Soil Organic Matter Fractions Reflect the Mycorrhizal Associations and Foliar Chemistry of Dominant Tree Species in a Northern Temperate Forest

Abstract:

Recent work suggests mycorrhizal fungi are important drivers of soil organic matter dynamics, but the extent of their influence on organo-mineral interactions remains unclear. We assessed the relationship between the dominant functional group of mycorrhizal fungi and the distribution and chemistry of mineral-associated organic matter (MAOM) and particulate organic matter (POM) in temperate forests of northern New Hampshire and Vermont, USA. Using density fractionation, we measured the proportion of soil organic matter (SOM) stored as MAOM and POM and the C:N of these soil pools beneath six tree species that vary in both their foliar chemistry and mycorrhizal associations. We found that tree mycorrhizal type was associated with important differences in the distribution and chemistry of SOM, with a higher proportion of total soil C found in the MAOM, rather than POM, fraction, and lower MAOM C:N beneath tree species with arbuscular mycorrhizal (AM) fungi compared to those with ectomycorrhizal (ECM) fungi. Foliar chemistry was associated with the distribution and chemistry of POM, but not MAOM: we found more POM C and higher POM C:N in soil beneath trees with higher foliar C:N. Higher values of soil clay content were strongly associated with a higher proportion of total C in the MAOM fraction. These results add to the growing body of support for mycorrhizal fungi as predictors of soil C and N dynamics, in addition to the more widely-cited effects of organic matter chemistry and soil mineralogy. Because MAOM decomposition is thought to be less responsive than POM decomposition to changes in soil temperature and moisture, our data suggest that forests dominated by AM-associated tree species may release less C than ECM-dominated forests in the warmer, wetter conditions projected for New England with climate change.