Fate and Surface Transport Of Urea In a Coastal Plain Soil: A Rainfall Simulation Study.

The surface transport of urea has rarely been studied since it is assumed to undergo rapid hydrolysis to ammonia. However, studies have shown urea to exist in estuarine and coastal waters. Urea in small amounts can trigger the diatom Pseudo-nitzschia spp. to produce the toxin domoic acid, which is of concern in Coastal waters. Our hypothesis was that urea in soils amended with manures and urea-based fertilizers can persist long enough to be transported to surface waters. Rainfall simulation experiments were performed using silt loam from the surface horizon of a Coastal Plain soil. Treatments included urea prill, super urea, surface applied poultry litter, incorporated poultry litter, dairy manure, and a control. The first simulated rainfall showed that super urea (10.77mg L^-1), surface applied poultry litter (6.73mg L^-1) and urea prill (4.11mg L^-1) generated the greatest concentrations of urea-N (nitrogen) respectively. The amounts from dairy manures (1.96mg L^-1) were intermediate, and concentrations from incorporated poultry litter were not significantly different from the control. By the second and third rainfall simulations, super urea, dairy manure, and surface applied poultry litter produced higher concentrations of urea-N compared to other treatments. During the fourth and fifth rainfall events, urea-N concentrations in all treatments, with the exception of dairy manure, had diminished to background levels. Another rainfall simulation experiment using urea prill only demonstrated that urea concentration in runoff was influenced by soil moisture content prior to the rainfall event. Flow weighted urea concentrations were positively correlated with total runoff volumes (R²=0.73). Urea loss under saturation excess runoff conditions was much greater than under infiltration excess runoff conditions. Indeed, quantitative information on the effect of fertilizer form, placement method, and time after application on the amounts of urea found in runoff can be used to identify management practices that have the potential to minimize urea losses to surface waters.