Tree mycorrhizal associations regulate relationships between plant and microbial communities and soil organic carbon stocks at local scales in a temperate forest

Forests store substantial amounts of soil organic carbon (SOC), but SOC stocks differ strongly between forest ecosystems dominated by arbuscular mycorrhizal (AM) or ectomycorrhizal (EcM) fungi. In temperate forests, nearly all tree species associate with either AM or EcM fungi, but it is unclear if variation in SOC stocks is linked to the dominance of AM versus EcM trees at local scales. However, SOC stocks are also influenced by many other factors, including plant diversity, plant traits, soil properties and microbial community composition, that can vary substantially at small spatial scales. Thus, elucidating how tree mycorrhizal associations interact with other drivers of SOC formation at local scales could improve our understanding of forest SOC storage. Using multivariate data from a 25-ha temperate forest plot, we hypothesised that SOC stocks would be greater in AM-dominated than in EcM-dominated subplots, due to the rapid decomposition of labile litter inputs from AM trees. We expected that variation in SOC stocks would be explained by interactions between AM associations, plant diversity or traits, and soil microbial communities. We also accounted for differences in soil abiotic conditions and used piecewise structural equation models to identify potential causal pathways. Forest stand attributes played a primary role in predicting forest SOC stocks (45% relative contribution), compared to microbial community composition (26%) and abiotic factors (20%). Forest SOC stocks increased with plant and litterfall diversity and root diameter but declined with soil clay-silt content. Importantly, forest SOC stocks were lower in subplots with a high ratio of EcM to saprotrophic fungi and subplots with a high predominance of bacterial functions related to carbon and nitrogen cycling. Our models suggest that smaller SOC stocks in stands with high EcM dominance are indirectly linked to lower tree species diversity, greater litter carbon input, and distinct soil microbial community composition. Our findings highlight that tree mycorrhizal type also influences SOC storage at local scales, but stand-level relationships between SOC stocks and mycorrhizal dominance differ from regional and global patterns. We show that mycorrhizal associations regulate complex relationships between producers, decomposers, and soil properties at small spatial scales. Considering these linkages could therefore improve estimates and management of temperate forest SOC stocks. Read the free Plain Language Summary for this article on the Journal blog.