CHARACTERIZATION OF DENITRIFICATION ACTIVITY IN ZONES OF GROUNDWATER EXFILTRATION WITHIN A RIPARIAN WETLAND ECOSYSTEM

Authors: McCarty, Gregory; Mookherji, Swati; ANGIER, J - US EPA

Submitted to: Biology and Fertility of Soils
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/9/2006
Publication Date: 11/1/2006
Citation: McCarty, G.W., Mookherji, S., Angier, J.T. 2006. Characterization of denitrification activity in zones of groundwater exfiltration within a riparian wetland ecosystem. Biology and Fertility of Soils. DOI 10.1007/s00374-0006-0151-0.

Interpretive Summary: Riparian buffers (vegetated areas around streams) may be effective natural remediation sites for agricultural nutrients such as nitrogen. These buffers would function most effectively if water moved uniformly through these ecosystems but we have found that water movement is spatially diverse with large amounts moving into the ecosystem in discrete upwelling zones. This can limit the interaction of nutrients such as nitrogen with the ecosystem processes that remove the nutrient. In this study we characterized the ability of soil within the upwelling zone to remove agricultural nitrogen by denitrification (formation of atmospheric gas). We found high potential for denitrification within these upwelling zones but that it was not uniformed distributed and that zones with highest rates of denitrification are associated with fresh carbon inputs. These studies provide important information for improved management of riparian buffers to reduce agricultural contamination of surface waters.

Technical Abstract: Movement of agricultural nitrogen (N) into riparian buffers often occurs within discreet seepage or upwelling zones which can limit ability of the ecosystem to process the nutrient. Characterization of the biogeochemical processing of N within these zones is important in assessing the of effectiveness of riparian buffers for mitigating soil water contamination. The biogeochemical potential for denitrification in groundwater upwelling zones of a riparian buffer wetland dominated by high carbon mucky soil was found to be highly stratified with profile depth with substantially higher active within the surface layer. The denitrification enzyme activity (DEA) within the wetland upwelling zone was closely related to the population size of denitrifying microorganisms as measured the most probable number (MPN) as well as the general microbial population as measured by substrate induced respiration (SIR). The addition of glucose to DEA assay tended to stimulate enzyme activity indicating that carbon substrate was limiting activity. The stratification pattern of microbial populations and DEA are consistent with new carbon inputs to the ecosystem being most important driver of biogeochemical activities such as denitrification. A survey of carbon inputs to the ecosystem under study identified two major sources skunk cabbage in early summer and tree leaf litter in the fall that result in most of the annual biomass carbon inputs to the wetland. Tests of the ability plant residues to stimulate denitrification and microbial respiration which was inversely related to the C/N ratio of these carbon sources.