Urea Transformation of Wetland Microbial Communities

Transformation of urea to ammonium is an important link in the nitrogen cycle in soil and water. Although microbial nitrogen transformations, such as nitrification and denitrification, are well studied in freshwater sediment and epiphytic biofilm in shallow waters, information about urea transformation in these environments is scarce. In this study, urea transformation of sedimentary, planktonic, and epiphytic microbial communities was quantified and urea transformation of epiphytic biofilms associated with three different common wetland macrophyte species is compared. The microbial communities were collected from a constructed wetland in October 2002 and urea transformation was quantified in the laboratory at in situ temperature (12°C) with the use of the ¹⁴C-urea tracer method, which measures the release of ¹⁴CO₂ as a direct result of urease activity. It was found that the urea transformation was 100 times higher in sediment (12–22 mmol urea-N m⁻² day⁻¹) compared with the epiphytic activity on the surfaces of the submerged plant Elodea canadensis (0.1–0.2 mmol urea-N m⁻² day⁻¹). The epiphytic activity of leaves of Typha latifolia was lower (0.001–0.03 mmol urea-N m⁻² day⁻¹), while urea transformation was negligible in the water column and on the submerged leaves of the emergent plant Phragmites australis. However, because this wetland was dominated by dense beds of the submerged macrophyte E. canadensis, this plant provided a large surface area for epiphytic microbial activity—in the range of 23–33 m² of plant surfaces per square meter of wetland. Thus, in the wetland system scale at the existing plant distribution and density, the submerged plant community had the potential to transform 2–7 mmol urea-N m⁻² day⁻¹ and was in the same magnitude as the urea transformation in the sediment.