Response of the natural abundance of15N in forest soils and foliage to high nitrate loss following clear-cutting

Export of microbially produced nitrate from an ecosystem is expected to increase δ15N 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 δ15N, we measured δ15N in a clear-cut watershed at the Hubbard Brook Experimental Forest, New Hampshire. δ15N ranged from –0.02‰ in the Oie horizon to 7.7‰ in the Bs2 horizon prior to clear-cutting and increased significantly by 1.3‰ in the Oie horizon and 0.9‰ in the Oa horizon 3 years after clear-cutting. Fifteen years after clear-cutting, δ15N in both O horizons decreased to near-initial values. No significant temporal changes in the Bs2 and C horizons δ15N were observed. Foliar δ15N was highest (1.7‰) the first 2 years after clear-cutting and was significantly higher than in the reference watershed (mean δ15N = –1.2‰), decreasing to 0.0‰ 3–5 years after clear-cutting and to –1.3‰ 9–11 years after clear-cutting. Increased foliar δ15N 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 δ15N may provide a practical alternative to long time series of stream chemistry for evaluating nitrogen saturation of forested ecosystems.