18O isotopic separation of stream nitrate sources in mid-Appalachian forested watersheds

The delta O-18 values of atmospheric nitrate deposition, microbe-produced nitrate, and stream nitrate were measured to determine the dominant source of stream nitrate in 27 mid-Appalachian headwater forested watersheds (12-771 ha) with varying bedrock geologies, land disturbance histories, and stand ages. The 12 monthly composite nitrate delta O-18 values of wet deposition and throughfall exhibited similar pronounced seasonal trends, with relatively depleted delta O-18 values during the summer. Wet deposition and throughfall nitrate delta O-18 values were not significantly different between northern (Leading Ridge, PA) and southern (Fernow, WV) regional sampling sites, indicating that delta O-18 values were spatially similar across the study area. Atmospheric nitrate delta O-18 values were significantly greater than microbe-produced nitrate delta O-18 values. allowing the two sources of stream nitrate to be separated. During four baseflow and three stormflow sampling periods, microbe-produced nitrate was the dominant (> 70%) source of nitrate in the study streams. This result does not mean atmospheric nitrogen deposition should be discounted as a source of forested stream nitrate, because atmospheric deposition is the primary external contributor to the long-term soil nitrogen pool that ultimately drives soil nitrate production rates. Stream nitrate delta O-18 values were greater during stormflow periods compared to baseflow periods, indicating greater contributions of atmospheric nitrate during storm events. Neither microbe-produced nitrate delta O-18 values from incubated forest soil samples nor stream nitrate delta O-18 values showed strong relationships with land disturbance history or stand age. However, watersheds dominated by Pottsville/ Allegheny bedrock and associated extremely acid soils had greater summer stream nitrate delta O-18 values than watersheds containing predominantly Catskill/Chemung/Pocono and Mauch Chunk/Greenbrier bedrock. Inhibited microbial nitrate production by low soil pH could account for the greater proportions of atmospheric nitrate deposition in streams draining Pottsville/Allegheny bedrock. (C) 2001 Elsevier Science B.V. All rights reserved.