Towards Closing Watershed Nitrogen Budgets: Incorporating Spatial and Temporal Scaling of Denitrification

Closing the nitrogen budget is a major scientific challenge at multiple scales. One of the largest sources of uncertainty is the amount lost via denitrification. Determining in situ rates of denitrification in elements of landscape that remove a disproportionately high amount of N from certain areas of catchment (hot spots) in response to seasonal and event driven conditions (hot moments) is critical to closing watershed nitrogen budgets. We present an approach to scale denitrification flux from seasonal soil cores collected in different landscape positions to the entire watershed using a combination of laboratory core experiments, terrain analysis and in situ soil oxygen and soil moisture content sensors.  In Pond Branch (the forested Baltimore Ecosystem Study reference watershed), nitrogen deposition values are relatively high (10kg/ha/yr) with very little export in streams (0.5 kg/ha/yr). Our data suggest that nearly 50% of this retention can be accounted for by denitrification.  We highlight the importance of riparian microtopography and the need to better link observations and models of coupled hydrological and biogeochemical processes.