Organic Nitrogen Runoff in Coastal Marshes: Effects on Ecosystem Denitrification

Since the 1970s, a shift from inorganic to organic nitrogen-based fertilizer has occurred worldwide, and now urea constitutes greater than 50% of the global nitrogenous fertilizer usage. As a result, concentrations of urea will likely increase in waterways, facilitating transport to coastal wetland habitats where microbial-mediated transformations have the ability to alleviate excess nitrogen (N) pollution. To assess this biological potential for N removal in a brackish marsh ecosystem, we conducted a 5-day laboratory experiment where we monitored denitrification rate potentials (DNP) in microcosms with intact, vegetated sods, testing treatments of different urea solutions (37.5 and 166.5 mM urea) and a nitrate solution (98.9 mM KNO3). The addition of urea, regardless of concentration, did not stimulate DNP, while nitrate additions did. Ammonium (NH4+) accumulated in the porewater in response to urea treatments, with approximately 80-90 % of urea being hydrolyzed during the experiment. Nitrate concentrations in the nitrate treatment were near zero by the end of the experiment, while measureable amounts of urea were still present in both urea treatments. An increase in DNP followed nitrate additions, but an accumulation of NH4+ after urea additions suggests that urea pollution may not be removed by coastal wetlands as efficiently as nitrate pollution, especially when nitrification is limited under anaerobic conditions. Further work exploring the most likely pathways for removal of excess NH4+ is necessary to describe the potential impact that increased urea concentrations could have on coastal ecosystems.