Partnering with Microbes to Predict Changes in Soil Carbon

February 28, 2023

In efforts to address climate change and identify ways to reduce the amount of greenhouse gasses like CO2 in the atmosphere, one key place to include is below our feet: the soil. Soil organic carbon is a central component of soil health. A 2020 study showed that soil carbon represents 25% of the potential of natural climate solutions. Protecting and, where appropriate, restoring soil carbon in places that it has been lost is an important piece of mitigating the effects of climate change, one that has co-benefits related to soil health and fertility.

Recognizing this, conservation-minded ranchers like those at TomKat Ranch in coastal Central California are finding ways to steward the soil, making ranching beneficial to ecosystems while producing high quality food for communities. To better utilize soil as a climate solution we need to understand the status of soil carbon across space and time, including how to measure the impact of management practices on carbon sequestration. To do that TomKat teamed up with conservation scientists at UC Santa Barbara and Point Blue.

Both plants and microbes drive carbon cycling in land-based ecosystems, and the researchers’ goals were to better understand whether these drivers could predict past, current, and future trends in soil carbon. Scientists from UCSB and Point Blue investigated how plant and microbial partners could help practitioners monitor—and importantly, forecast—carbon stored in the soil on rangelands at TomKat Ranch. In a newly published study in Soil Biology and Biochemistry led by Jacob Weverka, former Point Blue rangeland biologist and current UC Santa Barbara graduate student, they show exciting new field-tested ways of understanding carbon storage in the soil.

Why find biological indicators instead of just directly measuring carbon in the soil? The short answer is, seeing changes over time in response to a management action can often take a long time when directly measuring carbon in the soil. Meanwhile, ranchers want to understand how the time and money spent on a practice–like soil compost application, native grass restoration, or rotational grazing–translates to benefits for carbon storage, the ecosystem, and the ranching operation. Biological indicators can change more rapidly than soil carbon and show trends in a larger system. Because of how they metabolize carbon, the presence or absence of microbes can indicate, and also drive change and therefore be the cause of a trend we see over time.

Over a six year period, the scientists who led the new study took data on fungal, bacterial, and plant communities along with direct measurement of soil carbon at TomKat Ranch. They then analyzed which combination of factors best reflected soil carbon content in the past, present, and future, with a particular interest in future predictors. They found that certain combinations of plants, fungi, and bacteria predicted current and future carbon levels.

“We looked at a number of factors that are known to affect soil carbon storage, and tried to identify which aspects of the ecosystem are most important for predicting soil carbon at the scale that land management occurs, both in space and time,” shared lead author Jacob Weverka. “While there are still a lot of research needs in this area, I think our study is an important contribution to this growing body of knowledge.”

One notable result was that high carbon soils— which harbored greater amounts of bacteria that are known to thrive in high carbon environments— subsequently lost more soil carbon over time. The authors speculate that this is because those same bacteria are known to leak a lot of CO2 when they eat organic matter, leading to greater losses of carbon from the soil system. These types of microbes can be likened to the “hare” in the famous fable. Although they work fast, they leak a lot of carbon in the process and won’t “win” the carbon sequestration race in the long run on their own.

Another type of microbe that metabolizes carbon more slowly and is found in areas with less soil carbon–the “tortoise” in this analogy–was seen to store more carbon overall in the soil in the long-run. This is what we would expect based on current knowledge of these organisms, but the ability for these organisms to forecast future changes in soil carbon has rarely been demonstrated in the field.

Researchers also saw greater soil carbon sequestration in surface soils when more decomposing fungi were present. This makes sense given that these fungi are central players in incorporating plant material into the soil where it can be protected and stabilized. These findings were encouraging and may provide a new way to guide management actions that result in more carbon in the soil, less in the atmosphere, and therefore less severe climate impacts.

“We used to think carbon storage happened in the soil because it didn’t get eaten by microbes,” said co-author Dr. Chelsea Carey. “Now we know that the vast majority of carbon that stays in the soil has been consumed by microbes at some point. Microbial physiology is therefore hugely important for understanding soil carbon storage and should be incorporated into practices and analyses to address climate change.”

Dig in to the full publication for all the details here.

Questions?
Contact Chelsea Carey, Point Blue Director of Soil Research and Conservation