The influence of tree species, nitrogen fertilization, and soil C to N ratio on gross soil nitrogen transformations

To investigate controls on gross N transformations in forest soils, a pool dilution technique was used on soils of single-species plots of five major tree species (red oak [Quercus rubra L.], sugar maple[Acer saccharum Marsh.], hemlock [Tsuga canadensis (L.) Carr], beech [Fagus grandifolia Ehrh.] and yellow birch [Betula alleghaniensis Britton]) in the Catskill Mountains of New York State. Catskill forest soils had high rates of gross mineralization and NH ₄ ⁺ consumption, indicating rapid NH ₄ ⁺ cycling, a pattern not captured by net N mineralization assays. Sugar maple had the highest rates of gross mineralization and NH ₄ ⁺ consumption. Rates of gross nitrification were similar to rates of net nitrification for all species. Sugar maple had the highest gross nitrification rates, while hemlock and red oak had the lowest rates. There were no significant species differences in NO ₃ ^- consumption. Fertilization of the plots did not significantly alter N cycling rates with the exception of yellow birch, where N fertilization decreased NO ₃ ^- consumption. We observed a significant negative relationship between net nitrification and soil C/N ratio in both organic and mineral horizons, but our results indicate that the mechanism underlying that relationship was different in the two horizons. In the mineral horizon, limitation of net nitrification in soils of high C/N ratio probably resulted from low gross NH ₄ ⁺ production. In organic horizons, low NH ₄ ⁺ production was not a significant factor and higher NO ₃ ^- consumption explained some of the pattern. Understanding the roles individual tree species as well as excess N input play in regulation of the N cycle will improve forest management and prediction of forest responses to elevated N deposition.