A decades-old question in ecosystem science is “To what extent can we predict the biogeochemical consequences of plant-soil interactions? The Mycorrhizal-Associated Nutrient Economy (MANE) hypothesis, which builds on well-developed theories of leaf litter decay, soil organic matter dynamics and resource stoichiometry, predicts that plants and microbes that associate with different types of mycorrhizal fungi possess a suite of nutrient use traits that both reflect and determine soil attributes, leading to predictable biogeochemical syndromes in ecosystems. Here, we tested two predictions that arise from the MANE hypothesis: 1) trees that associate with arbuscular mycorrhizal (AM) fungi have faster decomposing leaf litters than trees that associate with ectomycorrhizal (ECM) fungi, and 2) plots dominated by AM trees have soils with a narrower C:N and greater inorganic N availability than plots dominated ECM trees. We tested these predictions by measuring plant and soil characteristics across 45 plots in south-central Indiana that vary in their relative abundance of AM and ECM trees, and by synthesizing plant trait data (for AM and ECM trees) and soils data (from AM-dominated and ECM-dominated stands) from temperate forests.
Overall, we found strong support for both predictions. In Indiana, AM litters decomposed ~50% faster than ECM litters (P < 0.05), with the magnitude of this effect greatest in AM-dominated plots. In the regional synthesis, we found AM litters decomposed nearly twice as fast as ECM litters (P < 0.05), and differences between AM and ECM decay rates were independent of phylogeny (e.g., gymnosperms versus angiosperms). In Indiana, soil C:N ratios were narrower in AM-dominated plots than in ECM-dominated plots, and the percentage of ECM trees in a plot was a strong predictor of soil C:N (r2 = 0.77; P < 0.01). Additionally, net nitrification rates and N leaching losses were more than five-fold greater in AM soils relative to ECM soils, and both processes were correlated to the relative abundance of AM and ECM trees in each plot. In the regional synthesis, soils dominated by AM trees had narrower C:N than those dominated by ECM trees (P = 0.06) and two-fold greater nitrification rates and leaching losses (P = 0.07). Collectively, our results demonstrate convergence in how AM- and ECM-associated plants affect C and N cycling in temperate forests, and indicate that mycorrhizal associations may represent “trait integrators” for a suite of plant and microbial functional traits involved in coupling C-nutrient cycles.