Earth’s carbon cycle works on a global scale. But it can be affected by the tiniest of organisms -- soil microbes. Those microbes decompose organic matter like plant litter and dead organisms, and create simple carbon compounds. The simple carbon compounds can then be used by other organisms, or turned into gases -- like carbon dioxide -- and released into the atmosphere.
Much like humans, soil microbes can be picky about where they live and work. Just as people may prefer a certain range of temperature and humidity, soil microbes have their preferable conditions too.
Alyssa Kim, researcher at Cornell University, is the lead author of a new study that explores how soil conditions, like moisture level and pore size, can affect soil microbes. Understanding how different soil conditions impact microbial activity can give researchers a better handle on ways to increase soil health and fertility, and help combat climate change. It can be a critical part in reducing greenhouse gas emissions from agricultural fields after harvests.
Colleagues at Michigan State University compared microbial activity near corn and switchgrass leaf litter. Corn is a vital crop, and farmers in the United States planted nearly 90 million acres in the 2022 growing year. Switchgrass is a promising bioenergy crop with an expanding footprint. Also corn and switchgrass have different litter characteristics. Litter chemistry affects how easily microbes can decompose different litters. The physical characteristics like texture can affect the water and air environment near litters.
The researchers found that corn and switchgrass litters differ in how they change moisture levels in the soil near them. They found distinct moisture depletion 0.1 to 1.5 millimeters away from switchgrass residues.
To study that moisture distribution, a method called X-ray and Neutron computed tomography was used. That method works similarly to medical CT scans. It’s a promising non-destructive way to study soils and water in them.
It turns out, moisture content is one of the most important factors influencing soil microbial activity. That’s because one way that microbes decompose organic material, like leaf litter, is by releasing chemicals called enzymes. Different enzymes break down different materials. An enzyme called beta-glucosidase can break down plant cell walls. Another enzyme called chitinase can break down the exoskeletons of insects and some fungi. Once the enzymes break down their target materials into simpler chemicals, soil microbes can feast.
When soil moisture levels are optimal for microbes, they tend to produce more enzymes. That can lead to faster decomposition of leaf litter and the release of larger amounts of carbon dioxide. That’s exactly what the researchers observed. Soil moisture levels were greater near corn litter, and decomposing corn litter released more carbon dioxide quicker than switchgrass litter.
Although the study focused on millimeter-scale observations, it has large-scale implications. Studying those microscale dynamics can help to understand what is actually happening in corn fields, and also, in promising bioenergy cropping systems like switchgrass.
The researchers also tested how soil pore size affects microbial enzyme activity. The pore sizes ranged from 10 to 30 micrometers, slightly smaller than the thickness of a single strand of most human hair. It is crucial to study soil pore structures because that’s where soil microbes live. Researchers used a method called Zymography, to map the activity of different enzymes. Some chemicals are added onto the soil surface. Such chemicals show fluorescence when decomposed, and that is how the location of enzymes are detected.
Soil pore size affects different enzymes differently. Beta-glucosidase – the enzyme that breaks down plant cell walls – worked more efficiently in soils with smaller pores. On the other hand, chitinase enzyme activity was greater in soils with larger pore sizes. Those contrasting results tell researchers that what is decomposed in soils can depend on soil pore architecture. That’s because there are different microbes living in pores of different sizes, producing different enzymes.
Soils in farm fields have a mix of large and small pores, which indicates a mix of moisture levels and different microbes. In the future researchers would like to look at soil pores and moisture levels on larger scales and test how differences in moisture distribution affects the decomposition process.
The research was supported by Great Lakes Bioenergy Research Center, U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research; National Science Foundation Long-term Ecological Research Program at the Kellogg Biological Station; Michigan State University AgBioResearch; and special thanks to Dr. Sasha Kravchenko and Kravchenko Lab at Michigan State University, as well as Anders Kaestner at Paul Scherrer Institute in Switzerland.