By Kathleen Wong, UC Natural Reserve System
A bucolic meadow in Marin is about to be catapulted into the next century. Climate researchers plan to heat its soils to temperatures the region won’t experience for decades. They want to see whether global warming will release more atmosphere-heating carbon from the vast stores now sequestered in the earth.
“There’s about three times as much carbon and organic matter in soil as there is carbon dioxide in the atmosphere,” says scientist Margaret Torn of Lawrence Berkeley National Laboratory (Berkeley Lab). The worrying part is, “we don’t know what happens in soils as the planet warms.”
Torn is concerned about soil microbes. These bacteria and fungi are nature’s janitorial crew, decomposing organic materials and returning that energy to the food web. The process also releases carbon dioxide into the atmosphere, which adds to the insulating blanket of greenhouse gases heating the globe.
This process makes soil microbes major carbon emitters. Their metabolic processes already release 10 times as much carbon as all of the fossil fuels humans burn every year. That amount is currently balanced by the carbon consumed by growing plants. The sheer scale of these amounts, however, means increases in soil emissions could have massive reverberations on the planet’s carbon budget.
“It’s like moving money from your savings account to your checking account: what was stored is now active in the carbon cycle,” says Elaine Pegoraro, a postdoctoral fellow working on the project.
Global warming is expected to shift the metabolisms of soil microbes into overdrive, which will increase the amount of carbon they pump into the atmosphere. Torn and her team want to know how surrounding environmental conditions, such as the availability of moisture and plant growth, interact to affect how soils impact the global carbon budget.
“We are attacking one of the big uncertainties in climate prediction: the feedbacks between ecosystems and climate that regulate greenhouse gases in the atmosphere,” Torn says.
On a sunny morning in late April, Pegoraro is picking her way through a meadow just north of the NRS’s Point Reyes Field Station. One of the 41 reserves in the UC Natural Reserve System, the station is located within Point Reyes National Seashore in Marin County. Maneuvering around the lab’s expansive warming experiment, Pegoraro is careful to place her feet atop the plastic grids that serve as walkways around the plots. It’s an example of the care the scientists are lavishing on this study: they’re trying to prevent human footsteps from compacting the ground, which could affect results.
“We want to understand how grasslands allocate carbon aboveground versus belowground as climate warms,” Pegoraro says. “Grasses don’t have a lot of stature aboveground. But they do have really deep root systems. Even if they die, their roots will stay in the soil as a store of carbon.”
By contrast, forests store much of their carbon aboveground in their trunks and leaves. If trees die from climate change, they are more likely to transfer their carbon to the atmosphere as they decay.
These, however, are just educated guesses. The real world is far more complex. Interactions between grasses, soil microbes, and the environment could throw these hypotheses out the window. The only way to know for sure is to do the science.
The soil warming experiment is “big” science in the sense that it sprawls across an entire acre of the national park. Yet aside from a few gray metal utility boxes, its outlines are just barely visible amid the green sea of grass blades.
Each of the 24 plots in the experiment is about the size of the center circle of a basketball court. Each circle is delineated by 22 hollow metal tubes pounded vertically into the ground. An additional rod has been driven into the circle’s center. The rods will be threaded with heating tape to warm the soil.
The rods have been punched 2.4 m into the earth—roughly the height of former Houston Rockets center Yao Ming. This enables the scientists to warm much of the profile of living soil.
This is not the first experiment to examine emissions from warmed earth. However, most previous studies only heated about 30 cm, or about a foot, deep. That omits a tremendous amount of active soil, says Torn. “More than half the world’s carbon is below that surface layer. While that deep carbon is older, it is actually just as responsive, and can get just as much of a boost in decomposition, as the soils closer to the surface.”
The effect of water
Moisture is likely to have just as much if not more impact on soil carbon as heat. To see these effects, the scientists have subdivided their Point Reyes site into three zones: one at the dry crest of the meadow, another at the midpoint of the slope, and the third at the bottom, where grasses give way to water-loving sedges.
As regards water levels, living things are like Goldilocks: they want conditions to be just right. “Warming adds more stress because it’ll dry at least the surface soil. And if it’s too dry, the microbes may not be able to absorb all the nutrients and carbon they want,” Pegoraro says. Likewise, “if it’s too wet, it may inhibit decomposition, because they won’t have enough access to oxygen.”
Each zone includes three types of plots: controls that will receive no heating, plots warmed 3°C above ambient temperature, and plots warmed 6°C above ambient temperature. Those temperatures bracket expected increases in local temperature by 2100.
All plots are outfitted with sensors that continuously measure carbon emissions, temperature, and moisture across the soil profile.
“If you tried to do this 45 years ago, the sensors would have been housed in an Airstream trailer. But now, thanks to the miniaturization of computer and battery technology, it’s all mobile,” Torn says.
Seven years ago, Torn’s lab installed an identical soil warming experiment at UC Berkeley’s Blodgett Experimental Forest, about 60 miles west of Lake Tahoe. Findings there suggest deep soil warming increases carbon emissions by a whopping 20 percent over findings from shallow warming experiments.
But at her forest site, Torn can’t fully account for a competing feedback activity: how soil warming affects plant growth. When soil microbes decompose organic matter, they release nitrogen alongside carbon dioxide. Nitrogen is a plant fertilizer. And plants pull carbon dioxide out of the air as they grow.
“We wanted to research warming on soil where we can measure plant growth and the uptake of nitrogen by plants. At Point Reyes, we can warm all the soil that the roots are in, and we can measure all of the biomass of the plants. We can’t do that in a forest,” Torn says, because the trees are too massive.
The extra nitrogen in warmed soils might perturb the ecosystem in other ways as well. Plants might allocate less carbon to their roots when nitrogen is easier to access. This shift can change the types of active microbes in the soil. To examine how this will play out, the researchers have added nitrogen to several additional plots around the main experiment, where they will observing changes to the microbial community as well as effects on plant tissue biomass and composition.
Before flipping the switch
Torn and colleagues laid out the soil warming plots at Point Reyes back in 2021. The team is in the midst of characterizing conditions at the site for a full year before turning on the heat in January of 2023.
This spring, the researchers have been recording the prevalence of local plant species, taking soil cores to measure organic matter and nitrogen content, and identifying the types of microbes living in the soil using genomic and other methods. They’ll repeat these measurements periodically throughout the life of the experiment.
As they work, the researchers have been using the field station as their home base. A historic former ranch house, the station has comfortable accommodations as well as some lab space. In addition, reserve staff have served as liaisons with the National Park Service for research permits and other services such as expanded internet service. Torn plans to bring representatives from the Department of Energy, which is funding the research with a $3.6 million grant, out to the field station to tour the site this fall.
Part of global efforts
Torn and colleagues hope to run the Point Reyes experiment for at least a decade. This will permit them to observe whether initial warming effects last, and for how long.
Ten projects around the world are now following similar soil warming protocols. The Berkeley Lab team plans to collaborate with those groups to obtain a global picture of soil warming impacts.
“The more we understand about different environments, the better we can make our predications about how places will respond,” Pegoraro says.