A new framework for predicting how roots and microbes influence soil organic matter dynamics in forests

While it is well-established that tree species modify soil organic matter (SOM) through differences in leaf litter chemistry, far less is known about the role of roots and their microbial associates in influencing SOM dynamics. We investigated the extent to which temperate hardwood trees which associate with arbuscular mycorrhizal (AM) fungi differ in their effects on SOM turnover from those associating with ectomycorrhizal (EM) fungi using 1) root and fungal ingrowth cores, 2) experimental tree girdling and 3) fertilization additions. We conducted our research in the central hardwood forests of southern Indiana where a rich assemblage of AM (e.g. maples, ashes, tulip poplar, black cherry) and EM (e.g. oaks, hickories, beech, pine) tree species co-occur on soils developed from similar parent materials. Our results indicate that EM trees likely play a greater role in contributing to SOM turnover than AM trees as rhizosphere enzyme activities were greater in EM soils than AM soils, and both girdling and fertilization reduced enzyme activities in EM soils but not in AM soils. Although girdling and fertilization had little effect on enzyme activities in AM soils, soil respiration decreased suggesting that much of the carbon (C) allocated belowground was likely derived from roots rather than from mycorrhizal fungi. Collectively our results suggest AM and EM trees influence SOM dynamics in fundamentally unique ways, and that categorizing forests based on the relative abundance of AM and EM trees may provide a useful framework for predicting complex biogeochemical interactions between roots, microbes and SOM.