Evaluating critical soil acidification loads and exceedances for a deciduous forest at the Turkey Lakes Watershed

Critical soil acidification loads (CL) and related exceedances, base cation leaching, N leaching, and forest biomass growth were evaluated for a well- studied deciduous forest site within the Turkey Lake Watershed (TLW). The a ssessment was done by way of steady-state mass balance considerations of pr imary inputs for N, Ca, Mg, and K. Critical soil acidification rates were f ound to be high at TLW. These rates amounted to about 900 or 1400 eq/(ha yr ) depending on the forest harvesting regime (selective harvest or maintaine nce of old-growth condition, respectively). The TLW soil substrate (till de rived from basaltic bedrock) appeared to weather well, thereby buffering ag ainst natural and anthropogenic soil acidification. As a consequence, soil acidification exceedances were estimated to be relatively low for both the selective harvest and old-growth scenarios. In comparing overall S and N in put/output data (atmospheric deposition data vs soil leaching losses), we f ound that the TLW site was essentially near or at S and N saturation. We al so found that atmospheric deposition and soil leaching rates have been decl ining since about 1980. The figures for CL and exceedance varied to some ex tent depending on the quality of input data and related uncertainties. Esti mated exceedances were increased when dry- as well as wet-deposition rates were considered. They varied depending on the yearly sulfate/nitrate/base-c ation mix, and the definition of "acceptable acid leaching." In addition, t hey were dependent on whether the forest was considered old growth or not.