| Three studies were conducted to explain the significant, existing variation in nitrogen export from mid-Appalachian forested watersheds. The specific objective of the studies was to determine if bedrock geology, soil chemistry, past land disturbance (fire, farming, and logging), vegetation type and age, and atmospheric nitrate deposition contributions had any control over stream nitrate concentrations. In the first study, stream nitrate was grab-sampled at baseflow on 27 forested watersheds in close proximity to Fernow watershed 4, WV, a stream nationally known for high stream nitrate concentrations and thought to be nitrogen saturated. The 27 watersheds showed significant variation in stream nitrate-N concentrations (0.005 to 1.540 mg L-1), with nearly half of the streams exhibiting nitrate concentrations higher than Fernow 4. The observed variation in stream nitrate-N concentrations was related to differences in bedrock geology. Watersheds containing primarily Mauch Chunk/Greenbrier (MCG) bedrock exhibited significantly higher stream nitrate-N concentrations than watersheds with Catskill/Chemung/Pocono (CCP) bedrock. In a second broader study, the variation in stream nitrate-N concentrations from 45 forested watersheds throughout the mid Appalachian region (northwest PA, southwest PA, and northwest MD and WV) was also related to differences in bedrock geology. Watersheds with the MCG, CCP, and Pottsville/Allegheny (PVA) bedrock types exhibited high, mid, and low stream nitrate-N concentrations, respectively. Stream organic N concentrations in the 45 watershed study were on the same order of magnitude as nitrate concentrations, but did not vary significantly among geology types or any other watershed characteristics. Stream nitrate-N concentrations were significantly correlated with the soil chemistry parameters pH, Ca, %N, and ON ratios. Soil chemistry differences were the most likely proximate cause of the stream nitrate differences, since they have the most direct control on microbial soil nitrate production. Bedrock geology was the most likely ultimate cause of stream nitrate differences, because it helps determine soil chemistry characteristics. In the final study, 18O in nitrate isotopic separation procedures indicated that the majority of stream nitrate (>70%) at high and low flows originated from microbial produced nitrate rather than atmospheric nitrate, which supported the finding that geology/soil complexes largely control nitrate leaching to streams. |
