Persistence and Surface Transport of Urea‐Nitrogen: A Rainfall Simulation Study

• Published in: Journal of environmental quality Vol. 45; no. 3; pp. 1062 - 1070
• Main Authors: Kibet, Leonard C., Bryant, Ray B., Buda, Anthony R., Kleinman, Peter J. A., Saporito, Louis S., Allen, Arthur L., Hashem, Fawzy M., May, Eric B.
• Format: Journal Article
• Language: English
• Published: United States The American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc 01-05-2016
• ISSN: 0047-2425; 1537-2537
• DOI: 10.2134/jeq2015.09.0495

Studies of harmful algal blooms and associated urea concentrations in the Chesapeake Bay and in coastal areas around the globe strongly suggest that elevated urea concentrations are associated with harmful algal blooms. The observed increased frequency and toxicity of these blooms in recent decades has been correlated with increased agricultural use of N inputs and increased use of urea as a preferred form of commercial N. This rainfall simulation study sought to assess the potential for different N fertilizers and manures to contribute to urea in runoff from a Coastal Plain soil on the Eastern Shore of Maryland. Under worst‐case conditions, ~1% of urea‐N applied as commercial fertilizer and surface‐applied poultry litter was lost in runoff in a simulated rainfall event, roughly equivalent to a 1‐yr return period rain storm in the study area, 12 h after application. Cumulative urea‐N losses, including four subsequent weekly rainfall events, approached 1.7% from urea‐N fertilizer containing a urease inhibitor. Urea‐N loss from incorporated poultry litter was negligible, and losses from dairy manure were intermediate. These losses are likely confined to hydrological contributing areas that extend several meters from a drainage ditch or stream for storms with frequent recurrence intervals. Cumulative dissolved N losses in runoff (urea‐N + ammonium‐N + nitrate‐N) as a proportion of total applied plant‐available N were <5%, suggesting that most of the applied N was lost by other pathways or was immobilized in soil. Results also highlight the potential for simple management options, such as shallow incorporation or timing, to greatly reduce urea runoff losses.
Core Ideas
Urea from terrestrial sources can increase frequency and toxicity of algal blooms.
The proportion of applied urea lost as urea in runoff was quite low (~1%).
Urea plus urease inhibitors yielded the most urea loss under worst‐case conditions.
Urea “wash‐off” soon after application poses the greatest risk to water quality.
Urea “wash‐off” can be mitigated by adjusting method or timing of application.