Radiological Damage to Microbes Hampers Plant Decomposition withinChernobyl Ecosystem

More than 25 years after an explosion and fire at the Chernobyl nuclear power plant spewed large quantities of radioactive material across much of the western USSR and Europe, microbes near the site of the accident remain compromised in their ability to decompose dead plant matter at normal rates.

Decades after the Chernobyl disaster, a sign warns of radioactive contamination amid undecomposed biomass in the Red Forest, located directly downwind of the nuclear reactor. Credit: Timm Suess

The result is a buildup of biomass that presents not just a local wildfire hazard, but could potentially spread radioactive beyond the immediate area if carried aloft by smoke.

Tim Mousseau, a professor of biology and co-director of the Chernobyl and Fukushima Research Initiatives at the University of South Carolina, and frequent collaborator Anders Møller of Université Paris-Sud became intrigued when they noticed something unusual in the course of their work in the Red Forest, the most contaminated part of the Chernobyl Exclusion Zone.

“We were stepping over all these dead trees on the ground that had been killed by the initial blast,” Mousseau said. “Some 15 or 20 years later, these tree trunks were in pretty good shape. If a tree had fallen in my backyard, it would be sawdust in 10 years or so.”

In an effort to assess the rate of plant decomposition relative to background radiation, the researchers placed hundreds of mesh bags containing uncontaminated leaf litter in different areas representing a range of radioactive contamination.

Nine months later, analysis of the samples showed that the degree of decomposition was inversely related to the level of radioactive exposure. In the most contaminated areas, decomposition was 40 percent less than in control regions with normal levels background radiation.

The team concluded that the bacteria and fungi that decompose plant matter in healthy ecosystems are hindered by radioactive contamination. They showed a smaller effect for small invertebrates, such as termites, that also contribute to decomposition of plant biomass.

According to Mousseau, slower decomposition is likely to indirectly slow plant growth as well, given that the products of decomposition provide nutrients for new growth. The team recently reported diminished tree growth near Chernobyl, which he says likely results both from direct radiation effects and indirect effects such as reduced nutrient supply.

“It’s another facet of the impacts of low-dose-rate radioactive contaminants on the broader ecosystem,” Mousseau says. “We’ve looked at many other components, namely the populations of animals in the area, and this was an opportunity for broadening our range of interests to include the plant and microbial communities.”
The results also show the potential for further spread of radioactivity.

“There’s been growing concern by many different groups of the potential for catastrophic forest fires to sweep through this part of the world and redistribute the radioactive contamination that is in the trees and the plant biomass,” Mousseau says. “That would end up moving radio-cesium and other contaminants via smoke into populated areas.

“This litter accumulation that we measured, which is likely a direct consequence of reduced microbial decomposing activity, is like kindling. It’s dry, light and burns quite readily. It adds to the fuel, as well as makes it more likely that catastrophically sized forest fires might start.”