Abstract: Tree mycorrhizal associations influence the composition and persistence of soil organic matter and could therefore shape forest biogeochemical responses to global change. Functional differences between under- and over-story plants could alter these dynamics. Yet how co-occurring mycorrhizal types associated with plants in different forest strata interact to collectively affect soil organic matter pools remains poorly resolved. We investigate the effects of understory ericoid mycorrhizal (ErM) shrubs on particulate and mineral-associated soil carbon and nitrogen stocks, as well as microbial variables that regulate these pools, at different depths under varying arbuscular (AM) versus ecto- (EcM) mycorrhizal tree dominance. We hypothesized that ErM shrubs would increase particulate organic matter stocks in the organic horizon and reduce mineral-associated stocks in the mineral horizons by suppressing saprotrophic microbial activity and that these effects would be highest under AM trees that lack fungal symbionts that can degrade organic matter. Ericoid mycorrhizal shrubs have a strong, positive influence on particulate organic matter stocks, and these effects are strongest in the organic horizon and in AM-dominated plots. In contrast, EcM tree dominance, has a more pronounced and negative influence on mineral-associated stocks and on the mineral horizons, whereas the effects of ErM shrubs on these pools are small. Tree mycorrhizal associations therefore have a stronger influence on cumulative stocks (~30 cm depth) since mineral horizons make up a greater proportion of the soil profile. Plant mycorrhizal effects on mineral-associated carbon are, however, relatively small compared to their effects on nitrogen. Despite their smaller effect on cumulative stocks, ErM shrubs strongly shape soil organic matter dynamics. Lower soil δ13C values suggest that ErM shrubs slow organic matter cycling across all depths. Our study highlights the importance of considering fungal guild interactions and their depth-dependent effects for understanding soil organic matter cycling and storage in forests where multiple mycorrhizal types co-occur.
