SummaryUnderstanding the context dependence of ecosystem responses to global changes requires the development of new conceptual frameworks. Here we propose a framework for considering how tree species and their mycorrhizal associates differentially couple carbon (C) and nutrient cycles in temperate forests. Given that tree species predominantly associate with a single type of mycorrhizal fungi (arbuscular mycorrhizal (AM) fungi or ectomycorrhizal (ECM) fungi), and that the two types of fungi differ in their modes of nutrient acquisition, we hypothesize that the abundance of AM and ECM trees in a plot, stand, or region may provide an integrated index of biogeochemical transformations relevant to C cycling and nutrient retention. First, we describe how forest plots dominated by AM tree species have nutrient economies that differ in their Cnutrient couplings from those in plots dominated by ECM trees. Secondly, we demonstrate how the relative abundance of AM and ECM trees can be used to estimate nutrient dynamics across the landscape. Finally, we describe how our framework can be used to generate testable hypotheses about forest responses to global change factors, and how these dynamics can be used to develop better representations of plant-soil feedbacks and nutrient constraints on productivity in ecosystem and earth system models.

SummaryAlthough much is known about how trees and their associated microbes influence nitrogen cycling in temperate forest soils, less is known about biotic controls over phosphorus (P) cycling. Given that mycorrhizal fungi are instrumental for P acquisition and that the two dominant associations -arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi -possess different strategies for acquiring P, we hypothesized that P cycling would differ in stands dominated by trees associated with AM vs ECM fungi.We quantified soil solution P, microbial biomass P, and sequentially extracted inorganic and organic P pools from May to November in plots dominated by trees forming either AM or ECM associations in south-central Indiana, USA.Overall, fungal communities in AM and ECM plots were functionally different and soils exhibited fundamental differences in P cycling. Organic forms of P were more available in ECM plots than in AM plots. Yet inorganic P decreased and organic P accumulated over the growing season in both ECM and AM plots, resulting in increasingly P-limited microbial biomass.Collectively, our results suggest that P cycling in hardwood forests is strongly influenced by biotic processes in soil and that these are driven by plant-associated fungal communities.

1.While it is well established that leaf litter decomposition is controlled by climate and substrate quality at broad spatial scales, conceptual frameworks that consider how local-scale factors affect litter decay in heterogeneous landscapes are generally lacking. 2. A critical challenge in disentangling the relative impacts of and interactions among local-scale factors is that these factors frequently covary due to feedbacks between plant and soil communities. For example, forest plots dominated by trees that associate with ectomycorrhizal (ECM) fungi often differ from those dominated by trees that associate with arbuscular mycorrhizal (AM) fungi in terms of their litter quality, microbial community structure and inorganic nutrient availability. Here, we evaluate the extent to which such factors alter leaf litter decomposition rates. 3. To characterize variations in decomposition rates, we compared decay rates of high-quality litter (maple; AM) and low-quality litter (oak; ECM) across forest plots representing a gradient in litter matrix quality and nitrogen (N) availability driven by the relative proportions of AM and ECM trees in each plot. In experiment two, we added litter from two AM and three ECM tree species to forest plots with either a high-quality litter matrix and high N availability (i.e. AM-dominated plots) or a low-quality litter matrix and low N availability (i.e. ECM-dominated plots). In both experiments, we found that AM litter decomposed more rapidly than ECM litter, and this effect was enhanced in AM-dominated plots. 4. Then, to separate the contributions of litter matrix effects from N availability effects, we added N fertilizer to a subset of plots from experiment two. Nitrogen addition increased decay rates of highquality litter across all sites, but had no effect on low-quality litter, suggesting that low N availability, not litter matrix quality, constrains decomposition of high-quality litters. Hence, N availability appears to alter litter decomposition patterns independently of litter matrix properties. 5. Synthesis. Our results indicate that shifts in the relative abundance of ECM-and AM-associated trees in a plot or stand have the potential to affect litter decay rates through both changes in litter quality as well as through alterations of the local-scale soil environment.

No abstract

Ecosystems often show differential sensitivity to chronic nitrogen (N) deposition; hence, a critical challenge is to improve our understanding of how and why site-specific factors mediate biogeochemical responses to N enrichment. We examined the extent to which N impacts on soil carbon (C) and N dynamics depend on microbial resource stoichiometry. We added N to forest plots dominated by ectomycorrhizal (ECM) trees, which have litter and soil pools rich in organic N and relatively wide C:N ratios, and adjacent forest plots dominated by arbuscular mycorrhizal (AM) trees, which have litter and soil pools rich in inorganic N and relatively narrow C:N ratios. While microbes in both plot types exhibited fairly strict biomass homeostasis, microbes in AM- and ECM-dominated plots differed in their physiological responses to N addition. Microbes in ECM plots responded to N enrichment by decreasing their investment in N-acquisition enzymes (relative to C-acquisition enzymes) and increasing N mineralization rates (relative to C mineralization rates), suggesting that N addition alleviated microbial N demand. In contrast, heterotrophic microbial activities in AM plots were unaffected by N addition, most likely as a result of N-induced increases in net nitrification (60% increase relative to control plots) and nitrate mobilization (e.g., sixfold increases in mobilization relative to control plots). Combined, our findings suggest the stoichiometric differences between AM and ECM soils are the primary drivers of the observed responses. Plant and microbial communities characterized by wide C:N are more susceptible to N-induced changes in decomposition and soil C dynamics, whereas communities characterized by narrow C:N are more susceptible to N-induced nitrate leaching losses. Hence, the biogeochemical consequences of N deposition in temperate forests may be driven by the stoichiometry of the dominant trees and their associated microbes.