Towards closing the watershed nitrogen budget: Spatial and temporal scaling of denitrification

Enhanced consideration of the hydrogeomorphic template of watersheds is critical to understanding watershed nitrogen budgets. We developed a framework to estimate the spatial distribution and temporal dynamics of soil moisture and soil oxygen in surficial soils to scale nitrogen transformations for a forested watershed (Pond Branch) in Maryland, USA. We sampled soil cores in upland, hillslope hollow, riparian hollow, and riparian hummock landscape positions in different seasons for biogeochemical fluxes including measurement of N-2 gas produced via denitrification. We extrapolated these rates in space and time with information derived from in situ soil oxygen and soil moisture probes to scale fluxes from plots to the catchment level. We addressed three questions: (1) How important are seasonal, daily, and storm event variations in soil oxygen for denitrification? (2) How is denitrification spatially distributed through the watershed? (3) How important is denitrification to the watershed nitrogen budget? We found that microtopography within the riparian zone is a significant influence on soil oxygen dynamics and therefore redox-sensitive biogeochemical processes such as denitrification. Riparian zone hollows (lower topographic positions) represented 0.5%-1.0% of the catchment area, but accounted for >99% of total denitrification. Interestingly, topography was a much stronger controller of oxygen than rainfall, which had little influence on temporal variation in soil oxygen levels. Spatial and temporal extrapolations of measured rates suggest that a minimum of 16%-27% of atmospheric nitrogen deposition is lost to denitrification. These results suggest that the importance of denitrification in the nitrogen budget of forested watersheds depends fundamentally on the presence of landscape elements, such as riparian hollows that function as hot spots of activity.