Milled cereal straw accelerates earthworm (Lumbricus terrestris) growth more than selected organic amendments

- Farmyard manure increased earthworm populations more than straw in the field.
- Milled straw increased earthworm growth more than milled manures in microcosms.
- Earthworm growth rates are positively correlated with amendment calorific value.
- Straw only benefits earthworm growth when milled.
- Incorporating milled straw in arable soils may increase earthworm populations.

Earthworms benefit agriculture by providing several ecosystem services. Therefore, strategies to increase earthworm abundance and activity in agricultural soils should be identified, and encouraged. Lumbricus terrestris earthworms primarily feed on organic inputs to soils but it is not known which organic amendments are the most effective for increasing earthworm populations. We conducted earthworm surveys in the field and carried out experiments in single-earthworm microcosms to determine the optimum food source for increasing earthworm biomass using a selection of crop residues and organic wastes available to agriculture. We found that although farmyard manure increased earthworm populations more than cereal straw in the field, straw increased earthworm biomass more than manures when milled and applied to microcosms. Earthworm growth rates were positively correlated with the calorific value of the amendment and straw had a much higher calorific value than farmyard manure, greenwaste compost, or anaerobic digestate. Reducing the particle size of straw by milling to <3 mm made the energy in the straw more accessible to earthworms. The benefits and barriers to applying milled straw to arable soils in the field are discussed.