EFFECTS OF LAND-USE, CLIMATE VARIATION, AND N DEPOSITION ON N CYCLINGAND C STORAGE IN NORTHERN HARDWOOD FORESTS

We hypothesized that much of the variability in dissolved inorganic ni trogen (DIN) loss from forested catchments can be explained by land us e history and interannual climatic variation, and that these factors d etermine the degree to which N deposition results in increased storage of C in forests. We used an existing model of C, N, and water balance s in forest ecosystems in conjunction with long-term climate and N lea ching loss data from several northern hardwood forest ecosystems to pr edict the effects of land use, climate variability and N deposition on C storage and N cycling and loss. Six sites from the White Mountains of New Hampshire with very different land use histories and annual str eam DIN losses were used. The only model parameter that varied between sites was land use or disturbance history. Each site was simulated us ing both mean climate data for each year and actual time series climat e data. Vegetation removal resulted in a period of increased DIN leach ing, followed by losses below those in control stands for both measure d and simulated data. One site with an extreme fire event over 170 yea rs ago still showed reduced N losses in both modeled and measured data . Significant interannual variation in DIN loss is evident in the fiel d data. Model predictions using actual climate time series data captur ed much of this variation. This high interannual variability along wit h the slow rate of change in DIN loss predicted by PnET-CN using mean climate throughout the simulations suggests that statistically signifi cant increases in DIN leaching losses due to long-term increases in N deposition will not be detectable for several decades, given current r ates of N deposition. N deposition increased C storage in all simulati ons, but the quantity stored was about 50% that predicted by another p ublished model. This difference results from differences in the effici ency with which added N is retained in the ecosystem. The;previous mod el used an 80% retention value, while retention was closer to 50% over most of the time period examined here.