Soil pH Affects Degradation of the Urease Inhibitor NBPT.

Soil pH is reported to be an important chemical property affecting N-(n-butyl) thiophosphoric triamide (NBPT) inhibition of urease. Only a few studies have followed NBPT decay kinetics by making direct measurements of the NBPT molecule or its oxygen analog (NBPTo) in soil or buffer mediums. This study was conducted to better characterize the effect of pH on NBPT metabolism in the soil under biotic and abiotic (conditions) including the appearance of NBPTo and decay products. Chemical buffer (200 mM) incubations were conducted with sodium acetate at pH 4.0 and 5.0, and  2-(N-morpholino)ethanesulfonic acid at pH 6.0 and 7.2, and degradation was quantified using ¹H- NMR. Soil incubations were conducted at pH 5.1, 6.1, 7.6 and 8.2 with sterilized (autoclaved) and non-sterilized soil collected (0-5 cm depth) from an agricultural field that exhibited a pH gradient; and NBPT degradation and metabolism products were measured with HPLC-MS. NBPT and NBPTo exhibited pseudo first-order rate kinetics in chemical buffers and exponential decay patterns in soils. NBPT hydrolyzed to n-butylamine and a phosphorus byproduct, ^-OP(S)(NH₂)₂. NBPT decay constants in soil were larger under acidic conditions and became progressively smaller as pH rose. NBPT half-life in non-sterilized soil was equivalent to 0.07, 0.59, 2.70, and 3.43 d at pH 5.1, 6.1, 7.6, and 8.2, respectively. Soil sterilization increased these half-lives by 0.13, 0.55, 1.03, and 4.07 d, respectively. Soil and buffer incubations indicated that NBPTo was more sensitive than NBPT to hydrolysis under acidic conditions, which was attributed to the greater electrophilicity of the P atom. Chemical hydrolysis will likely be a large contributing factor to NBPT and NBPTo degradation in acidic to slightly alkaline soils (pH 5.1 to 7.6), while microbial degradation will likely become more important in strongly alkaline conditions (pH 8.2).