Digging up the dirt on the planet’s largest carbon reservoir

Every day beneath our feet, microbial decomposers tussle with soil minerals over a vast reservoir of carbon stored in the ground — and scientists know almost nothing about how this jostling plays out at the global scale.

Yet that knowledge might prove invaluable for helping to mitigate climate change, by revealing a better understanding of the carbon cycle, allowing more accurate climate models, and even enabling scientists to pull more carbon from the air and store it in the planet’s abundant soils.

Yale’s Eric Slessarev says it’s time to get his hands dirty — figuratively speaking — and look for answers.

Slessarev, an assistant professor of ecology and evolutionary biology in the Faculty of Arts and Sciences, recently received a two-year, $400,000 federal grant to study the role of minerals in helping to store soil organic matter (SOM), the largest carbon reservoir on Earth’s continents. The grant comes from the U.S. Department of Energy’s Office of Science. Slessarev is a member of the Yale Center for Natural Carbon Capture, a part of Yale’s ambitious Planetary Solutions Project.

SOM is the carbon rich fraction of soil that is made up of decaying plant and microbial material, which scientists say may play a key role in addressing climate change.

In an interview with Yale News, Slessarev talked about the SOM analysis he has planned, the global database he intends to develop — and why it’s important. The interview has been edited and condensed.

How will this work potentially aid planetary solutions efforts?

Eric Slessarev: This work will provide knowledge infrastructure for applied climate solutions that rely on soil. The more we know about soil mineralogy, the better equipped we will be to reduce the carbon footprint of global agriculture and restore SOM where possible.

How much carbon does SOM store? Does the size of this carbon reservoir vary much over time?

Slessarev: We only know roughly how much organic carbon that soils store at a global scale — but the number is thought to be in the range of 1,500 to 2,000 billion metric tons of carbon, which is more than is stored in the atmosphere and all aboveground plant biomass combined. We develop these estimates by averaging the amount of carbon that is stored in different soil types and then using global soil maps to scale the averages up.

The size of the soil organic carbon pool has been dynamic over history because agriculture drives soil carbon loss. One landmark 2017 study estimated that soil carbon stocks have been depleted by roughly 130 billion metric tons since the advent of agriculture.

What are the things you most want to know about SOM’s carbon storage dynamics?

Slessarev: I am trying to learn about the role of soil minerals in controlling SOM storage at large scales. We know that minerals help to stabilize SOM — but our knowledge of how different mineral types are distributed across space at the global scale is vague at best. By developing a more precise understanding of which minerals occur in which environments, I hope to improve our ability to predict the fate of SOM.

Can you describe your strategy for analyzing SOM and assembling a database of information about it?

Slessarev: This is a synthesis project, so the main objective is to compile existing data and analyze them in new ways. I will focus on synthesizing soil geochemical data — measurements of the elements that compose soil minerals. These geochemical data can be used to estimate soil mineralogy, which is time-consuming to measure and often not quantified directly. Because geochemical data are relatively easy to collect, they have been reported extensively in national level “geochemical baseline” surveys, which will make the task of synthesizing the data relatively straightforward.

On a personal level, what drew you to this area of research?

Slessarev: I am drawn to this topic because it links geology and ecosystem ecology. I have always been fascinated by the way soil develops in different geologic environments. Soil formation is a sort of competition between geologic forces and the inexorable effects of water and biology, which gradually weather and transform soil minerals over time. This study will test the role of geologic factors in this competition, and ultimately help to identify their role in SOM cycling and hence the global carbon cycle.