Reducing tillage in cultivated fields increases earthworm functional diversity

Alternative cropping systems such as conservation agriculture have been implemented to limit the harmful effects of intensive conventional cropping systems. Conservation agriculture is known to modify the structural diversity of earthworm communities, but no data have been reported so far on their functional diversity. Structural and functional indices of community were used to study the effects of different soil tillage intensity on earthworm diversity in arable soils.Field data were collected in four agricultural trials across France representing different soiland climatic conditions. Three types of soil tillage were assessed: plowing, superficial tillage and direct seeding. Earthworm abundance, species richness and ecomorphological group abundance were investigated. Seven functional traits, i.e. body length, body mass/length ratio, epithelium type, cocoon diameter, typhlosolis type, carbon preferences and vertical distribution, were selected according to their hypothesized link with mechanisms of tillage impact. Functional diversity indices were then computed. Soil tillage intensity decreased functional diversity and modified the functional trait profile within the earthworm community whereas neither structural diversity (species number) nor abundance changed with tillage intensity. Differences between plowing and direct seeding were significant in each trial, and superficial tillage often showed intermediate trait values. Regarding ecomorphological groups, anecic abundance was positively influenced by a decrease in soil tillage, contrary to epigeic and endogeic earthworms that showed no response. Tillage acts as an environmental filter, and decreasing its intensity caused a lesser convergence of traits and thus higher functional trait diversity. We demonstrated that a trait-based approach better permitted comparisons of community responses across sites than species number or abundance.