Comparing Functional Assessments and Measurements of Nitrogen Processing in Wetlands

Thomas E. Jordan, Mary A. Pittek, Ryan P. Szuch, Dennis F. Whigham,
and Donald E. Weller

Abstract of a Paper Presented at the Sixth Wetlands Workshop, Sponsored by the U. S. Environmental Protection Agency, Atlantic City, New Jersey, October 2003

Wetlands can remove nitrogen from watershed discharges via denitrification, which is favored by the lack of oxygen and abundance of organic carbon in wetland soils. Denitrification can convert nitrate to nitrous oxide (N2O) and dinitrogen (N2 ) gases. N2 is a desirable end product but N2O is an atmospheric pollutant that promotes global warming and destruction of stratospheric ozone. Our objective was to find indicators or surrogate measurements of denitrification and N2O emission in non-tidal freshwater wetlands. We sampled the top 10 cm of soil in 7 flats and 11 riverine wetlands in spring, summer, and fall. Denitrification potential, measured as denitrification enzyme activity (DEA) decreased with increases in soil redox potential (Eh) and depth to the water table. DEA increased with increases in soil pH, ammonium concentration, and percentages of N, C, and water-filled pore space. The closest correlation was between DEA and Eh with Pearson r 2 =0.5. The relationships between DEA and the soil characteristics applied similarly to both flats and riverine wetlands, but, compared to riverine wetlands, flats tended to have lower DEA and soil characteristics correlated with lower DEA. N2O emission rates varied greatly among wetlands and seasons but, unlike DEA, showed no strong correlations with any of the soil characteristics we measured. We compared our measurements with hydro-geomorphic (HGM) assessments of habitat, hydrologic, biogeochemical and other functions of the wetlands. For flats, DEA increased with increases in the biogeochemical function score, especially in the summer when flats altered by artificial drainage became drier relative to un-drained flats. For riverine wetlands, the biogeochemical function score was a poor predictor of DEA. This may reflect the fact that maximum scores are assigned to wetlands that are least altered from pristine condition regardless of whether the alterations increase or decrease soil moisture. No HGM function scores correlated with N2O emission.