Scientists are investigating how to naturally reduce soil greenhouse gas emissions that are expected to increase with higher temperatures, and have identified an environment that could help them turn this idea into a reality. Rangelands, open areas used for animal grazing, cover 62% of California’s non-developed land. With such vast coverage throughout the state, rangeland environments provide a great opportunity to apply nature-based practices that can optimize carbon storage, and ultimately reduce greenhouse gas emissions. 

Nature-based carbon solutions take advantage of the natural benefits that ecosystems provide to reduce atmospheric CO2. One solution involves soil amendment, such as adding a thin layer of compost to soil to improve plant growth and decrease carbon released from soils over time. Previous research in California’s grasslands, the type of ecosystem that rangelands commonly are, shows this method can be effective. However, the positive effect of adding compost over a larger scale has not been explored and tested. EESA research scientist Housen Chu co-authored a recent study published in JGR Biogeosciences that investigated how adding compost to a 1.54 hectare rangeland – almost the size of four football fields – affected soil carbon storage and greenhouse gas emissions.

Because compost is made up of organic matter that contains nutrients often limited in California’s grasslands, adding it to soil can increase nutrients available to plants. The compost also helps soils retain water which aids vegetation growth. This ultimately increases vegetation productivity and helps ecosystems absorb more carbon dioxide from the atmosphere. So, when scaled up to a regional scale, this method could enhance soil carbon storage and reduce the release of significant amounts of CO2 into the atmosphere.

“The increase in vegetation productivity from the compost application was likely due to the compost allowing for a longer growing season–more than 60 days–than an adjacent grassland that did not receive the compost application,” explained Tommy Fenster, currently a UC Davis Ph.D. candidate and the lead author of the study. Fenster also received the Berkeley Lab-California State University, East Bay Pathbreaker Internship for conducting this study. 

The research team, led by California State University (CSU), East Bay associate professor Patty Oikawa, conducted the study at the Galindo Creek Field Station in Concord, California. The grassland, owned by CSU, East Bay, is leased to a rancher who grazes cattle at the site.

Back in 2019, the team installed an eddy covariance flux tower at the center of the rangeland, which measures greenhouse gas exchange between soils, plants, and the atmosphere across large expanses of an ecosystem. After collecting more than a year of baseline data, the team applied a thin layer of compost to 1.54 hectares of the field to the south of the tower and kept the rest of the grassland without compost addition for reference. 

The eddy covariance towers collect continuous data that allowed the team to capture the effect of compost addition – and lack thereof – over the course of a year after the compost application. The researchers paired this data with soil nutrient and moisture measurements, in addition to phenology data, which informs how vegetation growth, timing, and rate is impacted by seasonality.

They found that the rangeland with the added compost had overall lower CO2 emissions compared to the rangeland with no compost, confirming the positive effect of this natural strategy for reducing carbon loss from soil at the landscape level. 

“This study is unique, as it involves both field-scale soil treatment in addition to continuous monitoring of the treated and reference fields, which allowed us to detect when and how they diverged,” Chu said.

The data also reinforced the positive effect of compost on soil quality, as the field with the compost addition had more nutrients, which supported vegetation for a longer growing season compared to the site with no compost. The longer growing season led to more CO2 absorbed because vegetation was photosynthesizing at higher rates and for a longer time, taking in more CO2 from the atmosphere in the process. 

“These results demonstrate that compost application, considering its effectiveness and low cost, could play a key role in enhancing carbon sequestration across California rangelands by increasing grassland productivity,” said Patty. 

This study, the first of its kind at this large of a scale, emphasizes the potential of compost at reducing soil carbon loss and ultimately reducing greenhouse gas emissions from soils.