosphoric triamide (NBPT) has prolonged activity in alkaline relative to acidic soils. A laboratory incubation experiment was conducted at two temperatures (0.5 and 20°C) to determine whether NBPT degradation was affected by soil pH (alkaline vs. acidic) and to determine the impact on urea hydrolysis. Soil (10 g) from the surface horizon of a loam (pH 5.5) and a silt loam (pH 8.4) was placed in bottles and incubated up to 28 d in constant‐temperature chambers. A third soil was constructed by adding CaCO3 (0.3 g) to the loam soil (pH 8.2). Urea (20 mg) with and without NBPT (20 mg) was applied to each soil. High‐performance liquid chromatography–mass spectrometry analysis of KCl extracts revealed that NBPT degraded rapidly in the acidic soil compared with the alkaline soils (silt loam and loam with added CaCO3). Exponential decay constants for the acidic soil were 5.2 and 3.9 times larger than decay constants for the alkaline soils at 0.5 and 20°C, respectively. Urea hydrolysis rates were reduced by NBPT, and the response was greater for the alkaline soils (P < 0.0001). Hydrolysis was inhibited by 17.0 and 86.2% at 20°C and by 53.3 and 92.1% at 0.5°C for the acidic and alkaline soils, respectively. This study confirmed field observations that NBPT persistence and activity is greater in alkaline soils. Additional studies, including chemical hydrolysis and sorption, are needed to clarify the processes responsible for the rapid disappearance of NBPT from extracts of acidic soils.