Trees and Their Mycorrhizal Fungi Are the Key to Soil Microbes

The Science

Trees have different traits that affect soil organic matter and nutrients. This study looked at two main traits—tree leaf habit (either deciduous trees that lose their leaves in the fall or evergreen trees that keep their leaves year-round) and root mycorrhizal association. Trees almost always associate with only one type of mycorrhiza that grow either inside or outside the root tips. Mycorrhizae are key for helping plants get nutrients and water from soil and in turn get sugars from trees. Researchers found that mycorrhizal association was more important than leaf habit in affecting the other free-living fungi in the soil around roots. The other fungi are important in breaking down organic matter due to enzyme production. Much like people have enzymes in their digestive tracts, fungi and other microbes digest outside their bodies and then absorb smaller bits of organic matter. Researchers also found that many enzymes needed to break down plant and fungal tissue were affected more by mycorrhizal association than leaf habit.

The Impact

Mycorrhizal fungi could be considered keystone organisms in soil food webs. They connect trees to other organisms living in the soil that break down soil organic matter, releasing nutrients that plants need for growth. Findings suggest that trees are found with different communities of free-living microbes depending on mycorrhizal associations. The importance of mycorrhizal associations for breaking down organic matter means that trees and associated mycorrhizae can affect how organic matter decomposes and how much carbon remains stored in soil.

Summary

Forests in the northeastern United States are experiencing shifts in community composition due to the northward migration of warm-adapted tree species and certain species’ declines (e.g., white ash and eastern hemlock) due to invasive insects. Changes in belowground fungal communities and associated functions will inevitably follow. Therefore, a team of researchers sought to investigate the relative importance of two important tree characteristics—mycorrhizal type [ectomycorrhizal (EcM) or arbuscular mycorrhizal (AM)] and leaf habit (deciduous or evergreen)—on soil fungal community composition and organic matter cycling. Soil was sampled in the organic and mineral horizons beneath two AM-associated (Fraxinus americana and Thuja occidentalis) and two EcM-associated tree species (Betula alleghaniensis and Tsuga canadensis) with an evergreen and deciduous species in each mycorrhizal group. To characterize fungal communities and organic matter decomposition beneath each tree species, researchers sequenced the ITS1 region of fungal DNA and measured the potential activity of carbon- and nitrogen-targeting extracellular enzymes. Each tree species harbored distinct fungal communities, supporting the need to consider both mycorrhizal type and leaf habit. However, between tree characteristics, mycorrhizal type better predicted fungal communities. Across fungal guilds, saprotrophic fungi were the most important in shaping fungal community differences in soils beneath all tree species. The effect of leaf habit on carbon- and nitrogen-targeting hydrolytic enzymes depended on tree mycorrhizal association in the organic horizon, while oxidative enzyme activities were higher beneath EcM-associated trees across both soil horizons and leaf habits.

Principal Investigator

Caitlin Hicks Pries
Dartmouth College
[email protected]

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This work was supported as part of the Environmental System Science program by the Biological and Environmental Research (BER) Program within the U.S. Department of Energy’s Office of Science under contract DE-SC0020228.

Related Links

• ESS-DIVE Data set

References