Ectomycorrhizal-dominated temperate forests show faster decomposition of soil organic matter than neighbouring arbuscular mycorrhizal-dominated forests

Alexis Carteron, Fabien Cichonski and Etienne Laliberté

Soil microorganisms play a crucial role in nutrient cycling and carbon storage. Among them, arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) fungi contribute greatly to soil organic matter degradation but differ in their effects due to distinct enzymatic activities. Ectomycorrhizal fungi have been suggested to promote carbon sequestration by slowing down litter decomposition, due to the competition with free-living saprotrophs for organic nutrients (i.e. the “Gadgil effect” hypothesis). On the other hand, AM fungi may indirectly stabilize soil organic matter by promoting microbial products and soil aggregation (i.e. the microbial efficiency-matrix stabilization hypothesis). We simultaneously tested these two hypotheses by comparing the soil organic matter decomposition in adjacent patches of temperate forests dominated by AM or EcM trees. Over one year, we followed decomposition of the three organic horizons using a reciprocal transplant design experiment. Litterbags were composed of different mesh size that allow or exclude ingrowth of fungal hyphae. We estimated differences in mass loss using mixed linear models. After one year, litterbags from EcM-dominated patches showed higher mass loss. The exclusion of mycorrhizal fungi slowed down the decomposition in patches dominated by AM, but also in EcM patches, contrary to the Gadgil effect expectations. These results suggest that soil microbial communities, including mycorrhizal fungi but particularly EcM fungi, actually promote decomposition of organic matter. This experiment needs be extended in time, for the results to be better supported. Nonetheless, our study emphasizes that mycorrhizas should be taken into account to increase our capacity to predict ecosystem functioning.