Compost key to sequestering carbon in the soil

By moving beyond the surface level and literally digging deep, scientists at the University of California, Davis, found that compost is a key to storing carbon in semi-arid cropland soils, a strategy for offsetting CO2 emissions.

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By moving beyond the surface level and literally digging deep, @ucdavis scientists found that compost is a key to storing carbon in semi-arid cropland soils, a strategy for offsetting CO2 emissions. @globalchangebiohttps://t.co/A2Fjf1Q12N

— UC Newsroom (@UC_Newsroom) August 14, 2019

Soil carbon changes compared

For their 19-year study, published in the journal Global Change Biology, scientists dug roughly 6 feet down to compare soil carbon changes in conventional, cover-cropped and compost-added plots of corn-tomato and wheat-fallow cropping systems. They found that:

Conventional soils neither release nor store much carbon.
Cover cropping conventional soils, while increasing carbon in the surface 12 inches, can actually lose significant amounts of carbon below that depth.
When both compost and cover crops were added in the organic-certified system, soil carbon content increased 12.6 percent over the length of the study, or about 0.07 percent annually. That’s more than the international “4 per 1000” initiative, which calls for an increase of 0.04 percent of soil carbon per year. It is also far more carbon stored than would be calculated if only the surface layer was measured.

Overall net loss with cover-cropped systems

Many studies that investigated carbon change in the top foot of soil found that cover-cropped systems store carbon. The UC Davis study also found gains in the surface but, deeper down, enough carbon was released from cover-cropped systems that it resulted in an overall net loss.

“There are other benefits to cover crops that farmers may still enjoy, but in our systems, storing carbon is not necessarily one of them,” said co-first author Nicole Tautges, a cropping systems scientist with the UC Davis Agricultural Sustainability Institute. “We‘d make more progress by incentivising compost.”

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When both compost and cover crops were added in the organic-certified system, soil carbon content increased 12.6 percent over the length of the study, or about 0.07 percent annually. – Photos: Henk Riswick

The researchers did not compare composted systems without cover crops, but suspect the compost helped sequester carbon despite the cover crop, a notion they intend to investigate further.

Vital nutrients for microbes

Carbon has to filter through soil microbes to create stabilised forms of carbon in soil. Compost provides not only carbon but also additional vital nutrients for those microbes to function effectively.

“One reason we keep losing organic matter from soils is that our focus is on feeding the plant, and we forget the needs of others who provide important services in soil like building organic carbon,” said senior author Kate Scow, director of the UC Davis Russell Ranch Sustainable Agriculture Facility. “We need to feed the soil, too.”

The results indicate an opportunity for compost to provide multiple, interconnected benefits to farmers and the environment by improving soils, offsetting greenhouse gas emissions, and transforming animal and food wastes into a valuable product the soil needs.

Also read: The soil paradox