Noah W. Sokol, Emily D. Whalen, Andrea Jilling, Cynthia Kallenbach, Jennifer Pett-Ridge, Katerina Georgiou

This is a plain language summary of a Functional Ecology research article which is published here.

Earth’s soils hold a massive amount of carbon – more than the world’s vegetation and atmosphere combined. But the fate of this carbon is uncertain – particularly the portion that is bound up in close associations with soil minerals (‘mineral-associated organic matter’). This mineral-associated organic matter represents one of the largest pools of carbon on the planet, and tends to persist for decades or even thousands of years. What will happen to the carbon in mineral-associated organic matter as the climate changes? Will it be rapidly decomposed and lost back to the atmosphere, accelerating the pace of climate change, or will Earth’s soils maintain or increase their hold on this soil carbon, and help slow the pace of climate change?

To answer these questions, we point to the still major gaps in our basic knowledge of how much soil carbon currently exists as mineral-associated organic matter across Earth’s different biomes – particularly in tropical and boreal forests, and the tundra. In a data synthesis of published studies, we estimate how much mineral-associated organic carbon exists on the planet (840 – 1540 petagrams – or gigatonnes – of carbon), and the amount found in each biome.

We also propose a framework that distills the factors that cause formation and loss of mineral-associated organic matter. This framework describes different categories of influential plant and microbial traits; these traits (such as physiology or morphology) shape the amount of organic matter that enters or leaves the mineral-associated organic matter pool.  In our trait framework, we distinguish between ‘feedstock traits’ – traits that control the total amount of organic matter that enters the soil from dead plants and microbes, and ‘formation traits’ – traits that control which subset of organic matter ultimately associates with minerals.

Our trait framework can help researchers unpack the complex mechanisms that drive changes in mineral-associated organic matter as the Earth’s temperatures and carbon dioxide levels increase. Focusing on these traits may also help improve the accuracy of global-scale carbon models that we use to predict the future fate of the world’s soil carbon stocks, and how reactions within the Earth system will affect the trajectory of climate change.