PS 13-173 - Underrated understories: Effects of ericoid mycorrhizal shrubs on soil organic matter along a gradient of AM-ECM dominance in temperate forests

Abstract: In the terrestrial biosphere, soils contain the largest fraction of carbon (C) and organic nitrogen (N), and mycorrhizal associations have been identified as key drivers of these elemental cycles. Mycorrhizal associations influence soil C and N cycling based on their nutrient uptake strategy, and the decomposition rate of their plant and fungal necromass. For example, total soil C and N and their distribution between particulate (POM) versus mineral-associated (MAOM) organic matter fractions differ across ecosystems dominated by different mycorrhizal associations. Much of the previous literature investigating mycorrhizal effects on soil organic matter (SOM) have focused on ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) associations. However, ericoid mycorrhizal (ERM) shrubs, which commonly dominate in the understories of ECM-dominated forests, are often not included in these investigations, but may exert strong influence on biogeochemical processes as well. Therefore, our work investigated the following questions: 1) What are the effects of ECM and ERM associations on soil C, N, and C:N in forests where their host plants co-occur? And 2) Are differences in total soil C and N also apparent in fractions of POM and MAOM? We hypothesized that soil would have higher C:N ratios with increasing ECM-dominance and ERM shrub cover. We also hypothesized that the fraction of C and N in POM would increase with increasing ECM dominance, but would be even higher under high ERM shrub cover. We sampled the 0-10 cm mineral soil layer across 43 forest plots at Coweeta Hydrologic Laboratory, a temperate forest co-dominated by AM and ECM trees with a variable understory of evergreen ERM shrubs Rhododendron maximum and Kalmia latifolia.

Using linear-mixed models, we analyzed total % soil C and N and found that both ECM and ERM were positively associated with increasing soil C:N, but for different reasons. Increasing ECM tree abundance was associated with decreased soil N, while increasing ERM shrub biomass was associated with increased soil C. We also found that proportions of C and N in POM fractions increased with increasing ERM shrubs, while proportions of C and N in MAOM decreased with increasing ERM shrubs. Therefore, while ECM and ERM appear to have similar relationships with soil C:N in general, our findings suggest different underlying mechanisms behind total soil C and N storage and SOM formation. Future work will explore the role of ERM fungal necromass in driving high soil % C and the fraction of C and N in POM.