Scaling Denitrification Fluxes from Cores to Catchments: Spatial and Temporal Controls

Abstract:

The influence of spatial and temporal heterogeneity on nitrogen cycling can be profound but catchment scale understanding remains elusive. One of the largest sources of uncertainty is the importance of 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 develop 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 the Pond Branch watershed in the Piedmont region of Maryland, nitrogen deposition values are relatively high (9kg/ha/yr) with low stream export (0.5 kg/ha/yr). Our data suggest that at least 16-27% of this retention can be accounted for by denitrification in certain areas of the riparian zone. We highlight the importance of riparian microtopography and the need to better link observations and models.