The Science

Tree roots form partnerships with fungi to obtain soil nutrients. In forests, there are two main types of partnerships: arbuscular mycorrhizae (AM) and ectomycorrhizae (EcM). These types differ in how fungi interact with roots and acquire nutrients from soil. A team of researchers investigated how mycorrhizae affected soil organic matter across four sites representing distinct climates and tree communities in the eastern United States. Soil carbon (C)-to-nitrogen (N) ratios and the amount of carbon and nitrogen protected by soil minerals strongly correlated with species composition of trees and EcM fungi.

The Impact

A popular concept in soil ecology is that mycorrhizal type determines how soil carbon and nutrients are stored. Forests dominated by AM mycorrhizae are expected to have lower C:N ratios and more mineral-associated organic matter than forests dominated by EcM. However, this expected pattern was only seen in forests where EcM trees had low-quality leaf litter, like pines and oaks, and where EcM fungi had hard-to-decompose tissues and the ability to break up complex organic molecules. Thus, this concept needs to be adjusted to account for differences among EcM species.

Summary

Scientists have suggested that tree species forming a symbiosis with AM versus EcM fungi is a strong predictor of soil carbon storage, but EcM systems are highly variable. In this study, researchers investigated how mycorrhizal associations and species composition of canopy trees and mycorrhizal fungi relate to the proportion of soil C and N in mineral associations and soil C:N across four sites in the eastern United States broadleaf forest biome. Study sites were in New Hampshire, Wisconsin, Illinois, and Georgia, and researchers identified canopy trees to species in each site and collected soil from the top 10 cm of the mineral horizons.

In two study sites (New Hampshire and Georgia), researchers found the expected relationship of declining mineral-associated C and N and increasing soil C:N ratios as the basal area of EcM-associating trees increased. However, soil properties strongly correlated with canopy tree and fungal species composition across all sites. The expected pattern was observed in sites that were (1) dominated by trees with lower quality litter in the Pinaceae and Fagaceae families and (2) dominated by EcM fungi with medium-distance exploration type hyphae, melanized tissues, and potential to produce peroxidases. This observational study demonstrates that differences in soil organic matter between AM and EcM systems depend on the taxa of trees and EcM fungi involved. Important information is lost when the rich mycorrhizal symbiosis is reduced to two categories.

Principal Investigator

Caitlin Hicks Pries
Dartmouth College
caitlin.hicks.pries@dartmouth.edu

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This research was supported by the Biological and Environmental Research (BER) Program within the U.S. Department of Energy’s (DOE) Office of Science under Grant/Award Number DE-SC0020228.

References