Compost, cover crops increase carbon in soil

A study finds increasing soil organic carbon, such as compost, is critical to increasing long-term soil carbon storage

In a 19-year study at the University of California, Davis, scientists dug six feet down in a study plot to compare changes in soil carbon in conventional, cover-cropped, and compost-added plots of corn-tomato and wheat-fallow cropping systems.

Their findings showed that soil health and the importance of carbon is more complex than often realized but, with the right management, the soil is a huge natural resource where carbon can be sequestered.

The implications are that the right applications not only help to slow the rapid rise of carbon in the atmosphere but foster effective sustainable agriculture.

The study showed that conventional soils neither released nor stored much carbon. Cover-cropped conventional soils increased carbon storage in the top 30 centimetres but lost significant amounts of carbon deeper than that. However, when both compost and cover crops were added in the organic test plots, soil carbon content increased 12.6 percent over the 19 years of the study. That represents 0.07 percent annually.

The study underscored that increasing soil organic carbon, in this case compost, is critical to increasing long-term soil carbon storage and potentially improving climate change mitigation in agricultural systems.

“We used poultry compost,” said Nicole Tautges, cropping systems scientist with the UC Davis Agricultural Sustainability Institute.

Tautges said that the wheat-fallow system is a reflection of an older cropping practice where the fallow was used to bank up water in places where there was no irrigation.

“We think there was no carbon input into that system essentially,” she said. “There was a lack of crop production during that fallow year with plain soil and no cover crop. That’s very common in dry agricultural areas.”

Soil health is driven by micro-organisms, which are critical to decomposing organic residues and recycling soil nutrients. Carbon has to filter through soil microbes so that the stabilized forms of carbon can settle. Compost provides not only carbon but essential nutrients for the microbes to function effectively and maintain soil health. But if the microbes’ diet is out of balance, they search for missing nutrients from existing soil organic matter, which can result in a loss rather than a gain of carbon. As for cover crop roots, the thought is that, while they provide carbon, they may not provide all the nutrients needed to stabilize the soil.

“We are applying the compost at around the 20 cm depth,” said Tautges. “Carbon is being transported down into the soil by water. We measured higher levels of dissolved organic carbon in the composted system, many times more compared to the other non-composted systems.”

Their analysis showed that soil organic carbon in the wheat-fallow systems did not change with the addition of nitrogen fertilizer, winter cover crops, or irrigation alone and even decreased by 5.6 percent with no inputs at all. However, in corn-tomato rotations, soil organic carbons increased by 12.6 percent with both winter cover crops and composted poultry manure applications. But in a conventionally managed system, the addition of winter cover crops resulted in an increase in soil organic carbon by only 3.5 percent to a depth of 30 cm and then decreased by 10.8 percent in the 30 to 200 cm layer, resulting in an overall loss.

“We don’t know why the cover crops are losing carbon at the depths they are, especially between 60 and 100 cm down. But we do know that that is about where their roots end. We are investigating various mechanisms of carbon loss among the systems to try to elucidate what those mechanisms are.”

Tautges said that while there are benefits to cover crops that farmers enjoy, in their test system, storing carbon was not necessarily one of them. She stressed that farmers would make progress by the application of compost.

The study was conducted in California’s northern Central Valley at the Russel Ranch Sustainable Agriculture Facility and, so far, the research is showing that semi-arid climate regions may be able to store much more carbon than once thought. One encouraging aspect of the study’s results is that the science and the state’s policy efforts for healthy soils are dove-tailing.

“Carbon sequestration in soils through the addition of compost is a key practice in our Healthy Soils Program and we are delighted that the science and policy efforts are aligning and supporting each other,” Karen Ross, secretary of the California Department of Food and Agriculture, said in a news release.

So far, the results are showing that the appropriate application of compost will provide many interconnected benefits to farmers and the environment by improving soils, offsetting emissions and transforming animal and food wastes into valuable products the soil and the microbes need.

The study was published in Global Change Biology.

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