Global Warming Has a New Perpetrator

Posted on: 04 February 2013

Earthworm Implicated as an Agent in Global Warming

The lowly earthworm has been implicated as an agent in global warming, according to new research published in the current issue of Nature Climate Change.  “Earthworms increase emissions of carbon dioxide as well as nitrous oxide, but there are no indications that they affect soil organic carbon stocks in soils,” says Dr Kees Jan van Groenigen, a research fellow at Trinity College Dublin and a co-author of the study. 

The rise in carbon dioxide (CO2) and nitrous oxide (N2O) in the air is believed to be responsible for exacerbating the atmosphere’s natural greenhouse effect, and creating climate change in the form of global warming. Soil organic carbon (SOC) – the carbon stored within soil – is part of the organic matter, which is made up of plant and animal materials in various stages of decay. 

“Soil is a major player in regulating global warming,” explains Dr Jan Willem van Groenigen of Wageningen University, a co-author of the study. “There is more carbon in soil organic carbon than in atmospheric CO2, and agricultural soils are by far the largest source of N2O emissions, because these soils receive large amounts of nitrogen fertilisation. Small changes in soil greenhouse gas dynamics can therefore have important repercussions for global warming.”

This is bad news for those battling against climate change, explains Ingrid Lubbers, a PhD candidate in the soil biology group at Wageningen University in the Netherlands and lead author of the study. “Earthworms help us to produce more food through improving soil fertility, but by doing so they also contribute to global warming by increasing greenhouse gas emissions from soils.” 

The study analyses 57 experiments conducted around the world that include information on how earthworms influence the net soil greenhouse gas emissions of carbon dioxide and nitrous oxide, and their influence on SOC stocks. 

Along with colleagues from Trinity College Dublin, the University of California at Davis and the International Center for Tropical Agriculture in Cali, Colombia, Lubbers looked for overall patterns in the data. It was not easy to see initially what the findings might be.

“The claims on the effects of earthworms on soil greenhouse gas emissions in the literature varied widely,” explains Dr van Groenigen. 

“For nitrous oxide, the story appeared to be relatively clear: our own experimental studies, as well as those of other research groups, strongly suggested that earthworms increase emissions of this gas. For carbon, the publication record showed conflicting views. Some studies reported increases in carbon dioxide emissions, whereas others suggested that earthworms might stimulate carbon sequestration – or capture – in soil organic carbon, thereby neutralising or even overcompensating for the effects on nitrous oxide.”

Earthworm

After analysing the data, a clear pattern emerged however. Earthworms increase emissions of carbon dioxide by 33 per cent and nitrous oxide by 42 per cent. 

“This is important information for our fight against global warming,” says Dr van Groenigen. 

“It is rather unfortunate that the presence of earthworms, which we try to stimulate in sustainable agriculture due to their positive effect on soil fertility, at the same time has an unwanted effect on greenhouse gas emissions.”   

Earthworms probably increase greenhouse gas emissions in a number of ways. The worms mix organic plant residue in the soil, which may increase decomposition and CO2 emissions. The earthworm’s gut also acts as a microbial incubator, boosting the activity of N2O-producing microbes. Finally, by burrowing through the soil, earthworms make it easier for gasses in the soil to flow to the atmosphere. 

The research team found more detailed patterns. 

“Earthworms stimulated N2O emissions most strongly in soils with high SOC contents,” explains Prof Johan Six of the University of California and co-author of the study. “This suggests that earthworm-induced N2O emissions can be a side effect of increased SOC stocks, when the amount of dead plants being incorporated into the soil increases.”

Many questions remain on this topic, however. It is not yet clear to what extent the stimulating effects of earthworms on plant growth can negate earthworm-induced increases in greenhouse gas emissions. “Our literature search also pointed out a large gap in the published studies,” says Lubbers. “We need more experiments that include growing plants, as well as more long-term studies and more field studies before we can decide to what extent global worming leads to global warming.”

Greenhouse Gas Emissions from Soils increased by Earthworms written by Lubbers IM, Van Groenigen KJ, Fonte SJ, Six J, Brussaard L and Van Groenigen JW  appears in Nature Climate Change, a monthly journal dedicated to publishing significant and cutting-edge research on the impact of global climate change and its implications for the economy, policy and the world at large.