Baltimore Ecosystem Study Institute of Ecosystem Studies

2011 BES Annual Meeting Presentation and Poster Abstracts

Spatial and Temporal Controls of Nitrogen Cycling and Export
Duncan, Jonathan
Co-Authors: Groffman, Band

Abstract: Pond Branch is the 41 ha forested reference watershed of the Baltimore Ecosystem Study. The catchment exhibits peak streamwater nitrogen exports during the summer. The majority of nitrate is exported at baseflow, yet groundwater concentrations are depleted relative to the stream. Furthermore, detailed soil core studies have shown that the majority of nitrogen cycling occurs within surface soils, a zone not directly connected to the stream during summer baseflow conditions. We examine the role of microtopography and soil moisture variability as hydrologic controls on watershed nitrogen export by combining high frequency spatially distributed measurements and models. Analysis of soil cores at varying oxygen concentrations using a new gas flow method show that riparian locations have higher rates of N2 flux at 5 and 10% O2 than 0%, suggesting that denitrification is nitrate limited and that there is tight coupling between nitrification and denitrification. Stream nitrate concentrations reveal that the seasonal summer increase (0.04 mg/L to 0.18 mg/L) occurs during baseflow over the span of several days. Complimentary data of groundwater tables, soil moisture, and soil oxygen show marked diurnal variability and demonstrate a strong relationship to the rapid increase of nitrate concentrations. Riparian hollows become oxygenated within the same time period as nitrogen export increases. This work highlights the importance of hydrologic variability for nitrogen retention and saturation studies.