Most earthworms may be tiny, but a new study suggests their impact on the climate could be mighty.
Researchers had long assumed the creepy crawlers help store carbon in soils by consuming fallen leaves and other decaying plant matter, which they deposit in soil in their cast, or droppings. But newer studies suggest the worms may actually increase soils' output of two key greenhouse gases, carbon dioxide and nitrous oxide.
A new meta-analysis, published Sunday in the journal Nature Climate Change, found that the presence of earthworms appears to increase soils' output of CO2 by 33 percent and of nitrous oxide by 42 percent.
A bowl of exotic European earthworms species federal scientists recently found in National Wildlife Refuge sites throughout the Upper Midwest.
Credit: Lindsey Shartell/U.S. Fish and Wildlife Service
"We've known for only a couple of years that they can really increase nitrous oxide emissions, but it was not really clear how much," said study co-author Jan-Willem van Groenigen of Wageningen University in the Netherlands, whose work is based on a review of 57 previously published analyses.
As for claims that the worms help store carbon in soils, van Groenigen said that traditional argument has always seemed suspect to him.
"It's strange to claim on the one hand that earthworms are good for soil fertility by decomposing organic matter in soil, and on the other hand that they increase organic matter in soil," he said.
Van Groenigen was quick to note that the new study is not the last word on the earthworm question. Scientists, who often refer to earthworms as "ecosystem engineers" in recognition of the role they play in churning soil and improving its drainage, have been slow to understand the little organisms' role in the carbon cycle.
The current crop of studies suggest that earthworms that live in the upper layer of soil eat leaves, crop residues and other plant matter. When they excrete the remains, their droppings provide a feast for soil microbes that emit nitrous oxide. Their burrowing and churning also mixes plant matter into the dirt, where it decays and produces carbon dioxide.
But most of those studies were conducted over short periods of time, perhaps a few weeks, and many of them took place in laboratory conditions rather than in real-life fields and forests, van Groenigen said, which may distort the results. There are hints that over longer periods, CO2 emissions from worm-ridden soils may decrease while nitrous oxide emissions may rise.
Still, experts who were not involved in the new study said it was a solid analysis — though limited by the narrow scope of existing research on earthworms' role in the carbon and nitrogen cycles.
Earthworms "probably don't get enough attention," said Peter Groffman, an ecosystem ecologist at the Cary Institute of Ecosystem Studies. "They play a super-important role in climate change. There's all this carbon in soil, and there's a big flux of greenhouse gases from soil to the atmosphere. And those fluxes are very fundamentally affected by things like earthworms, millipedes and mites that live in the soil."
Though earthworms are a familiar and friendly presence in many gardens, fields and yards, many of the species found in the northeastern U.S., where Groffman works, are invasive foreign species. The region's native earthworms were wiped out thousands of years ago, during the last ice age, as glaciers formed, advanced and retreated in the Northeast, leaving a landscape devoid of the soil worms need to thrive.
Over time, that soil returned, and so did earthworms: European species began hitching rides to North America hundreds of years ago with the first colonists. More recently, Asian species have made the journey.
Groffman, who has studied invading earthworms' transformation of Northeast forests since the 1990s, said his own research bears out the new study's main conclusions.
"Their point is right," he said. "Earthworms have really big effects on carbon and nitrogen cycling in soil, and potentially big effects on greenhouse gases, and the distribution of earthworms is changing."
One big question, he said, is how a landscape invaded by earthworms will eventually adapt to their presence, and whether that will increase or decrease emissions of heat-trapping gases.
Josef Gorres, a soil scientist at the University of Vermont, said his own long-term field studies suggest that, at least in the Northeast, native plants grow better -- and presumably store more carbon -- when earthworms haven't invaded a landscape.
"There are sugar maple forests we've studied, one with this very strange Asian earthworm that has really reduced plant life on the forest floor to nothing," he said. "What the Dutch workers are saying in their paper is right on, if you look at the literature that is available."