A Comparison of a Maleic‐Itaconic Polymer and N‐(n‐butyl) Thiophosphoric Triamide as Urease Inhibitors

Abstract

A maleic‐itaconic polymer (MIP) is claimed to inactivate soil urease by nickel removal from the enzyme. Four studies were conducted to compare the urease inhibition properties of two commercial formulations of MIP with a commercial formulation of N‐(n‐butyl) thiophosphoric triamide (NBPT). In the first study, addition of MIP at 1, 5, or 50 mg kg–1 provided 0 to 2% inhibition of urea hydrolysis, averaged across three soils. At 500 mg kg–1, MIP inhibited urea hydrolysis by 7 to 9%, averaged across three soils. By contrast, addition of NBPT at 1 or 5 mg kg–1 inhibited urea hydrolysis by an average of 64 and 72%, respectively. In a second study, 13 carboxylic acids of known nickel sequestration properties, two MIP formulations, and NBPT were evaluated as soil urease inhibitors. Averaged across three soils, MIP and the other carboxylic acids provided <5% inhibition of urea hydrolysis when applied at 50 mg kg–1. Addition of NBPT at 1 and 5 mg kg–1 provided an average of 62 and 72% inhibition. In two additional studies using purified jackbean [Canavalia ensiformis (L.) DC.] urease in the absence of soil, NBPT provided complete inhibition at a concentration of 1 mg L–1 in the reaction mixture. Adding MIP at rates up to 100 mg L–1 either did not inhibit urease, or stimulated urea hydrolysis. It was concluded that MIP was not an effective inhibitor of soil or jackbean urease. It was also concluded that nickel sequestration by carboxylic acids is an unlikely mode of action for soil urease inhibition.