Effects of Nitrogen Enrichment, Wildfire, and Harvesting on Forest‐Soil Carbon and Nitrogen

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Abstract
Northern forest soils represent large reservoirs of C and N that may be altered by ecosystem perturbations. Soils at three paired watershed in Maine were investigated as case studies of experimentally elevated N deposition, wildfire, and whole‐tree harvesting. Eight years of experimental (NH4)2SO4 additions at the Bear Brook Watershed in Maine significantly reduced forest‐floor C/N ratios from 30.6 to 23.4. Forest‐floor C and N pools were lower in the treated watershed (38 Mg C ha−1, 1612 kg N ha−1) compared with the reference (75 Mg C ha−1, 2372 kg N ha−1). Fifty years after wildfire at Acadia National Park, the burned watershed with hardwood regeneration had significantly lower forest‐floor C and N concentrations (208 g C kg−1 soil, 9.9 g N kg−1 soil) than the reference watershed dominated by a softwoods (437 g C kg−1 soil, 12.8 g N kg−1 soil). Forest‐floor C and N pools were lower in the burned watershed (27 Mg C ha−1, 1323 kg N ha−1) compared with the reference (71 Mg C ha−1, 2088 kg N ha−1). At the Weymouth Point, the harvested watershed regenerated to spruce‐fir, the dominant stand type that existed before the harvest, and it had significantly lower forest‐floor C concentrations and pools (406 g C kg−1 soil, 24 Mg C ha−1) than the reference (442 g C kg−1 soil, 39 Mg C ha−1) after 17 yr. All perturbations studied were associated with lower forest‐floor C pools.