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Abstract
Temperate forests receive some of the highest rates of nitrogen (N) deposition in the world. While numerous studies have investigated the effects of N deposition on trees and soils, there is little consensus on why some forests respond positively to N enrichment while others respond negatively. Given that forests provide ecosystem services such as N retention and carbon (C) storage, a unifying framework that explains why forests differ in their sensitivity to N deposition represents a critical frontier in ecosystem science.
Much is known about tree species sensitivities to N enrichment; however, much less is known about N effects on soil microbes and the consequences of N enrichment on microbially-mediated C and N cycling. Here we propose a framework to explain the differential sensitivity of tree species and their associated microbes to N deposition. The mycorrhizal associated nutrient economy (MANE) hypothesis suggests that the type of mycorrhizal association of the dominant tree species in a forest - arbusular mycorrhizal (AM) or ectomycorrhizal (ECM) - unites tree and microbial characteristics with soil C and N dynamics. In this dissertation, I evaluate the hypothesis that variable soil responses to N deposition may be simplified by considering mycorrhizal associations.
Overall, this dissertation demonstrates that N addition leads to reduced ecosystem services in AM forests but increases ecosystem services in ECM forests. Specifically, AM forests exposed to N enrichment leach more nitrate (a pollutant) and store less soil C owing to accelerated leaf litter decomposition rates. In contrast, N deposition has little effect on nitrate leaching in ECM forests and increases soil C retention by decreasing decomposition rates. Thus, forest mycorrhizal associations may provide a useful framework for understanding and predicting variable soil responses to N deposition at landscape and regional scales. Given that the relative abundance of AM and ECM trees are evenly distributed across most forests in the Eastern and Midwestern US, the MANE framework may provide a lens for determining how forests with varying tree species compositions will respond to N deposition.
