Earthworms Increase Soils’ Greenhouse Gas Emissions

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 ofWageningen 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.

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