BioEd Online

Earthworms have an unexpected impact on forests, researchers have found: they can change soil chemistry, and so are expected to affect how quickly carbon dioxide is emitted from a forest floor.

The result is particularly interesting to those studying North American forests, because earthworms have historically been rare in this part of the world. Only two genera of earthworms are native to North America, and most of the worms now inching their way across the continent were introduced by European settlers in the 1800s. In some areas of northern Minnesota, for example, earthworms are spreading through the region for the first time since the last glaciation.

Timothy Filley, a biogeochemist at Purdue University in West Lafayette, Indiana, has been looking closely at the soil underneath worms in patches of forest, to see what changes are being affected as the invasion continues.

Filley looked at leaf litter from tulip poplar (Liriodendron tulipifera) trees in forest plots at the Smithsonian Environmental Research Center in Maryland, both with and without earthworms, and sampled the soil over a period of 6 months. He found that the worms chewed up certain leaf litter preferentially, and the breakdown of different types of organic matter then had a knock-on effect underground. "The soil below has been influenced by the earthworms, chemically," says Filley.

All chewed up

As leaf litter starts to break down and become soil, some of the material will be decomposed by fungi or bacteria and converted into CO₂. Other forms of carbon are more inclined to stay locked up in soils, including complex biopolymers that are hard to break down (such as the woody lignin that makes up stems, or the cutin in leaves). Other bits of carbon evade degradation by physically gathering in a lump of matter that is hard to break down, or sticking chemically to the surface of minerals.

Worms, it seems, prefer to munch on the softer, lignin-poor material found in leaves, Filley found. This means that areas hosting worms have soils poor in cutin and rich in lignin.

Changing the balance of lignin to cutin, and possibly the balance between carbon tied up in different ways in the soil, will probably have an impact on how much CO₂ a patch of forest releases and how quickly. Recent studies have shown that biopolymer components such as cutin in soft leaf tissue might be the form of carbon that is stored longest in soils, for example.

Global change

Filley hasn't measured CO₂ release from his forest patches, so he can't say yet how these chemical changes are affecting this - or by how much. But clearly earthworm spread needs to be taken into account when thinking of carbon sinks, he says. "This global change could alter the amount of carbon in soil, or the amount that is leached out into the soil," says Filley. "What we think we know about forests will change."

The questions Filley is looking into are important, says geochemist Anthony Aufdenkampe of the Stroud Water research Center in Avondale, Pennsylvania: "Earthworms are now rapidly expanding due to human activities into northern ecosystems," he says. These ecosystems hold large soil carbon pools that, if released, could have big effects on atmospheric CO₂, he adds.

Filley's work has been submitted to JGR:Biogeosciences and was presented at the American Chemical Society national meeting in Boston, Massachusetts, last Wednesday.