Global correlations between belowground ectomycorrhizal abundance and soil carbon
Soil carbon storage and decay is regulated by the activity of free-living decomposer microbes, which can be limited by nitrogen availability. Ectomycorrhizal fungi may be able to induce nitrogen limitation and reduce activity of free-living microbial decomposition by mining soil organic nitrogen. The implication is that ectomycorrhizal-dominated systems should have increased soil carbon storage relative to non-ectomycorrhizal systems, which has been confirmed at a global scale. However, past work is based on binary biome categorization and cannot measure the quantitative effect of variation in the relative abundance of ectomycorrhizal fungi within the soil community. To investigate these effects, we performed a meta-analysis of published studies, selected for the inclusion of molecular descriptions of soil fungal community composition with ITS sequences, as well as associated soil chemical data, sampling date, and location. The search returned 11 studies with a total of 1948 samples, accumulating in over 7 million ITS sequence reads. We assigned operational taxonomic units using the QIIME pipeline and UNITE fungal database and assigned fungal reads as ectomycorrhizal or non-mycorrhizal based on current taxonomic knowledge. We tested for associations between ectomycorrhizal abundance, climate, and soil carbon and nitrogen using a mixed effects model to account for variation among sites.
Preliminary results support past work. Sites with greater soil carbon had quantitatively more ectomycorrhizal fungi within the soil microbial community based on fungal sequence abundance, after accounting for soil nitrogen availability. This is consistent with our hypothesis that ectomycorrhizal fungi induce nitrogen-limitation of free-living decomposers and thereby increase soil carbon storage. The strength of the mycorrhizal effect increased non-linearly with ectomycorrhizal abundance: the greater the abundance, the greater the effect size. Mean annual temperature, potential evapotranspiration, soil moisture and soil pH were also significant predictors in the final AIC selected model. As we increase the diversity of sites included in the analyses, we will test for interactions between EM abundance as a predictor of soil carbon storage and climate.