Planting more trees does not automatically lock more carbon into forest soils—the type of fungi that tree roots partner with determines how much carbon stays underground. This matters because governments worldwide are betting on tree planting and forest management as a key strategy to offset carbon emissions. But the simple assumption that more trees equals more carbon storage ignores a critical variable: trees such as oaks and pines host ectomycorrhizal fungi, which build up soil carbon, while maples and ashes host arbuscular mycorrhizal fungi, which do not. No one knows exactly why this difference exists, or how rising atmospheric CO₂ will shift the balance. The researchers propose three linked mechanisms—how fungi compete for nutrients, how much carbon they demand from their host trees, and how their underground networks decompose. If the team succeeds, their findings will directly inform tree-planting schemes at local, national, and global scales. Rather than planting a generic mix of species, policymakers and land managers could select tree–fungus combinations that maximise long-term soil carbon storage. The project uses a unique long-term CO₂ enrichment experiment in an ancient oak woodland to test how these dynamics play out under future atmospheric conditions.
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This project will push the frontiers of understanding of the factors controlling soil carbon content in forest soils. Forest soils contain vast amounts of carbon, and planting new trees and managing existing forests is seen as a critical strategy to mitigate the effects of global climate change on our planet. The basic assumption is that more trees=more carbon, but this assumption is far too simplistic. For example, different types of tree have different impacts on the carbon content of soil, where most is stored. Trees whose roots associate with a particular group of soil fungi – called ectomycorrhizal fungi – (e.g. oak, pines, beech) tend to be associated with soils that have lots of carbon compared to trees that associate with a different type – called arbuscular mycorrhizal fungi (e.g. ash, yew, maple). The question is why? This challenge is especially important when considering on-going changes in plant carbon capture in response to elevated atmospheric carbon dioxide (CO2) concentrations, and the need to promote biodiversity. For example, tree biomass production generally increases in response to elevated atmospheric CO2 concentrations, but how such responses translate to affect soil carbon pools in forests dominated by different mycorrhizal types is unknown. In addition, the relative abundance of species that form either ectomycorrhizas or arbuscular mycorrhizas in forests often varies along a dominance-equal mixture gradient, but we know little about how ectomycorrhizal and arbuscular mycorrhizal fungi interact to affect soil carbon pools. We propose that the below-ground associations with these different types of soil fungi is critical to understanding why, because the fungi have different characteristics and both host tree and fungi likely responds to different environmental drivers. We propose there are three major inter-linked mechanisms contributing to differences in soil carbon contents in forests dominated by ectomycorrhizal and arbuscular mycorrhizal tree hosts: 1) greater capacity of ectomycorrhizal fungi to compete for, degrade and capture nutrients from organic matter relative to free-living microorganisms, 2) contrasting demands for host plant carbon (produced by photosynthesis) by different mycorrhizal fungi to fuel nutrient foraging activities, and 3) fundamental differences in the production, turnover and decomposition of networks of fungal hyphae and thus its contribution to soil organic carbon. The supply of carbon from host trees is critical in driving all these mechanisms, but the carbon ‘cost’ of ectomycorrhizal fungi is hypothesised to be greater compared to arbuscular mycorrhizal fungi, because of their ability to obtain key growth limiting nutrients (nitrogen and phosphorus) from organic matter. Therefore, we further hypothesise that increases in atmospheric CO2 concentrations will have greater impact on the contribution of ectomycorrhizal compared to arbuscular mycorrhizal host plants to soil carbon storage, because it may provide plants access to greater amounts of carbon to ‘pay’ for the cost of ectomycorrhizal fungal activity. In this adventurous proposal, we use a combination of in situ and ex situ stable and radio-isotope tracers, a network of woodlands with contrasting abundance of ectomycorrhizal and arbuscular mycorrhizal host trees, and a globally-unique long-term free air carbon dioxide enrichment experiment in mature ancient oak woodland (BIFoR FACE). Our findings will push the frontiers of understanding of forest soil carbon dynamics by providing robust mechanistic evidence, enabling local, regional, national and global tree planting and restoration efforts to achieve net zero.
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