Can Earthworms Harm The Planet?

BioCycle December 2008, Vol. 49, No. 12, p. 53

Clive A. Edwards

IN summer 2007, articles with the common title, “Worms are Killing the Planet,” appeared in a number of journals and newspapers including the United Kingdom’s Daily Telegraph, the Irish Sun and Materials Recycling Week, as well as and other media. As author of more than 140 papers and six books on earthworms, I have been deluged with questions on the validity of these claims that earthworms could be environmentally harmful. It appears that the articles were based on media interviews with Dr. Jim Fredericksonat The Open University in the United Kingdom.

The main theme of the articles was that earthworms, in vermicomposting systems, are producing significant amounts of greenhouse gas emissions. Among the statements by Dr. Frederickson quoted in the articles were, “Getting waste out of landfill and into worm composting systems can actually produce more greenhouse gases than landfill sites;” “Emissions that come from worms can actually be 290 times more potent than carbon dioxide and 20 times more potent than methane;” “Worms used in composting emit nitrous oxide, a greenhouse gas, 296 times more powerful, molecule for molecule, than carbon dioxide;” and “The amount of nitrous oxide emitted by large-scale worm composting is something we should be looking at before we go further down that route.” It appears that the claims made in the articles were based primarily on research reported by Dr. Frederickson and his colleagues, in 2003 and 2005, that had poor experimental designs, inadequate replication and unsatisfactory control of environmental conditions in the vermicomposting beds.

These claims need to be put into a broader context. In the U.S. in 2006, 84 percent of greenhouse gas emissions were carbon dioxide (CO2), 7.8 percent were methane (CH4), and 5.2 percent were nitrous oxide (N2O). Of the N2O emissions, 72 percent came from managing agricultural crop residues, 3.9 percent from animal manures and 0.5 percent from all forms of composting, including vermicomposting (Trends in Greenhouse Gaseous Emissions, 2006).

A great deal of key research over the last 20 years has described earthworm composting systems, all of which depend upon adding wastes regularly to beds in thin layers, and maintaining an aerobic mesophilic condition in the beds, which are usually under cover. In Dr. Frederickson’s 2003 vermicomposting experiments, he compared N2O emissions from heated and unheated outdoor beds containing between 0.5 and 1.0 kg.m2 of earthworms, with those from unheated beds with no earthworms. There was no control of moisture, which is a critical factor. Dr. Frederickson reported no correlations between earthworm densities in the beds and the amounts of N2O emissions produced, stating this might have been due to incomplete sampling or a failure of monitoring that process. This is hardly justification for the sweeping conclusions on N2O emissions that he reached.

In an ancillary laboratory microcosm in-vessel experiment, he concluded that larger populations of earthworms produced more N2O than smaller populations. However, the earthworm populations (1.0 — 5.0 kg.m2) used were much higher than those in the outdoor beds and again, there were no records of the aerobicity or moisture conditions in the chambers, which are critical issues relevant to N2O production.

Dr. Frederickson’s 2005 experiments compared methane and nitrous oxide emissions from an outdoor vermicomposting bed, with those from mechanically-turned composting windrows. This is an unjustifiable comparison, since the gases would inevitably escape from windrows during turning. In these experiments, calculations show mean methane fluxes of 24.10 mg.m2hr-1 from the turned windrows compared with 0.64 mg.m2hr-1 from the vermicomposting system.

Was that measurement taken right after turning or before turning and would that make a difference in what he reported? These data were based on taking gas samples weekly from both composting and vermicomposting systems for three weeks, then every two weeks for the following 10 weeks. He also presented data which represented mean N2O emissions of 1.36 mg.m2hr-1 from the composting windrows, compared with N2O emissions of 5.07 mg.m2hr-1 from the vermicomposting system; however, he stated that these levels were low. Again he did not record moisture contents, NH4 production, or the aerobicity of the systems – all critical issues relating to N2O production. And this takes us to the fundamentals of parameters for vermicomposting that need to be followed.

According to the articles published in the popular press, Dr. Frederickson also stated that “recent research by German scientists has found that worms produced a third of the N2O gases emitted when used for composting.” This is a total misquote! The two publications by the German scientists, Karsten and Drake (1997) and Matthies et al (1999) that he references, assessed nitrous oxide emissions from soils with and without earthworms – not from vermicomposting.

