Seasonal and temporal factors leading to Urea-N accumulation in surface waters of agricultural drainage ditches

Authors: KLICK, SABRINA - University Of Maryland Eastern Shore (UMES); PITULA, JOSEPH - University Of Maryland Eastern Shore (UMES); Bryant, Ray; COLLICK, AMY - University Of Maryland Eastern Shore (UMES); HASHEM, FAWZY - University Of Maryland Eastern Shore (UMES); ALLEN, ARTHUR - University Of Maryland Eastern Shore (UMES); MAY, ERIC - University Of Maryland Eastern Shore (UMES)

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2020
Publication Date: 12/11/2020
Citation: Klick, S.A., Pitula, J.S., Bryant, R.B., Collick, A., Hashem, F.M., Allen, A.L., May, E.B. 2020. Seasonal and temporal factors leading to Urea-N accumulation in surface waters of agricultural drainage ditches. Journal of Environmental Quality. 185-197. https://doi.org/10.1002/jeq2.20173.
DOI: https://doi.org/10.1002/jeq2.20173

Interpretive Summary: Urea is the most common form of nitrogen fertilizer used in agriculture. Its use supplanted that of ammonium nitrate following the 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City, where ammonium nitrate was used as the explosive. Coincident with the increased use of urea in agriculture, there have been increased occurrences of algal blooms in coastal areas. Furthermore, studies have shown that algal blooms are more toxic when urea is present as the nitrogen source for algae growth. In this study, we monitored urea concentrations and environmental conditions in surface waters of drainage ditches on the Eastern Shore of the Chesapeake Bay. Although we did observe elevated concentrations that appeared to be associated with spring-time applications of urea fertilizers, the highest concentrations occurred in summer months when no fertilizer had been recently applied. Urea is known to be rapidly converted to ammonium and nitrate following application to soils. Data from this study support the hypothesis that urea forms in stagnant ditch waters as a product of organic matter decomposition under conditions of warmer temperatures, low concentrations of dissolved oxygen, and low redox potentials. The same relationships were observed in stagnant pools of water in a forested area that was unaffected by nitrogen fertilization. If conditions that promote urea accumulation in stagnant ditch waters are followed by an intense summer rainfall event, urea may be flushed from agricultural and forested landscapes to coastal areas where it potentially contributes to a toxic algal bloom. Replacing urea with a different form of nitrogen fertilizer would have no effect on this summertime phenomenon, but drainage water management practices aimed at slowing agricultural drainage water release to streams, rivers and coastal areas may help mitigate the problem.

Technical Abstract: Urea-nitrogen (N) is commonly applied to crop fields, yet it is not routinely monitored despite its association with reduced water quality and ability to increase toxicity of certain phytoplankton species. The purpose of this work was to characterize temporal fluctuations in urea concentrations and associated environmental conditions to infer sources of urea in agricultural drainage ditches. Physicochemical properties and nitrogen forms in ditch waters were measured weekly during the growing seasons of 2015 through 2018. Fertilizer application was only associated with spring peaks of urea-N concentrations in ditches next to cornfields; whereas, summer peaks in ditches adjacent to both corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] fields were not associated with fertilizer applications. Environmental conditions of warmer temperatures, lower dissolved oxygen concentrations, and lower redox potentials were correlated with higher urea-N concentrations. In 2018, peaks of urea-N and ammonium-N during the summer co-occurred with peaks of dissolved organic N (DON) and total dissolved N (TDN), suggesting they might be associated with the breakdown of organic matter and turnover of the organic N pool. Although the highest urea-N concentrations occurred when ditch surface waters were hydrologically cut-off from nearby streams, heavy rainfalls can potentially flush accumulated urea-N into coastal waters where it may affect algal bloom toxicity. Therefore, implementation of available drainage ditch management practices are recommended, but these strategies also need to be optimized for targeting periods with high rainfall that coincide with fertilizer and non-fertilizer sourced urea-N accumulation in drainage ditches.