Abundance and stability of belowground earthworm casts influenced by tillage intensity and depth

In temperate soils, there has been little study of the abundance and water stability of belowground earthworm casts under different tillage systems and soil depths. The aim of this study was to determine the effects of earthworm activity on soil aggregate stability under various tillage systems. Three tillage treatments were compared (moldboard plowing (MP), surface tillage (ST) and no-tillage (NT)). We present here an original method by which earthworm casts were quantified via image analysis at 2- and 12-cm depths. In addition, soil aggregate stability expressed as the mean weight diameter (MWD) and factors involved in soil aggregate stabilization such as soil organic carbon, hot-water extractable carbohydrate content (HWEC) and water repellency (WR) of aggregates were measured in earthworm casts and bulk soil. In superficial soil layers, the relative cast abundance was small and more abundant under NT than the ST and MP treatments. At 12 cm, the relative cast abundance doubled irrespective of the tillage treatment and tended to be higher with conservation tillage (i.e., NT and ST). Moreover, the MWD increased in casts compared to the bulk soil in this deeper layer. The increase in MWD in casts was explained in part by the increase in soil organic matter (carbon content and HWEC) and the WR. For the two depths, we found a positive relation between the MWD from the bulk soil and casts among the tillage treatments. We conclude from this study that casts located at a 2-cm depth did not participate in soil aggregate stabilization, while at 12 cm the greatest structural stability observed under conservation tillage was explained in part by the production of casts.