Earthworms
Earthworms break-up, incorporate and mix organic materials into the soil. This is essentially a form of 'biological cultivation' which improves the structure of the soil, via the formation of aggregates and connected pore networks.
The resistance of the soil to penetration is reduced by the presence of earthworm burrows which makes it easier for plant roots to establish and for water to infiltrate. The larger, deeper root network that results from earthworm burrows increases the access that the plant has to water and nutrients.
There are three broad types of earthworm each with contrasting lifestyles:
Surface dwelling earthworms
Surface-dwellers, or epigeic earthworms, prevail in organic matter such as leaf litter and animal dung. They consume these materials and the microbes which grow on them. They break up the materials they consume into smaller fragments.
Vertical burrowing earthworms
Vertical burrowers, or anecic earthworms, form persistent vertical burrows in the soil, sometimes to depths in excess of several metres. Vertical burrowers feed on organic materials on the soil surface, pulling them down into their burrows before they consume them. They produce casts on the soil surface.
Horizontal burrowing earthworms
Horizontal burrowers, or endogeic earthworms, live in the bulk soil and search for food by burrowing in all directions. These burrows are often back-filled with cast material and are less persistent than those formed by anecic earthworms.
The work that earthworms do by ingesting soil, creating burrows, and improving the soil structure requires energy. Therefore if you want to get worms to do work for you, you have to feed them and protect their habitat.
Earthworm activity can be increased by
1. Feeding the soil with regular applications of organic manures and crop residues.
2. Minimising the length of time that the soil is left bare by planting cover crops.
3. Reduce tillage as much as possible.
During my PhD at the University of Reading I undertook experiments to determine if earthworms affect the mobility, speciation and bioavailability of metals in contaminated soils.
Results indicated that earthworms increase metal mobility, change the speciation to a more available form and increase bioavailability.
The mechanism for this effect is the degradation of organic matter due to passage through the earthworm gut and subsequent release of organically bound metals into solution.
When earthworms were inoculated into soil that had been remediated with biochar and/or compost they did not re-mobilise the sequestered metals.
Mobilisation was the greatest in soils with low (<2%) soil organic matter and non-contaminated soils. Therefore, our findings also highlight a promising phenomenon that, if harnessed, may help to alleviate micronutrient deficiencies in degraded soils.
Papers on Earthworms:
Garau, M., Garau, G., Sizmur, T., Coole, S., Castaldi, P. and Pinna, M.V., 2023. Biochar and Eisenia fetida (Savigny) promote sorghum growth and the immobilization of potentially toxic elements in contaminated soils. Applied Soil Ecology, 182, p.104697.
Garau, M., Sizmur, T., Coole, S., Castaldi, P. and Garau, G., 2022. Impact of Eisenia fetida earthworms and biochar on potentially toxic element mobility and health of a contaminated soil. Science of the Total Environment, 806, p.151255.
Omosigho, H.O., Swart, E., Sizmur, T.P., Spurgeon, D.J., Svendsen, C. and Shaw, L.J., 2022. Assessing the efficacy of antibiotic treatment to produce earthworms with a suppressed microbiome. European Journal of Soil Biology, 108, p.103366.
Duddigan, S., Fraser, T., Green, I., Diaz, A., Sizmur, T. and Tibbett, M., 2021. Plant, soil and faunal responses to a contrived pH gradient. Plant and Soil, 462(1), pp.505-524.
Kiss, T.B., Chen, X., Ponting, J., Sizmur, T. and Hodson, M.E., 2021. Dual stresses of flooding and agricultural land use reduce earthworm populations more than the individual stressors. Science of The Total Environment, 754, p.142102.
Duddigan, S., Gil-Martínez, M., Fraser, T., Green, I., Diaz, A., Sizmur, T., Pawlett, M., Raulund-Rasmussen, K. and Tibbett, M., 2020. Evaluating heathland restoration belowground using different quality indices of soil chemical and biological properties. Agronomy, 10(8), p.1140.
