Soil Biology & Biochemistry 39 e f R

d f ne 2 Earthworms are ecosystem engineers (Jones et al., 1997) Hence, either because individuals move to the best soil patches or because mortality is higher in unfavorable patches this would lead to spatial variations in earthworm soil properties (Phillipson et al., 1976; Poier and Richter, soil humidity due to microtopography or total content in organic matter). The second hypothesis is related to soil characteristics than can be easily changed by earthworms ARTICLE IN PRESS(soil distribution into aggregates size classes, content in the fraction of the total organic matter pool that can be assimilated by earthworms). This second hypothesi