The increasing usage of urea nitrogen in fertilizers is a growing concern because of its influence on harmful algal blooms and reduced water quality. The Eastern Shore of Maryland is known for intensive agriculture, and the associated nutrient run-off impacts the water quality of the Chesapeake Bay. Agricultural drainage ditches directly receive urea-N fertilizer run-off from crop fields in the spring. However, urea-N concentrations above 0.3 mg N L-1 are also observed later in the summer and 2-4 days after rain events. It is unclear why urea-N accumulates during the summer because environmental urea is rapidly broken down, and urease inhibitors only last about two weeks. We hypothesized that microbial communities contribute to the urea-N pool through the decomposition of dissolved organic nitrogen. To test this hypothesis, a mesocosm experiment was conducted using agricultural drainage ditch sediments. Sediments were flooded with solutions containing either dissolved organic nitrogen, urea, ammonium, nitrate, or no nitrogen; and incubated at 25oC for five days. Water samples were taken daily to measure nitrogen concentrations and physicochemical parameters. Sediment samples were collected at hours 24 and 96 to obtain microbial community composition and relative abundance using high-throughput sequencing.
Results/Conclusions
Preliminary results showed the consumption of urea and dissolved organic nitrogen (DON) substrates within the first 96 hours after sediments were flooded with the nitrogen solutions. Urea-N production started after 96 hours, and mesocosms enriched with DON substrates resulted in higher urea-N production (0.12 mg N L-1 day-1) than mesocosms enriched with inorganic nitrogen. The mesocosms enriched with DON also had the highest ammonium-N production throughout the entire experiment. Nitrate-N concentrations also peaked at 96 hours in all mesocosms when urea-N was lowest. Urea-N production was associated with a lower oxidation-reduction potential, lower dissolved oxygen concentration, and a pH level above 6. Analysis of the bacterial communities from high-throughput sequencing is on-going, but is predicted to reflect a shift from urea-N consumption to production. A pH level above 6 is favorable for bacterial ureases, but ammonium-N concentrations increased after 48 hours and reached concentrations above 3 mg N L-1 by the end. It is possible that elevated ammonium-N concentrations inhibited urease activity, and allowed urea-N production to outpace urea hydrolysis. Therefore, this experiment will be repeated and carried out to seven days to capture peaks of urea-N from production after 120 hours, and analyze urease activity throughout the experiment.