Effect of Soil Matric Potential on Urea Hydrolysis

Soil water availability affects NH 3 loss from surface applied urea, partly due to its influence on urea hydrolysis rates. However, the quantitative effects of soil water on hydrolysis rates are not well understood. Our objective was to quantify how soil matric potential affects urea hydrolysis rates across a wide range of soil matric potentials. Urea hydrolysis rates were measured based on disappearance of urea at various soil water potentials in soils limed to a pH of 8.2 to eliminate confounding by soil pH. Urea‐N was applied on a soil mass basis (1200 μg urea‐N g −1 soil, 0.2–2.4 mol urea L −1 ) in Exp. I and at a constant urea‐N soil solution concentration of 40,000 μg urea‐N g −1 soil solution (1.43 mol urea L −1 ) in Exp. II. Differences in relative rates of urea hydrolysis between both experiments suggested that in Exp. I there was a confounding effect of urea‐N solution concentration on the response of urea hydrolysis to soil matric potential. In Exp. I, relative rates of hydrolysis changed little at intermediate soil matric potentials between −0.1 and −1.0 MPa because urea‐N concentration in solution increased as matric potential decreased. In Exp. II, relative rates were greatest at the highest matric potentials (>−0.24 MPa) and decreased more rapidly as matric potential decreased, approaching zero at a matric potential of approximately −10 MPa. These results also suggest that research to describe urea hydrolysis rates as affected by soil water content should be based on urea‐N concentrations in soil solution and not urea‐N concentrations on a soil mass basis.