Urea-nitrogen concentration is influenced by humic-like dissolved organic matter in agricultural drainage ditches adjacent to corn fields

Authors: Klick, Sabrina; BRYANT, RAY - US Department Of Agriculture (USDA); COLLICK, AMY - Morehead State University; MAY, ERIC - University Of Maryland Eastern Shore (UMES); Pisani, Oliva

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2023
Publication Date: 6/27/2023
Citation: Klick, S.A., Bryant, R.B., Collick, A.S., May, E.B., Pisani, O. 2023. Urea-nitrogen concentration is influenced by humic-like dissolved organic matter in agricultural drainage ditches adjacent to corn fields. Journal of Environmental Quality. 00:1-15. http://doi.org/10.1002/jeq2.20498.
DOI: https://doi.org/10.1002/jeq2.20498

Interpretive Summary: Agricultural drainage ditches are important targets to reduce nutrient impacts from agriculturally managed lands because they capture excess water from bordering crop fields to prevent flooded fields and crop loss. As a result, they also receive excess nutrients from fertilizers applied onto crop fields which can be transported to the nearby waterways. Urea, a form of organic nitrogen (N), is a component of fertilizers and has been shown to increase the toxicity of harmful algal blooms, which can lead to fish kills and accumulated toxins in consumed shellfish. The purpose of this study was to examine the sources and mechanisms leading to the accumulation of organic forms of N, such as urea, in the surface waters of drainage ditches using a laboratory experiment and a rainfall field study. Results showed that non-fertilizer sourced urea is linked to increases of humic-like dissolved organic matter (DOM) and environmental conditions that supports the bacterial conversion of nitrate to N gas. Rainfall and simulated flooding in the laboratory experiment acted as a temporary disturbance to environmental conditions and nutrient cycling by microorganisms. The disturbance can lead to increases of various forms of N, which could get flushed into the nearby waterways. Additionally, the DOM released from agricultural drainage sediments during the fall season was similar to the types found in natural wetland and forest environments. However, future research is needed to investigate if the DOM released from sediments during spring could be more available to growing microorganisms. Given the ecological impacts of urea, other N forms and available DOM to microorganisms, we suggest that these nutrients be incorporated into future N budgets and agricultural drainage ditch management practices to meet water quality targets aimed to improve the health of downstream waters.

Technical Abstract: Agricultural drainage ditches accumulate high urea-nitrogen (N) concentrations even when urea fertilizers are not applied to the adjacent crop fields. Rainfall events have the potential to flush the accumulated urea and other bioavailable forms of dissolved organic nitrogen (DON) downstream, which can influence water quality and phytoplankton communities. However, the sources of urea-N and the mechanisms supporting urea-N accumulation in agricultural drainage ditches is poorly understood. A mesocosm experiment simulated a flooding event with a treatment solution containing either a N or carbon (C) source and were monitored for changes in inorganic (NO3-, NH4+) and organic (DON and urea) N concentrations, physicochemical properties, dissolved organic matter (DOM) composition, and N cycling enzymes. The DOM released from mesocosm sediments consisted of mostly terrestrial-derived, high molecular weight DOM, similar to wetland and forest environments. Urea-N concentrations increased with higher total dissolved N (TDN) concentrations and degree of humification. Field rainfall led to higher TDN concentrations (8 mg N L-1) dominated by NO3--N (80-96%). The low urea-N (0.02 mg N L-1) and NH4-N (<0.2 mg N L-1) concentrations post-rainfall suggested the seasonal slowing of DOM decomposition and biological inputs. Therefore, we hypothesized that environmental factors such as increasing humification, solubility of metals, and reduced substances may restrict urease activity and promote urea-N accumulation when biological activity and DOM decomposition is seasonally higher. This study provides further insights of sources and mechanisms leading to high urea-N concentrations and the potential contribution of DOM inputs from drainage ditches to nearby surface waters after hydrological events.