Response of the natural abundance of 15N in forest soils and foliage to high nitrate loss following clear-cutting

Export of microbially produced nitrate from an ecosystem is expected to increase delta(15)N in the remaining soil organic matter and NH4+. To test the hypothesis that nitrification and nitrate loss induced by clear-cutting cause an increase in soil and foliar delta(15)N, we measured delta(15)N in a clear-cut watershed at the Hubbard Brook Experimental Forest, New Hampshire. delta(15)N ranged from -0.02parts per thousand in the Oie horizon to 7.7% in the Bs2 horizon prior to clear-cutting, and increased significantly by 1.3parts per thousand in the Oie horizon and 0.9parts per thousand in the Oa horizon 3 years after clear-cutting. Fifteen years after clear-cutting, delta(15)N in both O horizons decreased to near-initial values. No significant temporal changes in the Bs2 and C horizons delta(15)N were observed. Foliar delta(15)N was highest (1.7parts per thousand) the first 2 years after clear-cutting and was significantly higher than in the reference watershed (mean delta(15)N = -1.2parts per thousand), decreasing to 0.0parts per thousand 3-5 years after clear-cutting and to -1.3parts per thousand 9-11 years after clear-cutting. Increased foliar delta(15)N coincided with increased stream-water nitrate concentration, suggesting that the increased nitrification responsible for elevated stream-water nitrate may also have caused an enrichment of the plant-available ammonium pool. The response observed in this catchment also suggests that sampling of soil or foliar delta(15)N may provide a practical alternative to long time series of stream chemistry for evaluating nitrogen saturation of forested ecosystems.