From Dirt to Digital: The Story of Soil Carbon Science
How DOE-funded science is tracking the carbon that underpins soil's productivity and impact on climate
Introduction
The vast stores of carbon in Earth’s soil can greatly impact our environmental future, acting as either a useful carbon sink or a climate-warming carbon source to the atmosphere. It’s also a critical element for healthy, productive soil.
Scientists across many of the Department of Energy (DOE)-funded national labs are urgently trying to answer the question of how soils, which store about three times more carbon than the atmosphere, might be affected by changes to Earth and environmental systems.
Plants absorb carbon dioxide from the atmosphere during photosynthesis, then store much of it in their roots. But soil carbon is extremely vulnerable as Earth’s average temperature warms.
Rising temperatures increase the activity of microbes that break down this organic matter and release greenhouse gasses, which contribute to further warming the planet. This results in a feedback loop that will amplify warming–but we don’t yet understand exactly how, or how fast.
DOE scientists are closing this knowledge gap in three ways—field experiments, lab analysis, and models—to holistically understand this relationship that could amplify the impacts of atmospheric warming.
Chapter 1: In the Field
One of the best ways to understand how soils will respond to higher temperatures is to warm up some soil and see what happens.
That’s why Berkeley Lab scientists have constructed large, experimental heating plots in the carbon-rich soils at Point Reyes National Seashore and the Blodgett Forest Research Station near Eldorado National Forest in California.
At Blodgett, these plots warm the soil by an average of four degrees Celsius (C). At Point Reyes, the plots heat the soil by three degrees C or six degrees C to mimic possible warming scenarios. Both sites also have control plots that are not heated to compare the impact of warmer temperatures.
This experimental setup allows scientists to see how heat affects the amount of carbon being released as CO₂ versus how much carbon is stored in the soil.
The field experiments are also a great way to test and understand ecosystems in-situ, with all their naturally occurring influences at play.
The Point Reyes site gives insight to the functioning and warming response of a coastal grassland ecosystem, while the Blodgett site gives insight to that of a coniferous forest ecosystem.
Keeping the existing carbon in the soil is also a priority for healthy soil and carbon storage. That’s why field scientists study how it moves above and below ground. This transformation of carbon from soil to the atmosphere, and vice versa, is called carbon flux.
Scientists capture carbon flux in the field with instruments that seal off a small section of surface soil and measure gases being released and absorbed.
To track carbon fluxes belowground, they also pull air samples from a network of tubes buried at different depths and store the samples in bottles for later analysis.
Samples of soil and air collected from the field are taken to the lab for the next major phase: lab analysis.
Chapter 2: In The Lab
On-site at Berkeley Lab, researchers analyze their field samples and perform entirely new experiments under more controlled conditions.
The network of microbes, plants, and minerals that all affect the amount of carbon in the soil. Illustration by: Celisa Cortes
The carbon stored in soils mostly takes one of three forms: underground plant biomass, mineral-bound carbon, and microbial biomass.
Much of the lab work is to quantify each of these forms in samples from the field, figuring out how their total carbon storage changes in response to warming.
Before analysis can begin, samples must be processed, which requires sieving soil samples to separate the sediments into different sizes, removing roots and other plant matter, and recording the physical properties of the soil.
Underground Plant Biomass
To measure how much carbon is associated with plant matter, roots are hand-picked from the sieved soil samples.
The roots are placed in an oven to dry, then weighed to determine the total root biomass.
Mineral-bound Carbon
In another lab, scientists analyze field samples to determine how much carbon is bound up in mineralized forms.
Carbon can exist within the molecular structure of rock minerals like limestone (CaCO3). It can also exist in the form of mineral-associated organic carbon, which is carbon from living things that bonds or adheres to minerals in a way that makes it much less available for microbes to eat, and therefore less likely to be released to the atmosphere.
A sequential extraction exposes the sample to a series of increasingly powerful solvents that dissolve different kinds of minerals.
As carbon is released at each step along the process, scientists can determine how much carbon was bound to specific minerals.
