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Soil Carbon Sequestration is No Silver Bullet

A recent study shows that soil can’t capture enough carbon to outweigh emissions.

By H Conley

1/26/2023

Farmers are on the frontlines of the climate crisis as extreme temperatures, flooding, droughts and other disasters exacerbated by human-caused climate change threaten their livelihoods.

They have also been touted as linchpins in solving this crisis, thanks to soils’ ability to sequester carbon. Plants, both naturally occurring and agricultural, pull carbon out of the atmosphere as part of photosynthesis and then sink it into the soil via their roots and by decomposition.

If farmers can maximize the carbon sequestered through sustainable management, a practice known as carbon farming, it would be monumental for the climate movement. But new research suggests the big promise of soil carbon sequestration doesn’t hold water.

Dr. William Schlesinger has studied soil since the early 1970s, when a professor of his pointed out soil’s natural ability to hold carbon out of the atmosphere. He wanted to understand how much carbon can be sequestered, compared to how much is released when soil is disturbed. Schlesinger, President Emeritus of the Cary Institute of Ecosystem Studies, has published multiple papers since the 1990s that have used available data to make these calculations.

His research has found that soil is slow to accumulate carbon, limited by plants’ capacity for photosynthesis. Meanwhile, farming causes soil to shed much of what was sequestered. Any carbon that is captured by soil is then “lost fairly rapidly,” Schlesinger said. “That juxtaposition made me want to look into how you could manage soils to increase the rate of accumulation above the slow natural rate.”

His review study, published in the journal Biogeochemistry in June, compiled data from 26 studies that looked at different farming practices, including fertilization, irrigation, and manure application.

The studies also examined cover cropping (planting a field that would otherwise be left bare that season with a crop that reduces erosion, improves soil health and smothers weeds, but isn’t harvested), as well as reduced tillage (where farmers disturb the soil as little as possible by not tilling as deep or only tilling small areas). In addition, the studies covered biochar (charcoal made from burning wood chips, plant matter or manure, which becomes a more solid form of carbon and can be applied to soils).

Schlesinger’s analysis shows how little impact these practices have on rates of soil carbon sequestration.

“The amount of carbon you’re going to accumulate with better [soil] management is not going to be huge,” he said. “In fact, I’d use the word trivial.”

That small amount cannot fully offset the total greenhouse gas emissions released by agriculture globally. This is why Schlesinger said it’s important to make our agricultural system carbon neutral.

By his calculations, not even the best management practices could sequester 5% of the U.S.’s current emissions, so we must end our reliance on fossil fuels and switch to green energy. Only by severely reducing emissions can carbon sequestration have any significant impact on fighting the climate crisis, he said.

An industry built around carbon offset credits (the practice of corporations, governments or individuals paying for carbon to be sequestered elsewhere to outweigh their emissions) has taken off in recent years, with some looking to soil as a potential source of credits. 

According to Acadia Tucker, a regenerative farmer, author, and climate activist, Schlesinger’s findings are significant, because they show that soil cannot sequester enough carbon to make the credits system viable. 

However, there are still many unanswered questions about soil and how carbon accumulation works that could change our understanding of these findings. For example, Tucker argues that climate scientists should be looking at the role microbes play in carbon sequestration.

“A lot of these models that are used to either measure or predict carbon sequestration completely leave microbes out of the question,” Tucker said. ”It’s this black box that’s happening within soil and I don’t think we’ll come to a clear understanding of what’s possible or impossible until we understand those interactions more.” 

Microbes are responsible for breaking down plant matter and fixing the carbon they digest into the soils. Scientists don’t have a clear picture of how these organisms in different types of soil, climates, regions and management systems vary. This gap in knowledge means the available data can only show part of the picture of carbon accumulation in soils.

Tucker agreed with Schlesinger when he said we must, “immediately move away from trying to capture carbon that’s being emitted, or has been emitted, and focus on reducing new emissions.” 

Schlesinger says policy aimed at combating the climate crisis must prioritize solar and wind power first and foremost. There are still many benefits to sustainable farming practices for farmers, consumers, local communities and the environment, so policies promoting these practices remain important.

“I think agricultural soils are some of the most abused soils on earth and better management of them would be great,” Schlesinger said. “You may accumulate some carbon and that would be good, but it shouldn’t be touted as the solution to global climate change.”