Recent studies demonstrate a strong relationship between the dominance of ectomycorrhizal (ECM)- versus arbuscular mycorrhizal (AM)-associated trees and soil organic matter (SOM) properties in temperate forests. However, the mechanisms driving SOM patterns along such “mycorrhizal gradients” remain unclear. Contemporary theory emphasizes the central role of microbial growth in linking plant and soil traits with the formation and stability of SOM. Thus, our objective was to examine the extent to which microbial growth dynamics mediate mycorrhizal effects on SOM properties. To do this, we quantified biogeochemical traits of plants and soils, soil carbon (C) and nitrogen (N) pools and fluxes, and microbial growth traits across mycorrhizal gradients in six temperate forests spanning a range of climatic and edaphic properties. Given previous evidence that AM dominance is associated with N-rich soils and high-quality leaf litter, we expected to observe more efficient microbial growth in AM soils leading to greater amounts of SOM in association with soil minerals and SOM overall.
Results/Conclusions
Our results agree with previous work demonstrating that ECM-dominated plots tend to be less fertile. ECM dominance was associated with lower soil pH, higher soil C:N, and higher leaf litter lignin:N across all six forests. SOM also changed across the mycorrhizal gradient. While ECM dominance was consistently associated with lower soil N, the relationship with soil C varied among sites. Patterns in mineral-associated SOM similarly varied among sites though this pool tended to be greater in AM-dominated plots. Microbial respiration per unit biomass (qCO2) generally increased with ECM dominance supporting our hypothesis of higher microbial growth efficiency in AM-dominated soils. However, our data suggest that the linkage between growth efficiency and SOM depends on site-level factors. The relationship between qCO2 and soil C was negative at four of the six sites, but was strongly positive at the two coldest sites with the greatest soil C concentrations, likely reflecting overflow respiration of the high-C:N SOM at these sites. In sum, our results demonstrate linkages between plant and soil traits, microbial growth, and SOM, but suggest that the importance of microbe-mediated SOM formation pathways may strongly depend on climatic factors.