Natural abundance N in soil and litter across a nitrate-output gradient in New Hampshire

Stable isotopes of nitrogen are potentially a valuable tool for regional assessments of nitrogen saturation because they provide an integrated measure of the past nitrogen cycling history of a site. We measured dN of soil and litter, as well as net nitrification potential, at three sites across a nitrate-loss gradient in the White Mountains, New Hampshire to test the hypotheses: (1) that dN in soil and litter increase across a spatial gradient of nitrate loss; and (2) that dN in soil and litter is elevated when nitrification is elevated. dN was found not to vary significantly among the three sites. Patterns of leaf litter and forest floor dN, however, were strongly influenced by species composition in individual plots. Beech litter had significantly higher dN than yellow birch, sugar maple, and red maple. The conifer-dominated plots had significantly lower dN in both the organic soil horizons and in litter than did the hardwood-dominated plots. When we adjusted for spatial heterogeneity in mineral soil dN values by using an enrichment factor, dNfoliar dNBs, in place of absolute soil dN values, a positive relationship was found with net nitrification for hardwoods. dN may also be a useful tool for evaluating species differences in nitrogen cycling and nitrogen uptake. The distinct pattern we observed of decreasing dN across the continuum from hardwood-dominated to conifer-dominated sites may suggest that local drivers (for example, nitrification rate) regulate the absolute value of foliar dN, while species-driven factors (e.g., timing and type of uptake) control the foliar dN value of one species relative to another in the same plot. # 2007 Elsevier B.V. All rights reserved.