Into the Rhizosphere: Soil Fungi and Carbon Dynamics
Underneath the Earth’s surface, water, nutrients, and chemical signals are shuttled through a sprawling network between tree roots and soil fungi. “Many forest trees depend on their associated soil fungi for nutrients, as the fungi are better at absorbing nitrogen, phosphorous, and other nutrients,” says U.S. Forest Service ecologist Melanie Taylor. “The trees return the favor by sharing their sugars with the fungi.”
Some trees form associations, called arbuscular mycorrhizas, where the fungi grow inside plant roots by penetrating the cell wall of a tree’s roots. Other trees form ectomycorrhizas, where the fungi grow on the outside of the root, eventually covering it in a dense sheath of fungal tissue.
Taylor and her colleagues suspected that these relationships between trees and soil fungi could affect decomposition in soils. When trees die – or when their branches and leaves fall to the forest floor – these tissues are gradually broken down by decomposers. Many of the nutrients released through this process are then acquired by mycorrhizas and provided to trees.
Decomposition is the main process that returns terrestrial carbon to the atmosphere – and soil contains more carbon than all of Earth’s atmosphere and vegetation combined. “Understanding the factors that enhance or suppress soil carbon content is critical to understanding the overall carbon budget,” says Taylor.
The scientists investigated patterns of soil carbon and microbial activity in a mixed pine-hardwood forest in northern Georgia. “We found that the relationship between trees and their associated fungi indirectly affects decomposition in soils around the trees,” says Taylor. “These relationships affect the rate of decomposition occurring around the tree, which impacts the amount of carbon lost from the soil.” The study was published in the Journal of Ecology.
Taylor and her colleagues studied the relationships between the soil fungi and eight tree species. Half of the trees – tulip poplar, sweetgum, American holly, and Eastern red cedar – form arbuscular mycorrhizas. The other four – mockernut hickory, American beech, white oak, and loblolly pine – form ectomycorrhizas.
The scientists collected soil and leaf litter from all the tree species, as well as the small, fine roots that absorb water and nutrients. In the forest, they characterized patterns in soil chemistry and soil microbial biomass, and in the lab, they conducted a microcosm experiment with soils, leaf litter, and fine tree roots.
“We found that trees with arbuscular mycorrhizas had less carbon in the soil around them than trees with ectomycorrhizal fungi,” says Taylor. “Trees with arbuscular mycorrhizas also had higher decomposition activity.” The study suggests that the type of mycorrhiza a tree forms has implications for decomposers in the vicinity of the tree and the release of carbon.
The findings may change the way scientists estimate carbon storage in forest soils. “Our findings refine existing estimates of how forest landscapes store carbon in soils,” says Taylor. “Understanding these processes may be crucial to predicting how new assemblages of tree species will store carbon in the future.”
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For more information, email Melanie Taylor at melanietaylor@fs.fed.us.