The 1997 publication concluded that N2O emissions by earthworms could account for 33 percent of the N2O emitted by garden soils. However, this conclusion was based on in vivo laboratory measurements with one earthworm in each 38 ml vial without soil. In the 1999 publication, based on adding earthworms to forest soils, N2O emissions from limed soils with earthworms were actually 8 percent lower than the controls without earthworms, hardly a significant conclusion. Clearly, neither of these publications supports Dr. Frederickson’s extravagant claims about N2O emissions from vermicomposting.

In 2007, a group of Dutch scientists (Bertora et al 2007) concluded that in plowed grassland field studies, N2O emissions increased slightly for up to 12 days when earthworms were present, compared with when they were absent. After this transitional phase, the presence of earthworms decreased emissions to 50 percent of that from soils with no earthworms. In other field experiments involving crop residues, the same group of Dutch scientists (Rizhiya et al, 2007) concluded that earthworms had no effects on N2O production from crop residues incorporated into the soil.

Other research from Canada (Speratti and Whalen 2008), which used research on laboratory microcosms, concluded that earthworms did not affect N2O fluxes in soils in any way. Finally, researchers from Mexico (Contreras-Ramos et al (in press)) reported that addition of sewage sludge to soil increased the emission rate of nitrous oxide. However, when earthworms (Eisenia fetida) were added together with the sludge, the nitrous oxide emissions decreased significantly. Additions of E. fetida without sludge had no significant effects on N2O emissions.

Based on these various reports, any possible emissions of greenhouse gases by earthworms from soil or vermicomposting systems is extremely small when compared with the well-documented emissions of nitrous oxide, methane and carbon dioxide from inorganic fertilizer manufacture, landfills, manure heaps, lagoons, crop residues in soils and manure from pigs and cattle in housed systems. While there will be N2O emissions from all these sources, there is no justification for suggesting that environmentally-friendly and energy-efficient systems for producing vermicomposts and composts should be restricted because of their potential to produce greenhouse gases. The global production of nitrogenous greenhouse gases in agriculture should be compared from all sources before vermicomposting is publicly condemned in such a sensational way.

Clive A. Edwards is a Professor at the Soil Ecology Laboratory at The Ohio State University. His colleague, Norman Q. Arancon, who is an Assistant Professor at the University of Hawaii, assisted in the research and writing of this commentary.

Bertora, C., van Vliet, P.C.J., Hummelink, E., Willem van Groenigen, J. (2007) Do earthworms increase N2O emissions in ploughed grassland? Soil Biology & Biochemistry, 39, 632-640.

Contreras-Ramos, S., Dendooven, L, Alvarez-Bernal, D., Montes-Molina, J, Van Cleemput, O. (2008) Emission of nitrous oxide from hydrocarbon contaminated soil amended with waste water sludge and earthworms. Applied Soil Ecology (in press).

Frederickson, J., Howell, G. (2003) Large-scale vermicomposting: emission of nitrous oxide and effects of temperature on earthworm populations (The 7th International Symposium on Earthworm Ecology, Cardiff, Wales 2002). Pedobiologia, 47, (5-6). 724-730.

Hobson, A.M., Frederickson, J., Dise N.B. (2005). CH4 and N2O from mechanically turned windrow and vermicomposting systems following in-vessel pretreatment. Waste Management, 25, 345-352.

Karsten, G.R., Drake, H.L. (1997) Denitrifying bacteria in the earthworm gastrointestinal tract and in vivo emission of nitrous oxide (N2O) by Earthworms. Applied and Environmental Microbiology, 63, 1878-1882.

Matthies, C., Griebhammer, A., Schmittroth, M., Drake, H.L. (1999) Evidence for involvement of gut-associated denitrifying bacteria in emission of nitrous oxide (N2O) by earthworms obtained from garden and forest soils. Applied and Environmental Microbiology, 65, 3599-3604.

Rizhiya, E., Bertora, C., van Vliet, P.C.J., Kuikman, P., Faber, J., Willem van Groenigen, J. (2007) Earthworm activity as a determinant for N2O emission from crop residue. Soil Biology & Biochemistry, 39, 2058-2069.

Speratti, A., Whalen, J. (2008) Carbon dioxide and nitrous oxide fluxes from soil as influenced by anecic and endogeic earthworms. Applied Soil Ecology, 38, 27-33.

U.S. Greenhouse Gas Inventory Reports: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006. Published April 2008, USEPA #430-R-08-005.

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