Predicting the ecological consequences of shifting forest composition requires understanding how trees species alter the soils beneath them. However, given the huge number of tree species on earth, it is critical to assess the extent to which easily observable traits facilitate the prediction of soil biogeochemistry. Both aboveground and belowground traits are important drivers of forest function. The objective of this study was to assess the relative importance of leaf habit (evergreen or deciduous) and root mycorrhizal association (arbuscular [AM] or ectomycorrhizal ECM]) on biogeochemistry. Disentangling the relative importance of these two traits for carbon (C) and nitrogen (N) dynamics have proven difficult as most deciduous tree species associate with AM fungi and most evergreen tree species associate with ECM fungi. Thus, an open question remains – to what extent do trees with different leaf habits and/or mycorrhizal associations create sites with distinct soil properties? In this study, we used nearly 100-year-old planted, single-species forestry plots at The Morton Arboretum (DuPage County, IL) to evaluate the relative importance of mycorrhizal association and leaf habit on soil C and N dynamics.
Results/Conclusions
Overall, mycorrhizal association was a better predictor of tree trait-driven differences in soil C and N dynamics than leaf habit. While both leaf habit and mycorrhizal association drove variation in total soil C:N ratios (myco: P <0.001; leaf: P =0.013), mycorrhizal association alone predicted differences in soil pH, available N pools, extracellular enzyme activities, N cycling rates, and C mineralization rates (P≤0.024). Surprisingly, ammonium concentrations were higher and net nitrification rates were faster in ECM plots than in AM plots. Higher levels of N-degrading enzyme activity in ECM plots and intrinsically high soil pH across the garden likely drove more rapid N cycling in ECM plots. Thus, we show that mycorrhizal association is more important than leaf habit for predicting soil biogeochemistry, which supports the inclusion of mycorrhizal associations in terrestrial biosphere models. However, our study also complements a growing body of literature demonstrating that mycorrhizal effects on soil processes, and N dynamics in particular, are not uniform across sites.