The behaviour of earthworms, their role in organic matter incorporation into the soil, and the influence of aridity in such processes in arid and semiarid regions have scarcely been studied. In this study, physico-chemical analyses of the casts and the surrounding no-till agricultural soils of three experimental sites representing an aridity gradient in Navarre (NW Spain) were done. The casts were formed by the activity of the only anecic species, Scherotheca gigas (Dugès, 1828), ubiquitous in no-till soils in this region. We observed a significant depletion of clay and higher concentration of total organic C and labile C in the form of particulate organic matter (POM) in the casts as compared to the surrounding soil, suggesting selective ingestion of soil by S. gigas. This, together with the observation of increased concentration in POM with increasing aridity, suggests a major role of this species in the observed progressive gains of organic C stocks in no-till soils in the region. 1. Introduction It is well known that the abundance of earthworms in the soil depends on soil properties and on climate. Most of the studies related to the earthworms behaviour are from the tropical and subhumid temperate soils. Their behaviour in more limiting conditions, such as agricultural soils in semiarid or arid regions, has been less studied [1]. Some particularities of earthworms such as large body size, slow growth rate, ability to feed on soils poor in organic matter, occupying deeper soil profile, have been described in Mediterranean soils [2]. However, our knowledge is still limited. For instance, in the Iberian Peninsula, much less information on earthworm occurrence and activity exists for the Mediterranean domain than for the more humid NW area [3]. The reality is that in the majority of agricultural soils with Mediterranean type of climate, like in the Ebro Valley of Spain, located in the Mediterranean-Iberian-Levantine biogeographical superprovince [2], earthworms are scarce and often absent when tillage is intense [4]. It is also well known that, in general, when tillage is reduced earthworm activity increases. No-till and other methods of conservation tillage such as chisel plowing can thus increase earthworm populations [5]. Decreased tillage disturbances particularly benefit anecic species, which can move in the same burrow between deeper soil layers and the soil surface in search of food. This is so because when tillage destroys these burrows, some earthworms may not have the energy reserve needed to form a new burrow to their food source [6].
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