Global environmental changes are shifting the distribution and abundances of species in nearly all ecosystems on earth, yet the biogeochemical consequences of such changes are poorly understood. In forests of the northeastern and midwestern US, tree species that associate with arbuscular mycorrhizal (AM) fungi are becoming increasingly dominant relative to those that associate with ectomycorrhizal (ECM) fungi. Given that AM and ECM trees differ in their nutrient use traits, we sought to explore whether AM- and ECM-dominated forest plots would support distinct soil microbial communities that have divergent functions. To do this, we collected soils from AM- and ECM-dominated forest plots at a single site in south-central Indiana, across mycorrhizal gradients (plots differing in their relative abundance of AM vs. ECM trees) in three forests in southern Indiana, and across mycorrhizal gradients (six sites) throughout the northeastern and midwestern US. We examined bacterial and fungal communities from upper surface soils in each plot using multiple techniques, and investigated the microbial taxa most involved in fungal turnover and nitrification – two important N cycling processes in these forests.
Results/Conclusions
Across the regional mycorrhizal gradient, increases in the percentage of ECM trees in a plot correlated positively with fungal to bacterial ratios as estimated by the glucosamine:galactosamine ratio (Pearson’s r = 0.62; P < 0.001). This same pattern – greater fungi:bacteria in ECM-dominated plots – was also observed across our Indiana mycorrhizal gradient and in our AM and ECM dominated plots using different methods of quantification (qPCR and PLFA). Additionally, we found evidence that AM- and ECM-dominated plots were associated with distinct microbial communities and activities. AM-dominated plots contained more proteobacteria and fewer acidobacteria, and more Ascomycota and fewer Basidiomycota, relative to ECM-dominated plots (P < 0.05). In AM-dominated plots, fungal decay was dominated by saprotrophic fungi and putative endophytes, whereas in ECM-dominated plots fungal decay was dominated by saprotrophic fungi and ECM fungi. AM-dominated plots contained greater abundances of N-cycling microbes (estimated by copy numbers of N-cycle genes found in soil metagenomes; P < 0.05) and greater net nitrification rates in AM soils (P < 0.0001). Collectively, our results suggest that shifts in the relative abundance of AM- and ECM-associated trees owing to global changes may have profound implications for soil microbial communities and hence, how forests cycle carbon and nutrients.