Associations with ectomycorrhizal fungi do not lead to greater soil carbon storage than arbuscular mycorrhizal fungi in temperate forests
The roles arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi play in mediating levels of soil carbon and nutrient cycling around the globe is an increasingly important and highly debated topic. Recent research has indicated that EM fungi may be associated with increasing capacity for soil carbon storage. Yet, additional studies have also emphasized that this result may not be common across all EM species and sites. In this study we used an extensive dataset combining Forest Inventory Analysis data, and spatially explicit products describing soil carbon content and soil total nitrogen in temperate forests throughout the eastern half of the United States. We then addressed the following questions; (1) does increasing dominance of species associating with EM fungi result in increasing amounts of soil carbon stored per unit nitrogen? and alternatively (2) do differences in soil carbon per unit nitrogen differ based on dominant plant species groups irrespective of mycorrhizal association? Importantly, the study area encompassed a wide range of temperate forests including forests dominated by AM associates, EM associates, and forests including roughly equal representation of both mycorrhizal associations.
Across all forests we found no evidence that increasing dominance of species associating with EM fungi consistently results in higher levels of soil carbon storage per unit soil nitrogen. Instead, there was generally a wide range of soil carbon per unit nitrogen across all temperate forests with no differences among forests dominated by AM or EM fungi. There was, however, a slight trend of increasing dominance of EM associates with decreasing soil total nitrogen. Further tests among individual groups plant species (Acer, Cupressaceae, Pinus, Populus, and Quercus) further indicated a lack of consistent pattern between soil carbon storage per unit nitrogen and mycorrhizal association. The highest levels of found in the Cupressaceae and Acer (AM associates) compared with lower values observed with Quercus (considered to be primarily EM associates), but these were also not significantly different from levels observed with Pinus (EM associate) and Populus (associates with both EM and AM). Based on these findings and other recent studies we suggest that patterns of soil C storage per unit nitrogen are likely the result of a combination of plant and fungal traits as well as background ecosystem properties rather than broad associations with either AM or EM fungal groups.