Local land use effects and regional environmental limits on earthworm communities in Finnish arable landscapes

In many arable soils, earthworms form the key component of the soil animal community and greatly contribute to soil quality. Our goal was to identify variables that can explain the variation of earthworm communities across Finnish arable fields by focusing on both regional aspects, such as climate and inherent soil properties, and local aspects, such as field management practices and field margins. We sampled the earthworms during the autumn of 2004 and 2005 at 53 sites of cultivated fields and margins, distributed to 11 localities ranging from the southern parts of Finland to the Arctic Circle. Simultaneously, information was collected on topsoil properties, cultivation history, and climatic factors. We found that soil type was by far the best regional variable to explain the variation in total earthworm density, the density being highest in medium coarse soils, i.e., silts and very fine sands. However, soil type had a significant interaction with soil C:N ratio. After accounting for the effect of soil type, no clear geographical trends remained in the regional variation, except that coastal areas with thin snow cover and deep frost had lower earthworm density than inland areas with deep snow cover and shallow frost. Of the local variables, tillage frequency, grazing, and field margins were the most influential. Frequent tillage led to earthworm communities heavily dominated by endogeic species, while grazed pastures had more than three times the earthworm biomass in comparison to non-grazed fields. Field margins harbored over twice the density and almost double the number of species in comparison to the adjacent cultivated fields, and eight of the nine species had wider regional distribution in the margins than in the fields. Inside the fields, species richness tended to decrease gradually with increasing distance to the margin. Our results suggest that, while soil texture and climatic factors set the general limits to earthworm communities in Finnish arable soils, field management practices can override these limits at the local scale. Cultivation in general simplifies the earthworm community, and this is particularly clear in the fields that are frequently tilled, whereas pastures and field margins seem to effectively preserve the biomass and diversity of earthworms.