|Photo Credit: LeRoy N. Sanchez|
While many scientists who study climate change look up to the sky for clues about the Earth’s future, one researcher has spent her career looking down—at the abundance of life in the soil below. Innumerable microorganisms such as bacteria and fungi live in harmony with plant roots, decomposing fallen leaves and dead animals. In addition to acting as the ultimate recyclers, they also stabilize the soil and help to retain water.
Cheryl Kuske, a microbiologist at Los Alamos National Laboratory, has focused the last two and a half decades on studying this microbial environment. “By decomposing organic matter,” she explains, “microorganisms help cycle carbon and nitrogen through the ecosystem.” Some of the carbon and nitrogen released from the organic matter goes into the soil and is assimilated into roots to help new plants grow—the carbon is incorporated into sugars, and the nitrogen atoms are used to build proteins. But some of these molecules are also released as CO2 and N2 gases into the atmosphere.
The soil ecosystem functions in a delicate balance. Although some organisms release gases into the air, others—including certain bacteria and leafy plants—remove harmful CO2 from the atmosphere for food production.
Kuske and her colleagues at Los Alamos National Laboratory have been investigating the roles of these microbes in carbon and nitrogen cycling to help make better predictions about terrestrial ecosystem responses to climate change. Using a technique called metagenomics to sequence the DNA of all the microbes at once, the team can study the organisms’ genes and the enzymes they produce.
These microrganisms’ lifecycles are so intertwined that their single genomes cannot be isolated for sequencing. However, analyzed jointly, they yield important clues about their collective functions in the environment. Scientists can identify things such as which bacteria or fungi are responsible for fixing nitrogen or carbon, the ratio of bacteria to fungi in the soil, and which microbes are closely associated with root health or plant growth. The researchers can even figure out which enzymes are currently being used through a technique called meta-transcriptomics; this approach sequences only the transcripts of genomic data that are actively being made and used for protein synthesis.
|Photo courtesy of Cheryl Kuske|
By sampling microbes from various soil environments over long periods of time, Kuske’s team and collaborators are able to understand what happens under the surface when things change aboveground. For instance, in a recent long-term study in Utah, the scientists discovered that slight changes in the summer precipitation pattern, combined with a 2°C rise in soil temperature, resulted in significant changes in the population of microbes below: the types of organisms completely changed, thus altering their overall role in the environment. For example, cyanobacteria—bacteria that create energy through photosynthesis—were no longer present. As a consequence, the new population of microbes no longer had the ability to pull carbon out of the air and had a decreased capacity for fixing nitrogen for protein synthesis.
Increased nitrogen from industrial runoff or fertilizer from agriculture can also have significant effects on the composition of organisms in the soil, as nitrogen is an essential molecule for the growth of both plants and bacteria. A comparison of 15 recent field experiments where nitrogen deposition was measured showed that in an arid environment, an increase in nitrogen had a positive effect on soil health at low concentrations, but too much was toxic to the soil community . In a field experiment in Nevada, higher nitrogen concentrations changed the species composition of bacteria—but not fungi—leading to a fungi-dominated community [2,3].
Although the ramifications of these changes to the microbial world are not yet completely understood, Kuske’s team is continuing their studies, both in the laboratory, under controlled conditions, as well as at various field sites in the American Southwest. What they do know is that the feedback loop is strong. Changes in the aboveground environment—such as rising temperatures, altered precipitation, and increased nitrogen runoff—lead to changes below ground that can have far-reaching consequences.
“The studies being conducted at Los Alamos provide an understanding of the interactive biological processes that are inherent in all types of terrestrial ecosystems and that tightly control carbon and nitrogen fluxes to the atmosphere,” says Kuske. Climate warming and altered weather patterns will disrupt this balance. When the diversity of soil microbes change, the feedback loops that ensue could have lasting effects on the amounts of carbon and nitrogen in the soil and the atmosphere.
Rebecca McDonald is a science writer at Los Alamos National Laboratory specializing in the communication of bioscience research. She has also worked as a freelance writer, and volunteers her time as a communications consultant for a science education non-profit.
Disclaimer: Elena E. Giorgi is a computational biologist in the Theoretical Division of the Los Alamos National Laboratory. She does not represent her employer’s views. LA-UR-16-22406.
 Steven B, Kuske CR, Gallegos-Graves LV, Reed SC, & Belnap J (2015). Climate change and physical disturbance manipulations result in distinct biological soil crust communities. Applied and environmental microbiology, 81 (21), 7448-59 PMID: 26276111
 Sinsabaugh RL, Belnap J, Rudgers J, Kuske CR, Martinez N, & Sandquist D (2015). Soil microbial responses to nitrogen addition in arid ecosystems. Frontiers in microbiology, 6 PMID: 26322030
 Mueller RC, Belnap J, & Kuske CR (2015). Soil bacterial and fungal community responses to nitrogen addition across soil depth and microhabitat in an arid shrubland. Frontiers in microbiology, 6 PMID: 26388845