Mechanisms driving the seasonality of catchment scale nitrate export: Evidence for riparian ecohydrologic controls

Abstract

Abstract Considerable variability in the seasonal patterns of stream water nitrate ( ) has been observed in forested watersheds throughout the world. While many forested headwater catchments exhibit winter and early spring peaks in concentrations, several watersheds have peak concentrations during the summer months. Pond Branch, a headwater catchment in Maryland monitored for over 10 years, exhibits recurrent and broad summer peaks in both concentrations and watershed export. Higher export from June to September is particularly surprising, given that these summer months typically have the year's lowest discharge. A key challenge is identifying the source(s) of and the mechanism(s) by which it is transported to the watershed outlet during the summer. In this study, we assessed multiple hypotheses (not mutually exclusive) that could account for the seasonal trend including proximal controls of groundwater‐surface water interactions, instream processes, and riparian groundwater‐N cycling interactions, as well as two distal controls: geochemical weathering and senescence of riparian vegetation. A combination of long‐term weekly and limited duration high‐frequency sensor data reveals the importance of riparian ecohydrologic processes during base flow. In this watershed, patterns of seasonal stream water concentrations and fluxes depend fundamentally on interactions between groundwater dynamics and nitrogen (N) cycling in the riparian zone. Groundwater tables control nitrification‐denitrification dynamics as well as hydrologic transport. Our results suggest that in many watersheds, a more sophisticated exploration of production and transport mechanisms is required to identify critical points in the landscape and over time that disproportionately drive patterns of watershed export.