Plant and Soil Natural Abundance delta-15N: Indicators of Nitrogen Cycling in the Catskill Mountains, New York, USA

We examined the potential use of natural abundance ¹⁵N of plants and soils as an indicator of forest nitrogen (N) cycling rates within the Catskill Mountains, NY. These watersheds receive among the highest rates of N deposition in the northeastern United States and are beginning to show signs of N saturation. Many studies have shown a link between increased N cycling rates and ¹⁵N enrichment of soil and plant pools. Faster rates of N cycling processes, especially nitrification, lead to fractionation of ^14/15N, creating N products that are relatively depleted in ¹⁵N. This can lead to enrichment of soil pools, as lighter ¹⁴N is lost from the system via leaching or denitrification. Plant N pools can become increasingly enriched as they take up ¹⁵N-enriched soil N. Despite similar amounts of N deposition across the Catskill Mountains, forests dominated by different tree species appear to vary in the amount of N retained or lost to nearby streams. To determine if plant and soil ¹⁵N could be used as indicators of N cycling rates, we collected foliage, wood, litterfall, organic and mineral soil, and fine roots from single species stands of American beech (Fagus grandifolia), eastern hemlock (Tsuga canadensis), red oak (Quercus rubra), and sugar maple (Acer saccharum). Fine roots and soil ¹⁵N were highest within sugar maple stands (p<0.05). Sugar maple soils also had the highest rates of net nitrification and N leaching. Therefore, soil ¹⁵N appears to correlate with forest N retention and loss. However, ¹⁵N enrichment was highest within foliage, litterfall and wood of beech trees (p<0.05). The decoupling between foliage ¹⁵N and N cycling, as well as between ¹⁵N of foliage and fine roots, illustrates that it may not be possible to use a single plant pool as an indicator of N cycling rates.