Difference in Gross Nitrogen Cycling in Dominant Southern Appalachian Forests: Implications for Climate Change.

The different dominant forest types in the southern Appalachian Mountains (pine-oak, cove hardwood, mixed-oak and northern hardwood) are all considered N-limited, however, continued N-deposition could drive these systems into nitrogen saturation. Previous studies have identified nitrogen mineralization rates in pine-oak and northern hardwood to be most sensitive to shifts in soil temperature and soil moisture, while overall rates are controlled by soil moisture, temperature and C:N. With inorganic nitrogen accumulation below rooting depth, northern hardwood has shown indications of approaching N-saturation. With no clear correlation between net and gross estimates of nitrogen cycling processes, an assessment of gross process rates is needed to unravel the role of different N cycling pathways in these different forest types. We conducted a lab incubation to assess how N-mineralization, nitrification, denitrification, dissimilatory reduction to ammonium (DNRA) and N immobilization vary along a vegetation and elevation gradient in a southern Appalachian basin. We measured higher rates of N-mineralization, nitrification and denitrification in northern hardwood forests and mineralization and NH₄⁺-immobilization rates in northern hardwood were similar (5.7 ± 2.3 and 6.6 ± 2.1 mg N gsoil^-1 d^-1, respectively). Nitrification rates, however; were always higher than DNRA rates with nitrification being tightly coupled to denitrification. In northern hardwood, nitrification was found to be responsible for 33 ± 23 % of the N₂O production suggesting an important potential role for nitrification in emitting a greenhouse gas that is 300 times as potent as carbon dioxide on a 100 year timespan. Higher inorganic nitrogen concentrations often result in increased N₂O production. The high rates of nitrification and denitrification observed for northern hardwood and the potential for N-saturation could indicate that these systems could be on the threshold of becoming a major greenhouse gas source.