Urea Hydrolysis in Soils: Enhancement and Inhibition By Ascorbic, Gallic, Benzoic, and Cinnamic Acids.

Urea-N fertilizer is used extensively in agriculture, and the rate of its hydrolysis influences nitrogen availability in soils. High concentrations of carbon in soils have been correlated with increased rates of urea hydrolysis, but the components of total carbon that are the most influential for high rates of urea hydrolysis are unknown. Therefore, the objective of this research was to investigate the fractions of soil carbon that contribute to increased rates of urea hydrolysis. Lab studies tested A and B horizon soils sampled from an agricultural field and a riparian zone in Maryland’s Coastal Plain. Soil samples were mixed with a urea solution and different concentrations of organic carbon substrates and the amount of urea hydrolyzed in 24 hours was measured. We found that ascorbic and gallic acids exhibited antioxidant effects and increased urea hydrolysis at low (100 mg kg^-1) and high (4,000-10,000 mg kg^-1) concentrations of added C, but decreased urea hydrolysis at intermediate levels (1000 mg kg^-1). These results may be tied to interactions among pH, soluble metals, and native C content that influenced oxidative damage caused by Fe²⁺-dependent Fenton reactions. In contrast to the complicated effects of added ascorbic and gallic acids, benzoic and cinnamic acids appeared to be mores straightforward. The measured enhancement of urea hydrolysis for these sources of C were likely a result of the addition of a labile C source that enhanced microbial activity and urea hydrolysis up to a threshold at which pH dropped and metals came into solution. Further studies will need to be completed to confirm the mechanisms behind the observed patterns, but this study provides important preliminary information for explaining how different fractions of soil C could have differing effects on important microbial processes in soils.