Abundance and stability of belowground earthworm casts influenced by tillage intensity and depth N. Bottinelli a,b, *, V. Hallaire a,b , S. Menasseri-Aubry a,b , C. Le Guillou a,b , D. Cluzeau c a INRA, UMR 1069 Sol Agro et hydrosyste `me Spatialisation, F-35000 Rennes, France b Agrocampus Ouest, UMR 1069 Sol Agro et hydrosyste `me Spatialisation, F-35000 Rennes, France c Universite ´ Rennes 1, UMR CNRS ECOBIO, IFR CAREN, Station Biologique, 35380 Paimpont, France 1. Introduction With the intensification of agriculture over the last five decades, deterioration in soil structure and increases in soil erosion have emerged as major issues. Therefore, new cultural practices such as conservation tillage (i.e., no-tillage or surface tillage) have been developed. Because soil aggregates are less disrupted and soil organic matter (SOM) is concentrated near the surface in this system, soil structural stability increases (Six et al., 1999). Soil aggregate stability refers to the capacity of aggregates to conserve their structural organization during water disturbance events. A decline in structural stability increases aggregate slaking and induces the clogging of soil pore spaces. Consequently, soil aggregate stability in the topsoil is closely linked to runoff rates and mean annual soil losses at the landscape scale (Barthe `s and Roose, 2002). In conservation tillage systems, many reports have documented higher earthworm numbers and biomasses compared to those observed under conventional tillage systems (reviewed in Chan, 2001). Through their casting activities, earthworms significantly affect soil aggregate processing within the top layers of soil (Lavelle and Spain, 2001). The accumulation of casts below the soil surface results in modification of soil matrix properties with the formation of granular aggregates (Jongmans et al., 2001) and ‘‘mammilated vughs’’ (Vandenbygaart et al., 2000). Many workers agree that earthworm casts, when aged or dried, contain more stable micro- aggregates than the surrounding soil (Shipitalo and Protz, 1988; Marinissen and Dexter, 1990). This increase is usually explained by SOM and especially microbial polysaccharides (Zhang and Schrader, 1993; Flegel et al., 1998; Oyedele et al., 2006). These act as binding agents (Tisdall and Oades, 1982) and/or increase the water repellency of aggregates and the extent of slaking (Jouquet et al., 2008). There is a surprising dearth of information on how tillage intensity influences the capacity of earthworms to stabilize soil aggregates. We hypothesized that tillage intensity, by affecting the Soil & Tillage Research 106 (2010) 263–267 ARTICLE INFO Article history: Received 25 June 2009 Received in revised form 31 August 2009 Accepted 17 November 2009 Keywords: Earthworm activity Conservation tillage Water repellency Carbohydrates ABSTRACT 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. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author at: INRA, UMR 1069 Sol Agro et hydrosyste `me Spatialisation, 65 Route de Saint Brieuc CS 84215, 35042 Rennes Cedex, France. Tel.: +33 2 23 48 54 26; fax: +33 2 23 48 54 30. E-mail address: nicolas.bottinelli@rennes.inra.fr (N. Bottinelli). Contents lists available at ScienceDirect Soil & Tillage Research journal homepage: www.elsevier.com/locate/still 0167-1987/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2009.11.005