Stream nitrate response to different burning treatments in Southern Appalachian Forests
Southern Appalachian forests are undergoing considerable change due to altered disturbance regimes. For example, fire exclusion has had a major impact on the structure and function of pine-hardwood ecosystems. Recently, fire has been prescribed for a variety of applications: 1) stand-replacement in the form of a mimicked wildfire, 2) site-preparation as part of a fell-and-burn prescription, and 3) understory burning for fuels reduction and wildlife habitat improvement. Assessing watershed-scale responses to burning requires identification of key parameters indicative of changes in structure and function. In the southern Appalachians, nitrogen in the form of NO3 is a key indicator of ecosystem change or response to disturbance. We compared stream NO3-N responses among stand-replacement fires (Winespring Creek and Hickory Branch), a fell-and-burn prescription (Jacobs Branch), and a wildfire in an old-growth deciduous forest (Joyce Kilmer). Nitrate-nitrogen concentration increased following two of the four fires. Concentrations following the fell-and- burn prescription fire increased from <0.01 to a maximum of 0.075 mg L-1 and remained elevated for 8 months. Similarly, stream NO3 concentration increased approximately 2 weeks following the old-growth deciduous wildfire from 0.04 to a maximum of 0.50 mg L-1 and remained elevated for 6 weeks. There were no significant differences in NO3 following one of the stand-replacement fires or between treatment and control or pre- and post-burn following the other stand-replacement fire due to maintenance of an unburned riparian area. Although the old-growth deciduous wildfire was essentially an understory burn, the magnitude of stream N response suggests that unavailable recalcitrant forms of N may have been released during the wildfire, as well as a reflection of the potential inefficiency of old-growth forests at sequestering mobilized nutrients. In all cases, hydrologic losses of NO3-N were insignificant with respect to effects on water quality and site depletion of N.