Kade Wilbourne pulls a lever, and several tonnes of volcanic rock shoot out onto the field behind us in a fan of blue-grey dust. We are sitting in the cockpit of a tractor on the Wilbourne Farm in Mecklenburg County, Virginia. Normally, soybean and corn yields are the metrics that matter most here. But today, what counts is carbon.
The soil on farms like this is already a major reservoir of carbon, contained in organic form in the bodies of microbes and plants and in inorganic form within minerals. Farmers have long added soil amendments like compost to their fields, which boosts both crop yields and the amount of organic carbon in soil. In recent years, researchers and start-ups around the world have begun paying farmers to spread basalt and other types of rock dust in order to increase inorganic carbon storage as well. Such silicate rocks undergo a process called rock weathering: they react with atmospheric carbon dioxide dissolved in water, stabilising it in mineral form. But how do these different ways of storing carbon in soil interact?
Wilbourne Farm is part of an experiment to find out. After laying down crushed basalt collected from a nearby quarry, Wilbourne’s trailer will spread different combinations of more typical soil amendments: compost and carbon-rich biochar, which enrich the soil with nutrients, and crushed limestone, which reduces acidity. Over the next few years, researchers will be able to measure how these carbon storage methods enhance or detract from each other. Other federally funded trials in Minnesota and California will test the same interactions across different climates.
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“There’s all these places where these cycles interact that might promote formation of organic and inorganic carbon,” says at Lawrence Livermore National Laboratory in California, who is leading the experiment.
The organic carbon from living organisms in soil is the planet’s largest reservoir of carbon outside of the ocean. But research on rock dust so far has focused on the inorganic side, says Sokol. That is because  carbon can remain out of the atmosphere for longer in mineral form. While organic carbon will stick around in soil for centuries before decomposing, much of the mineralised carbon from rock weathering will wash out to sea, where it can stay out of the atmosphere for millennia or even millions of years.
Rock weathering happens naturally with erosion and time – about two hundred million tonnes of CO2 are removed from the atmosphere this way each year – and crushing the rocks speeds things up. If rock dust were spread on much of the world’s farms, researchers this “enhanced” rock weathering (ERW) could remove between 0.5 and 2 billion tonnes of CO2 per year, a good portion of the total removals needed to reach climate targets. It could remove as many as per year on US farms alone.
But accounting for the interactions between rock dust and other soil amendments – especially their effects on organic carbon – could change these numbers, says Sokol.
For instance, the minerals added to soil when rock dust dissolves can increase the surface area available to particles of organic carbon, helping it accumulate and remain out of the atmosphere for longer. Adding compost along with rock dust might spur microbes to grow more quickly and leave behind more carbon when they die. That microbial activity may, in turn, speed up the weathering process, as fungi and bacteria release acids and physically break down the rock.
Conversely, these interactions could have negative effects on soil carbon. For instance, in an unpublished study, Sokol and his colleagues found adding rock dust to a field in California resulted in a loss of organic carbon from the soil – but only at first. After three years, the effect reversed, and the plot with crushed rocks ended up with more organic carbon than the one without.
Until the trials are complete, the overall effect of these interactions remains unclear. But observing an increase in net carbon removal due to the combination of rock dust and other soil amendments would be “very promising” for enhanced rock weathering projects, says Sokol, who has studied in theory. It would mean these projects could remove more carbon using less land.
The right combination might have additional benefits beyond carbon, like better crop yields and fewer emissions of the potent greenhouse gas nitrous oxide from soil. “You might get one plus one equals four,” says Sokol.
The day I visit marks the beginning of the experiment. First, nearly 20 tonnes of basalt goes down, scattered evenly over the field. Then Wilbourne makes several more passes, spreading other material in different combinations. Some plots get bone white limestone dust. Others get rich black biochar and chicken litter compost that reeks of ammonia. A few areas get everything.
Up in the tractor, I ask what Wilbourne, whose father Adam runs the farm, thinks about all the buzz around rock dust and CO2 removal. He shrugs. Aside from a few more passes with the spreader and the gaggle of soil scientists taking notes, this is a regular day’s work.