Sizmur, T. and Richardson, J., 2020. Earthworms accelerate the biogeochemical cycling of potentially toxic elements: Results of a meta-analysis. Soil Biology and Biochemistry, p.107865.
Richardson, J.B., Görres, J.H., and Sizmur, T. 2020 Synthesis of earthworm trace metal uptake and bioaccumulation data: Role of soil concentration, earthworm ecophysiology, and experimental design. Environmental Pollution 262, 114126.
Tibbett, M., Gil, M., Fraser, T., Green, I., Duddigan, S., De Oliveira, V., Raulund-Rasmussen, K., Sizmur, T. and Diaz, A. 2019 Experimental acidification of pasture: effects of long-term pH adjustment on soil biodiversity, fertility and function in comparison to heathland and acidic grassland. Catena. 180, 401-415
Sizmur, T., Martin, E., Wagner, K., Parmenteir, E., Watts, C. and Whitmore, A.P. (2017) Milled cereal straw accelerates earthworm (Lumbricus terrestris) growth more than selected organic amendments. Applied Soil Ecology. 113, 166-177.
Hodson, M.E., Black, S., Brinzac, L., Carpenter, D., Lambkin, D.C., Mosselmans, J.F.W., Palumbo-Roe, B. Schofield, P.F. Sizmur, T. and Versteegh, E.A.A. 2014 Biology as an agent of chemical and mineralogical change in soil. Procedia Earth and Planetary Science 10 114-117.
Sizmur, T., Wingate, J., Hutchings, T. and Hodson, M.E. 2011 Lumbricus terrestris L. does not impact on the remediation efficiency of compost and biochar amendments. Pedobiologia. 54S, S211-216.
Sizmur, T., Watts, M., Brown, G.D., Palumbo-Roe, B. and Hodson, M.E. 2011 Impact of gut passage and mucus secretion by Lumbricus terrestris on mobility and speciation of As in contaminated soil. Journal of Hazardous Materials. 197, 169-175. (Awarded SETAC Europe/Eurofins Best Publication Prize in Chemical Analysis and Environmental Monitoring)
Sizmur, T., Palumbo-Roe, B. and Hodson, M.E. 2011 Impact of earthworms on trace element solubility in contaminated mine soils amended with green waste compost. Environmental Pollution. 159, 1852-1860.
Sizmur, T., Tilston, E.L., Charnock, J., Palumbo-Roe, B. Watts, M. and Hodson, M.E. 2011 Impacts of epigeic, anecic and endogeic earthworms on metal and metalloid mobility and availability. Journal of Environmental Monitoring.13, 266-273. (Artwork on the cover of the journal issue due to manuscript being ‘highly rated’ by reviewers)
Sizmur, T., Palumbo-Roe, B. Watts, M. and Hodson, M.E. 2011 Impact of the earthworm Lumbricus terrestris (L.) on As, Cu, Pb and Zn mobility and speciation in contaminated soils. Environmental Pollution. 159, 742-748.
Gomez-Eyles, J.L., Sizmur, T, Collins, C. and Hodson, M.E. 2011 Effects of biochar and Eisenia fetida on the bioavailability of polycyclic aromatic hydrocarbons and potentially toxic elements. Environmental Pollution. 159, 616-622.
Sizmur, T., Palumbo-Roe, B., Hodson, M.E., 2010. Why does earthworm mucus decrease metal mobility? Integrated Environmental Assessment and Management. 6, 777-779.
Sizmur, T., Hodson, M.E., 2009. Do earthworms impact metal mobility and availability in soil? – A review. Environmental Pollution. 157, 1981-1989. (2nd most downloaded paper from Environmental Pollution April-June 2009)
Sizmur, T., Hodson, M.E., 2008. Impact of Eisenia veneta on As, Cu, Pb, and Zn uptake by ryegrass (Lolium perenne L.). Mineralogical Magazine. 72, 495-499.