Comparing functional assessments of wetlands to measurements of soil characteristics and nitrogen processing

Abstract

One beneficial service of wetland ecosystems is the improvement of water quality through nitrogen (N) removal. However, one important N-removal process, denitrification, can produce the atmospheric pollutant nitrous oxide (N2O). Wetland biogeochemical functions, such as N processing, can be assessed by the hydrogeomorphic (HGM) approach using a suite of simple field observations made in a single visit to a wetland. HGM assessments score functions on a scale of 0-1 where 1 equals the functionality of an undisturbed reference standard wetland and 0 equals the functionality of a completely degraded wetland. We compared seasonal measurements of potential denitrification, N2O emissions, and related soil characteristics to HGM assessments of nine non-tidal riverine wetlands and seven flats wetlands in the Nanticoke River watershed in Delaware and Maryland, USA. Denitrification potential, measured as denitrification enzyme activity (DEA), was higher in riverine wetlands than in flats. DEA increased with increases in percent water-filled pore space, pH, ammonium concentration, and the percentages of N and organic carbon. DEA decreased with increases in oxidation-reduction potential (Eh) and water-table depth. The difference in DEA between riverine and flats wetlands was attributable to the differences in the correlated soil characteristics. N2O emission rates were higher on average in riverine wetlands than in flats, but the difference was not statistically significant. N2O emission rates were generally less predictable than DEA and showed only weak correlations with pH, water-table depth, and the percentage of water-filled pore space when data from riverine wetlands and flats were combined. HGM biogeochemistry function scores ranged from 0.18 to 1 for the riverine wetlands and from 0.24 to 0.98 for the flats. The scores did not correlate with N2O emission or DEA, except for summer DEA in flats, which increased with increasing score. Wetland alterations that increase soil moisture relative to reference standard conditions decrease biogeochemistry and hydrology function scores but increase DEA. Biogeochemistry function scores would more closely reflect denitrification potential if the scoring incorporated measurements of soil characteristics that correlate with DEA.