The Nitrate Inventory of Unsaturated Soils at the Barrow Environmental Observatory: Current Conditions and Potential Future Trajectories

Studies conducted in the Barrow Environmental Observatory under the auspices of the United States Department of Energy Next Generation Ecosystem Experiment (NGEE) - Arctic have demonstrated measurable nitrate concentrations ranging from <1 to 17 mg/L in the unsaturated centers of high-centered polygons. Conversely, nitrate concentrations in saturated areas of polygonal terrain were generally below the limit of detection. Isotopic analysis of this nitrate demonstrates that it results from microbial nitrification. The study site currently comprises mostly saturated soils. Several factors, however, could lead to drying of soils on different time scales. These include 1) topographic inversion of polygonal terrain associated with ice-wedge degradation, 2) increased connectivity and drainage of polygon troughs, similarly related to the thawing and subsidence of ice-wedges, and 3) near-surface soil drainage associated with wide-spread permafrost thaw and active layer deepening. Using a GIS approach we will estimate the current inventory of nitrate in the NGEE intensive study site using soil moisture data and existing unsaturated zone nitrate concentration data and new concentration data collected in the summer of 2016 from high- and flat-centered polygons and the elevated rims of low-centered polygons. Using this baseline, we will present potential future inventories based on various scenarios of active layer thickening and landscape geomorphic reorganization associated with permafrost thaw. Predicted inventories will be based solely on active layer moisture changes, ignoring for now potential changes associated with mineralization and nitrification of previously frozen old organic matter and changes in vegetation communities. We wish to demonstrate that physical landscape changes alone could have a profound effect on future nitrate availability. Nitrate data from recent NGEE campaigns in the Seward Peninsula of Alaska will also be presented.