Site-level importance of broadleaf deciduous trees outweighs the legacy of high nitrogen (N) deposition on ecosystem N status of Central Appalachian red spruce forests

Abstract

Atmospheric nitrogen (N) deposition can influence forest ecosystem N status, and the resilience of forests to the effects of N deposition depends on a number of co-occurring environmental factors that regulate N retention or loss. In this study, we test the idea that N deposition may have important and long-lasting impacts on patterns of N cycling by using field and laboratory techniques to assess N status in seven high-elevation Central Appalachian red spruce (Picea rubens Sarg.) forests located at sites that historically received moderate to high inputs of N atmospheric deposition. During 2011 and 2012, we measured multiple indices of N availability (e.g. foliar/soil C:N and δ15N, resin ion-exchange, and N transformation rates) that integrate N cycling over seasonal to decadal time scales. Using a model selection approach, we compared the strength of the association between various environmental factors and temporally-integrated indices of N status in a series of regression models. Site-level differences in the relative importance value of broadleaf deciduous (BD) trees consistently explained most of the observed variation in N status. Soil C:N was significantly lower for sites with greater BD importance (R 2 = 0.67–0.77), and there was a strong positive relationship between BD importance and soil δ15N content (R 2 = 0.64–0.85). Despite a four-fold difference in historic deposition across the seven forest sites, we did not observe any significant relationships between site N status and N deposition. These findings suggest that potential legacy effects of N deposition were obscured by the influence of BD importance on N status at these sites. Our results add strong support to the idea that predicting the resilience of forests to the effects of N deposition requires detailed knowledge on the contribution of tree species composition to soil N cycling and retention.