Nitrate Sorption As a Potential Mechanism to Buffer Watershed N Export.

Atmospheric N deposition has increased over the last century, a trend predicted to continue into the future. These additions can exceed the N retention capacity of terrestrial ecosystems, resulting in increased watershed N export and a decline in surface water quality. This study sought to characterize soil nitrate sorption and relate those observations soil solution nitrate fluxes in order to assess the potential for this abiotic mechanism to buffer watershed N export. Two sets of paired watersheds of the Coweeta Hydrologic Laboratory were chosen to provide contrasts in land use history and N deposition. Within each of the four watersheds, 6-12 plots were located to maximize spatial variability with each watershed. At each plot location, resin lysimeters were used to quantify soil solution nitrate fluxes at 0, 30, and 60 cm on a seasonal basis for one full year. Soils were sampled by genetic horizon and analyzed for pH, carbon, extractable sulfate, and selective dissolution of Fe, Al, Mn, and Si. Nitrate sorption was determined for each soil sample using batch equilibration techniques. Soil A horizons showed very little capacity to sorb nitrate. Sorption of B horizon soils appeared to follow a sigmoid curve and were positively correlated with oxalate extractable forms of Al and Mn and negatively correlated with carbon. Intrinsic characteristics of the watershed pairs drove maximum sorptive capacity; however, land use history exerted a strong influence over other sorption characteristics. Further, soils observed to sorb nitrate were positively correlated with subsurface (30-60 cm) soils where resin lysimetry demonstrated a significant nitrate sink. This work suggests that sorption can increase the residence time of nitrate in some soil systems relative to that of water, thereby providing additional time for biological transformations. Further characterization of this mechanism will aid in the prediction of watershed nitrate fluxes across spatial and temporal scales.