The tubes are then spun in a centrifuge to separate the liquid containing the dissolved carbon so it can be measured.
Microbial Biomass
Scientists also quantify how much microbial mass is in soil samples, which contributes to the overall amount of soil carbon, and how microbes regulate the transformation and storage of carbon in soil.
At the same time, they study the DNA of microbes in the soil samples.
This helps us understand genetic traits, for example how efficiently they use carbon or their biomass, that affect microbes’ role in the decomposition of organic matter that releases CO₂.
The network of microbes, plants, and minerals that all affect the amount of carbon in the soil, with the addition of carbon moving in and out of the soil. Illustration by: Celisa Cortes
By getting an estimate of how much carbon is in soil before it warms, we can compare how much carbon has been emitted as greenhouse gases after the warming experiment.
But scientists also collect gas samples in real time to study the speed of gas release and how that may change.
To monitor gas fluxes on shorter timescales across days and seasons, scientists collect air samples around soils in the field and analyze them back at the lab.
Once in the lab, these gas samples are analyzed to determine the concentration of CO₂ and methane in the sample.
Air is extracted from the vials and injected into a gas chromatography machine, which measures compounds in the air sample.
The data collected in the lab greatly increases the trove of information these soil warming projects offer.
All that data provides the basis for the final tool in DOE’s toolbelt for understanding soil carbon: modeling.
Chapter 3: The Digital World
Analyzing soil in the lab gives an understanding of the present, but scientists also need to predict what soil carbon flux will look like in the future.
In order to turn past and present data into future predictions, scientists build numerical models.
A model uses a mathematical relationship between inputs (like temperature) and outputs (like soil carbon) to generate predictions about the future. For example, “With a 4°C increase in temperature, how much carbon will the soil lose in 50 years?”
When the math that predicts model outputs is hidden, they are referred to as “black boxes.” This means that the models rely on certain assumptions, which increases the likelihood of unreliable predictions.
But Earth’s climate is not a prediction that should be highly uncertain. To solve this problem, researchers are reducing uncertainty by developing models that consider what black box models ignore: the actual processes that form the “why” behind fluctuating soil carbon levels.
In other words, scientists are replacing the unknown “black boxes” with equations that represent the actual factors that affect soil carbon: microbes, nutrients, plant activity, minerals, and soil chemistry.
These improved models are more accurate in their predictions and more flexible to adapt to the ever-changing conditions brought on by a warmer climate.
Certain processes, for example, decomposition, may change in a warming world. By representing these in models, scientists are making their models rely on fewer assumptions.
All of this work is part of improving the Energy Exascale Earth System Model (or E3SM), the flagship model that the Department of Energy, Office of Sciences has developed to predict the future of Earth system change.
E3SM is one of the world’s most advanced publicly available Earth system models, designed to run on exascale computers. These computers have much more powerful hardware compared to super computers and can process information uniquely fast.
This makes E3SM able to incorporate models of not only soil but also ice, oceans, rivers, the atmosphere, and more to tell a complete story about Earth’s future.
Trying to accurately model soil across the globe is hard — from the tropics to the arctic, our world has a wide range of soil and ecosystems, and how these all respond to a changing climate varies greatly.
Accurate models give us the predictive power to make informed decisions.
The better we understand and represent soils, the stronger their predictive power. Which means that everyone from farmers and scientists, to city planners and government officials will be able to make more informed decisions about our future.
UC Berkeley's Sather Tower with a backdrop of the Bay Area.
Made up of billions of microscopic players working together, soil is key to regulating our world’s biogeochemistry–and ultimately our climate.
Understanding and predicting that system requires a similarly rich web of players across field, lab, and computer-based research. Together, these experts form a small part of the vast ecosystem of science within DOE, working together to produce public-sector science for the good of society.
Berkeley Lab scientists Kelsey, Steve, and Jing analyzing soil in different ways that all ultimately help to paint a clearer picture of the future of Earth's soil carbon.
