Abstract:
Urea is an organic form of nitrogen that has seen enormous commercial growth as agricultural fertilizer. With this expansion, ecosystems such as wetlands are more likely to experience elevated concentrations of urea via runoff. Once in the environment, the enzyme urease, produced by plants, fungi, bacteria, and archaea, controls the degradation of urea. However, within wetlands, little is known about how these organisms or their urease will respond to increased urea levels. These dissertation studies were conducted in coastal wetlands along the northern Gulf of Mexico to better understand spatial and temporal patterns of bacterial communities and urease activity in wetland soils. To examine urea removal pathways, microcosm experiments found removal of urea via denitrification in wetland soils to be limited. Microcosms supplemented with urea were not capable of hydrolyzing all added urea within 5 days. A survey of wetland soils showed urease activity varied considerably over space and time, with urease rates at the intermediate site significantly lower than sites with the lowest and highest elevation. Along with the lowest urease rates, the highest concentrations of inorganic nitrogen were also measured at the intermediate site, a parameter that has previously been linked to inhibition of bacterial ureases. To explore the differences in urease activity between sampling sites, soil bacterial communities were monitored over space and time using culture independent methods. The composition of soil bacterial communities differed over space, time, and depth at the four sampling sites. Using cultivation approaches, soil ureolytic bacteria were isolated and identified as being affiliated with eight genera, all of which were prevalent in sequence libraries prepared from the four sites. As environmental urea concentrations are expected to increase in coming years, information on the distribution and activity of ureolytic bacteria will be critical to better predict the ability of affected ecosystems to manage the additional nitrogen. Furthermore, if elevated ammonium levels inhibit urease activity, the capacity of ammonium-rich wetland soils to hydrolyze additional urea may be limited.
