Belowground carbon and fungi

Fungi play an indispensable role in terrestrial ecosystems due to their roles in nutrient cycling, soil formation and as symbionts with plants. Fungi externally digest and decompose organic matter by producing extracellular enzymes and other compounds, breaking down complex carbon compounds into simpler molecules. These simpler molecules contribute to soil organic carbon, which binds soil particles, improves soil structure and stability which are crucial processes for carbon sequestration and overall ecosystem stability.
Globally, tree planting schemes have been implemented to offset carbon emissions and the prices of carbon credits is on the rise. While much research has focused on the aboveground carbon stored in woody biomass, the significance of belowground carbon is often overlooked. In the UK, belowground carbon in soil, root biomass, and fungi is estimated to be 3-4 times greater than aboveground carbon in forest ecosystems (Ostle et al., 2009).

At Kew Gardens, we are quantifying carbon stored in UK habitats by measuring aboveground, belowground and soil respiration rates to gain a comprehensive understanding of ecosystem carbon dynamics. This research, primarily conducted at Kew's Wakehurst site in West Sussex, utilises multispectral imagery, LiDAR, gas flux exchange, and randomly sampled soil cores for testing soil nutrients and sequencing of mycorrhizal fungi to generate high-resolution data across the landscape. 

The majority of carbon sequestration by fungi occurs through their symbiotic relationships with plants and approximately 90% of plant species worldwide can form mycorrhizal associations (Smith and Read, 2008). These mutualistic relationships enhance seedling survival, plant nutrition, growth, and resistance to disease, pollutants, and droughts by improving water uptake and extending the reach of plant roots. In exchange for these benefits, plants provide mycorrhizal fungi with 20-30% of their photosynthetically fixed carbon (Smith and Read, 2008).

Preliminary findings of the research at Kew indicate that ectomycorrhizal fungi dominate forest ecosystems, which cover 13% of the UK and store 200-300 tonnes of carbon per hectare, whereas, arbuscular mycorrhizal fungi dominate in grasslands which cover approximately 40% of the UK (ONS, 2022) and store 10-30 tonnes of carbon per hectare.

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Arbuscular mycorrhizal fungi produce a glycoprotein called glomalin, which binds soil particles together and enhances soil structure. Glomalin is highly resistant to decomposition, allowing carbon to be locked in the soil for decades or more. This stable form of soil organic carbon significantly reduces the amount of CO2 released back into the atmosphere (Treseder and Turner, 2007). Ectomycorrhizal also secrete various organic compounds that can contribute to soil aggregation, as well as their hyphae which extends far into the soil, creating networks that stabilize soil aggregates and protect carbon within these structures.

Soil type is another important factor for belowground carbon sequestration. The soil’s physical and chemical properties influence the amount of organic matter it can hold and how stable that carbon is within the soil. For example, soils with high clay content can protect organic carbon from decomposition by binding it tightly to mineral particles. In contrast, sandy soils tend to store less carbon due to their lower organic matter content and quicker drainage, which can accelerate the breakdown of organic materials.
Biodiversity also plays a crucial role in carbon storage. Ecosystems with greater plant species diversity tend to sequester more carbon, as diverse plant communities can utilize resources more effectively and enhance overall ecosystem stability (Tilman et al., 2006; Cardinale et al., 2012). This biodiversity-driven resilience is essential for maintaining resilience within ecosystems in the face of climate change and land use pressures.

Land management practices play a critical role in an ecosystem's ability to store carbon long-term. A recent study in Scotland measured the long-term impact of afforestation on upland heathland with native birch and pine trees, suggesting that some habitats are better left untouched from a carbon ecosystem services perspective (Friggens et al., 2020).

Preserving natural ecosystems and understanding the intricate relationships within them is essential for enhancing carbon sequestration efforts. As we continue to explore the intricate relationships between aboveground and belowground carbon, it becomes increasingly clear that fungi play an indispensable role in carbon storage, and the dynamics of which are yet to be fully understood.

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References
Cardinale, B. J., Duffy, J. E., Gonzalez, A., Hooper, D. U., Perrings, C., Venail, P., Narwani, A., Mace, G. M., Tilman, D., Wardle, D. A., Kinzig, A. P., Daily, G. C., Loreau, M., Grace, J. B., Larigauderie, A., Srivastava, D. S., & Naeem, S. (2012). Biodiversity loss and its impact on humanity. Nature, 486(7401), 59-67. https://doi.org/10.1038/nature11148
Friggens, N., et al. (2020). Global Change Biology, 26(9), 5178-5188.
Office for National Statistics (ONS). (2022). UK natural capital accounts: 2022. Retrieved from https://www.ons.gov.uk/economy/environmentalaccounts/bulletins/uknaturalcapitalaccounts/2022
Ostle, N. J., Smith, P., Fisher, R., Woodward, F. I., Fisher, J. B., Smith, J. U., ... & Bardgett, R. D. (2009). Integrating plant–soil interactions into global carbon cycle models. Global Change Biology, 15(3), 979-993.
Smith, S.E., & Read, D.J. (2008). Mycorrhizal Symbiosis.
Tilman, D., Reich, P. B., & Knops, J. M. H. (2006). Biodiversity and ecosystem stability in a decade-long grassland experiment. Nature, 441(7093), 629-632. https://doi.org/10.1038/nature04742
Treseder, K.K., & Turner, K.M. (2007). Ecology, 88(3), 694-701.
Vanguelova, E.I., et al. (2016). Environmental Monitoring and Assessment, 188(11).