LAUSR

Abstract Fresh casts of endogeic earthworms are considered a hotspot of microbial activity that exhibit greater carbon mineralization (Cmin) than the bulk soil. While cast properties depend on the ingested soil material, little is known about how earthworm feeding behavior and digestive processes interact with those properties to determine the Cmin in fresh casts egested by endogeic earthworms. Two laboratory experiments were designed to (i) assess the short-term changes in Cmin of soil and fresh casts after the soil or soil physical size fractions passed through the gut of Aporrectodea caliginosa, a common endogeic species in temperate agroecosystems, and (ii) to determine whether these changes depended on initial properties of the ingested materials. In the first experiment, we determined how Cmin was affected by gut passage (i.e., casts vs. bulk and surrounding soil) and soil type (three sandy-loam soils with variable content of light fraction organic matter (LF): the Courval, St. Amable and Chicot soil series). In the second experiment, we related Cmin in casts to the interactive effect of the gut passage and soil fraction size (Courval soil series only). Six soil treatments were examined: whole soil and five-soil fraction size classes (2000–1000 μm, 1000–500 μm, 500–250 μm, 250–53 μm and