Nitrate Sorption in the Soils of the Coweeta Hydrologic Laboratory

Abstract

Atmospheric deposition of reactive nitrogen (N) from anthropogenic sources to forested systems may have acute and long-term deleterious impacts on tree species and water quality. Understanding how nitrate (NO3-) moves through the soil system and if it has the potential to be retarded from vertical or lateral leaching allows for a better understanding of the processes important for NO3- movement and export from forested watersheds. We examined four watersheds at Coweeta Hydrologic Laboratory (CHL) and determined that soil NO3- sorption is a mechanism for abiotic NO3- retention. NO3- sorption was best described with an S-shaped, sigmoidal model for B horizons that suggests that NO3- sorption to soil colloidal surfaces has a higher affinity for soil solution at low equilibrium concentrations. Parameter a, the sorption maximum, was most strongly correlated to ammonium oxalate extractable Al (Alo) and Mn (Mno), suggesting that amorphous Al and Mn oxides may be the primary source of positively charged sorption sites. Parameter b, the width of the sigmoid curve slope, was best predicted by %C in the soil; suggesting that C compounds may bind to and reduce the availability of positively charged exchange sites for NO3- sorption. Previously harvested watersheds exhibited larger variability in parameter values Xo, the inflection point of the curve, and b. High elevation watersheds had higher median values for Alo, Mno and the ration of oxalate to dithionite extractable Fe (Feo/Fed), suggesting that the soils at higher elevations are at earlier stages of pedogenic development and have more poorly crystalline Fe and Al oxides. The greatest potential for sorption maybe at an intermediate soil depth between where there is a significant decrease in biologically cycled C, phosphate and sulfate yet there is enough mineral weathering to provide the mineralogical structures that can support positively charged surfaces.