The dead trees and fallen leaves near Chernobyl aren’t decaying

457 The dead trees and fallen leaves near Chernobyl aren’t decaying
Radioactivity warning sign on a hill at the east end of Red Forest / Timm Suess via Wikimedia
It’s been nearly 30 years since the catastrophe at Chernobyl, and as the cleanup grinds on, the far-reaching effects continue to be documented. Birds with smaller brains, increasing spiders, decreasing butterflies, all these and more have been reported from the areas surrounding Chernobyl. One group you don’t hear very much about are the decomposers -- those bugs, microbes, fungi, and slime molds who nourish themselves by consuming the remains of dead organisms. Without these recyclers, carbon, nitrogen, and other elements essential to life would be locked in plant corpses. 
The effects of radioactive contamination on the decay of plant material remains unknown… until now. Scientists examining the forests around Chernobyl have found that radioactive contamination has reduced the rate of litter mass loss. The dead leaves on the forest floor, along with the dead pine trees in the infamous Red Forest, don’t seem to be decaying -- even a couple decades after the incident. 
“Apart from a few ants, the dead tree trunks were largely unscathed when we first encountered them,” study researcher Timothy Mousseau of the University of South Carolina tells Smithsonian. “It was striking, given that in the forests where I live, a fallen tree is mostly sawdust after a decade of lying on the ground.” 
Mousseau and an international team led by Anders Pape Møller from Université Paris-Sud decided to investigate the accumulation of litter, which was two to three times thicker in the areas where radiation poisoning was most intense. They predicted that decomposing rate would be reduced in the most contaminated sites due to the absence or reduced densities of soil invertebrates and microorganisms. 
To test this, the team filled 572 small mesh bags with dry leaves from four species of trees -- oak, maple, birch, pine -- collected from uncontaminated sites. They deposited the bags in the leaf litter layer at 20 forest sites around Chernobyl in September 2007; these sites varied a ton in background radiation, some by more than a factor of 2,600. All the bags were retrieved about a year later in June 2008, dried, and weighed to estimate litter mass loss. 
They found that the litter loss was 40 percent lower in the most contaminated sites; that is, there was a lot more litter left over in those bags than in the bags placed in normal Ukraine radiation levels. (In those areas with no contamination, 70 to 90 percent of the litter in the bags were gone.) The thickness of the forest floor increased with the level of radiation and decreased with loss of mass from all litter bags. Simply put, the more lingering radiation, the fewer the decomposers, the more dried leaves left in the bags. 
Additionally, a quarter of the bags deposited were made of a fine mesh (like pantyhose) that prevented access by soil invertebrates. By comparing the normal mesh bags with the fine mesh bags, they found that litter loss was slightly greater in the presence of large soil invertebrates than in their absence. So while insects played some role in breaking down the leaves, microbes and fungi played a much more important role. 
“The gist of our results was that the radiation inhibited microbial decomposition of the leaf litter on the top layer of the soil,” Mousseau explains. The accumulation of litter means that nutrients aren’t being efficiently returned to the soil, he adds, which could explain why tree are growing at a slower rate around Chernobyl. 
The work was published in Oecologia this month. 
Image: Timm Suess via Wikimedia


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