Rising Droughts Disrupt Soil Microbes' Ability to Capture Carbon

Microbial ecologist Steven Allison warns in a viewpoint published on April 12 in the journal Trends in Microbiology that soil health and future greenhouse gas levels could be harmed if soil microorganisms adapt to drought faster than plants. He contends that we need to learn more about how bacteria respond to drought in order to control the issue in both agricultural and natural environments.

"Soil microbes are beneficial, and we couldn't live without their carbon and nutrient cycling," Allison of the University of California, Irvine says. "However, climate change and drought can upset that balance, and we need to be aware of how it's changing."

Some soil bacteria absorb carbon from decomposing plants and store it in the soil, while others release carbon back into the atmosphere. Carbon that ends up in the soil is advantageous in a variety of ways. "The carbon in soil has these reverberating effects out to the rest of the world in terms of infrastructure in our natural and managed ecosystems," Allison adds.

"Carbon-rich soils hold more nutrients, so plants growing in those soils tend to be more productive, and the carbon changes the physical properties of the soil, which prevents erosion. In California now, we have this system where droughts are more intense, and then rainfall is more intense," he explains. "So, if you're losing soil carbon, when it rains really hard, it could carry your soil away and cause erosion, landslides, mudslides, sediments, and all kinds of problems that we're actually seeing right now."

Another narrative is the carbon that is released back into the atmosphere. "From a climate mitigation standpoint, what we want is for more carbon to be in plants and soils and less carbon to be in the atmosphere, so the more carbon we can absorb into plants through photosynthesis and the more carbon we can transfer and keep in the soil, the better off we're going to be in terms of climate change," Allison says. "That's why it's critical to understand how the balance of incoming versus outgoing carbon changes in response to drought, warming, or any other climate factor. Drought will have an impact on both plants and bacteria, but Allison believes that microbes will be able to recover faster."

"Microbes are extremely adaptable—they can change their physiology, their abundances so that more drought-adapted microbes take over, and they can potentially evolve—so we expect them to resist or recover from drought," Allison adds. "All of those different processes can happen pretty quickly with microbes, much faster than plants. If more carbon-releasing bacteria survive than carbon-sequestering microbes, we risk having carbon-depleted soils, which would have major consequences for plant productivity and future greenhouse gas levels."

Allison believes that additional research is needed before we can tilt the balance in the right direction. "There is still much to be done. Right now, we have data that suggests that when there is drought, something changes that results in carbon loss, but we don't know exactly how or why that is happening, whether drought is changing the abundance of beneficial plant associated microbes versus carbon releasing microbes, or if it's causing the evolution of one of the microbe groups, or if it's more determined by changes to their immediate physiology," Allison says. Some bacteria may be able to assist plants deal with drought. We could try to swing the balance in their favour if we knew which bacteria were most useful to plants and most likely to retain carbon in soil.

"There's a lot of potential for us to manage or engineer soil microbes," Allison explains. "In agricultural systems, we can consider manipulating the soil or reintroducing beneficial microbes." Management in more natural systems would most likely be on the plant side: soil microorganisms are frequently entangled with plants, so managing the plants can benefit the microbial element of the ecosystem. We also need more measurements to get a good sense of how drought affects soil carbon change in different ecosystems," Allison explains. "There's a lot of landscape out there- from Arctic tundra to deserts- and we could use more research across those diverse habitats."