Urease activity and inhibition in flooded soil systems

Abstract

Ammonia volatilization is an important mechanism of N loss from flooded rice soils. Inhibition of urease may delay the formation of conditions favorable to NH₃ volatilization in the floodwater, thus giving the soil and plant a better chance to compete with the atmosphere as a sink for N. The experiments reported here were designed to identify the site of urea hydrolysis in flooded soils and to attempt selective urease inhibition with some of the inhibitors reported in the literature.

Studies with three flooded soils using¹⁵N-labeled urea showed that 50–60% of the urea was found in the floodwater, despite incorporation. This floodwater urea is hydrolyzed largely at the soil—floodwater interface and subsequently returns to the floodwater (> 80%) or is retained by the soil (< 20%). Of the following urease inhibitors (K-ethyl-xanthate; 3 amino-1-H-1, 2, 4-triazole; phenylphosphorodiamidate) added at 2% (w/w of urea), only the latter was able to delay the appearance of NH₃ (aq) in the flood-water and thus delay NH₃ volatilization. Use of an algicide addition to the floodwater depressed NH₃ (aq) levels during the entire period studied, but in the presence of PPD the algicide had little additional effect.