Changes in earthworm populations during conversion from conventional to organic farming

The development of earthworms was investigated from 2001 to 2008 on the 176 ha arable fields of Ritzerau Farm (northern Germany) converted 2001–2004 from conventional to organic farming. Earthworms were recorded six times per year on a grid of 85 sampling points, whereas pH was determined in 2001 and 2005 and carbon content in 2001. Weak correlations between earthworm numbers and soil pH or organic matter of soil were found for few species, except for Lumbricus castaneus which was more abundant at pH > 6.5. Lumbricus rubellus benefited most from grain cultivation, Lumbricus terrestris from organic management. Aporrectodea caliginosa and Aporrectodea rosea were mainly related to rainfall and more abundant under 700–900 mm rain per year. Compensation of declined abundance after a period of heavy dryness lasted 0.5–1 year. The enhancement after conversion lasted 2–4 years and numbers increased from 0.2 to a maximum of 4.5 ind/m2.