A team of researchers has been studying the effects of adding extra leaf litter and other organic matter to the soil in Harvard Forest for more than 25 years. The original purpose was to test the limits of how much carbon the soil could store, because scientists believed that the soil had an infinite capacity to hold carbon. But the results so far have been completely unexpected.
The data appear to show that, as temperatures rise, the organic matter in forests breaks down more quickly, accelerating the release of carbon dioxide into the atmosphere. While the study intended to explore a promising avenue to mitigate global warming, the results seem to indicate that additional organic material in the soil may exacerbate it.
The leaf litter manipulation study, which is still going on at Harvard Forest and other sites around the world, involves doubling the leaf litter in 3x3-meter plots for 20 years. “With climate change, one of the current projections is that, as temperatures warm, we will have longer growing seasons, larger trees, a more productive forest, and more plant material going into the soil,” said Myrna Simpson of the University of Toronto. Researchers also installed root barriers to prevent belowground organic matter from making its way into the soil of the test plots.
The scientists expected to see an increase in carbon in the mineral soil layer, the place where most long-term carbon is stored. “But after 20 years we didn’t see any carbon gain at all,” said Simpson.
Using laboratory techniques that Simpson developed to examine the molecular composition of the soil, the researchers found evidence that the bacteria and fungi in the soil processed the excess organic matter and converted it to carbon dioxide. “We didn’t get any more carbon in the soil because the bacteria and fungi ate that carbon,” she said. “Rather than sequestering carbon in the soil, we saw an increase in soil respiration, which could contribute to global warming even more.”
Though the scientists admitted that their results were a bit depressing, they aren’t giving up. They will continue the study for at least another five years and process data from other sites to see if the results elsewhere are different. Simpson said that the balance between carbon storage and degradation in the soil is a function of temperature, moisture, nutrients, the type of bacteria and fungi living in the soil, and the physical characteristics of the soil, like pH and drainage.
“These characteristics can alter how the processes happen and the rate that they happen, so we’re looking at other forests, because not every forest is the same,” Simpson said. “At some point, the microbes will probably run out of usable substrates and slow down, but it didn’t happen after 20 years. We’ll keep monitoring to see if the trend continues.”