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 vs.
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 hypothesized 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 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.