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

Abstract

The δ 18O 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 δ 18O values of wet deposition and throughfall exhibited similar pronounced seasonal trends, with relatively depleted δ 18O values during the summer. Wet deposition and throughfall nitrate δ 18O values were not significantly different between northern (Leading Ridge, PA) and southern (Fernow, WV) regional sampling sites, indicating that δ 18O values were spatially similar across the study area. Atmospheric nitrate δ 18O values were significantly greater than microbe-produced nitrate δ 18O 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 δ 18O values were greater during stormflow periods compared to baseflow periods, indicating greater contributions of atmospheric nitrate during storm events. Neither microbe-produced nitrate δ 18O values from incubated forest soil samples nor stream nitrate δ 18O 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 δ 18O 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.