Stream nitrogen responses to fire in the Southeastern U.S.
Fire can play a significant role in runoff, sediment yield, and nitrate transport in aquatic and terrestrial ecosystems in the southeast US. The typical impact of fire is an immediate change in the physical properties of the soil and forest floor surface, followed by mid- and long-term changes in biological pools and cycling processes. Depending upon the severity of the fire and pre-burn conditions, there is a potential for wildfire and prescribed burns to pose risks within the regions to water quality. There has been little effort to specifically model the effects of prescribed burning and wildfire on forest hydrology. Our approach was to combine field measurements and modeling to quantify the impacts of fire on water quality and hydrology in two sites characteristics of the mountain and piedmont regions of the southeastern US. we used the nutrient cycling model NuCM (Nutrient Cycling Model) as our platform for predicting ecosystem nitrogen response. Study sites were located in the Nantahala National Forest in the southern Appalachians and the Uwharrie National Forest in the piedmont region. Portal automated samplers were installed to sample stream water N; and soil solution lysimeters and overland flow collectors were installed to sample surface vs. subsurface N. We focused inorganic nitrogen (NO3-N, NH4-N) in soil solution and streams draining the burned areas because they are key indicators of ecosystem response to disturbance and important water quality parameters. The Nantahala and Uwharrie sites were burned 7 and 5 months (Respectively) after sampling began. NuCM was parameterized and calibrated with pre- and post-burn data from the Nantahala and Uwharrie sites. In addition, more severe and intense prescribed fires and wildfire scenarios were modeled by increasing fire effects on parameters that are directly or indirectly altered by fire. In general, both stream NO3-N and stream NH4-N concentrations were unaffected by prescribed fire at any level of intensity or severity. Slight increases in stream and soil solution NO3-N concentration were observed under the wildfire scenario, but responses were well below levels of concern for aquatic resources and drinking water.
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