Examining the mechanistic basis of summer nitrate peaks: a coupled biogeochemical and hydrological approach
Contrary to behavior predicted by the nitrogen saturation hypothesis, Pond Branch (the Baltimore Ecosystem Study forested reference watershed) exhibits peak nitrogen export during the growing season. 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 explore possible sources of stream nitrogen along with hydrologic transport mechanisms by combining high frequency spatially distributed measurements and models. Upland, hillslope hollow, and riparian hummock and hollow locations were identified as fundamental landscape units. We instrumented each landscape unit with soil oxygen, soil moisture, and groundwater wells and collected soil cores during each season. Limited duration high temporal resolution (15 minute) measurements of stream nitrate concentrations at the outlet were used to assess how landscape heterogeneity of soil moisture and oxygen relate to summer nitrate export. 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 on groundwater tables, soil moisture, and soil oxygen show marked diurnal variability that appears to drive the rapid increase in nitrate concentrations. Riparian hollows become oxygenated within the same time period as nitrogen export increases and diurnal riparian groundwater fluctuations provide enough hydrologic variability to increase net nitrification and provide a transport mechanism to the stream. This work highlights the importance of hydrologic variability for nitrogen retention and saturation studies.