Food waste that ends up on a landfill site is the third-largest contributing factor to climate change. Converting food waste into compost is an essential part of combatting climate change. Compost added to soil has a significantly higher carbon content compared to soils that have not been composted. This carbon sequestration can save our planet.

Soil and climate change

It’s no secret that organic farmers believe in compost, but just what role compost plays in soil’s ability to store carbon – and keep it out of the atmosphere, where it contributes to climate change – has been less clear.

A recent study out of the University of California, Davis suggests that compost plays a more significant role than once thought in building soil carbon.

It also found that carbon levels fluctuate more in deeper soil than most evaluation methodologies tend to account for. In practical terms, the findings could mean agricultural incentive programs have undervalued compost and that we’ve measured carbon levels in soil all wrong.

Nicole Tautges, a co-author of the study, says it wasn’t a surprise to find that compost is good at helping soil store carbon – it’s where exactly it does this that was revealing.

“The surprising piece was that it raised soil carbon between one and two meters deep. Because the big question is, ‘How does the carbon get down there when we’re only applying it in the top foot?'” adds Tautges, the chief cropping systems scientist at the UC Davis Agricultural Sustainability Institute.

Her team has hypothesized that it has to do with how water moves through the soil, and they plan to continue studying its movement. But just demonstrating the importance of soil depth could be significant, both for increasing the value that farmers and policymakers place on compost application and, eventually, for how soil carbon measurements are taken.

“When you only measure the top foot, there’s potential to both over-and underestimate carbon storage in our agricultural soils,” says Tautges.

Soil research

In the study, which was published this summer in the journal Global Change Biology and conducted at a long-term research site the university initiated in 1993, the researchers measured soil organic carbon at five different depths down to two meters over 19 years.

They compared the carbon levels across several different cropping systems – conventional, conventional with cover crops, and one that included the application of compost alongside cover crops.

Researchers found that systems using cover crops alone not only failed to store more carbon, they actually lost significant amounts of carbon in the soil below about a foot deep. However, the system that used both cover crops and compost had significantly increased soil carbon content over the length of the study – about 0.7 percent annually.

That may sound like a small number, but it’s enormous in the context of soil, where change is slow and gradual. The “4 per 1,000” initiative has called for a 0.4 percent increase in soil carbon annually around the globe as a way to combat climate change.

The UC Davis researchers didn’t study the effect of compost alone, without cover crops. That may be a focus of future research, but Calla Rose Ostrander, director of the Carbon Project at the People, Food & Land Foundation, who has worked with the Marin Carbon Project for the last six years, likes that they didn’t separate it out. She sees compost as crucial, but only if it’s used as part of an approach to farming that’s sustainable in a holistic way.

“I think we have this desire to say, ‘What’s the quick fix?'” says Ostrander. She doesn’t want people to see compost as a silver bullet – spreading it on their farm without doing anything else to improve the soil. “You still need to manage for the whole ecosystem,” she adds.

Regenerative agriculture

As interest in regenerative agriculture and carbon farming picks up, this kind of research will be vital to ensure it’s done right. “When you have these very degraded systems, adding the carbon and the nutrients and the biology that comes with compost,” says Ostrander, “helps heal the soil and get the system back up to the point where it can be regenerative.”

Tautges hopes that the study will be part of a growing body of evidence that will lead to compost being included more often in the list of practices farmers are being paid to adopt. California already does this through its Healthy Soils Incentives Program, and a handful of other states have put similar efforts in place.

And, Leibowitz points out, the importance of good soil management cannot be overstated. The UN Food and Agriculture Organization has estimated that if current trends continue, the world has less than 60 years’ worth left of topsoil.

“For human beings to survive, we need to have the capacity to grow food in the soil,” says Leibowitz. “These issues are really urgent.”

This article originally appeared in Civil Eats. It was republished as ScienceAlert’s partnership with Covering Climate Now, a global collaboration of more than 250 news outlets to strengthen coverage of the climate story.

(Rachel Cernansky, 2021)

Rachel Cernansky, C., 2021. 19-Year Study Shows We’ve Been Undervaluing How Much Compost Can Boost Carbon Capture. [online] ScienceAlert. Available at: <> [Accessed 17 April 2021].