Long-term impact of conservation tillage on stratification ratio of soil organic carbon and loss of total and active CaCO 3 F. Moreno a, * , J.M. Murillo a , F. Pelegrı ´n b , I.F. Giro ´n a a Instituto de Recursos Naturales y Agrobiologı ´a de Sevilla (IRNAS-CSIC), P.O. Box 1052, 41080 Seville, Spain b Escuela Universitaria de Ingenierı ´a Te ´cnica Agrı ´cola, University of Seville, Spain Received 3 October 2003; received in revised form 26 November 2004; accepted 13 December 2004 Abstract Under semi-arid conditions, the properties of many soils are influenced by the presence of organic matter and calcium carbonate (CaCO 3 ). However, the influence of different tillage systems on the development of these properties has scarcely been studied under semi-arid Mediterranean conditions. We studied the effect of long-term conservation tillage (CT) and traditional tillage (TT) on the stratification ratio of soil organic carbon and on CaCO 3 content. The study was conducted in a wheat (Triticum aestivum L.)–sunflower (Helianthus annuus L.) crop rotation established in 1991 under rainfed conditions in Southwestern Spain. As is traditional in this area, wheat was fertilised, but sunflower was not. Conservation tillage was characterised by reduced number of tillage operations and leaving crop residues on the soil surface, while TT was with mouldboard ploughing. Stratification ratio of soil organic C was calculated from C contents in the 0–5 and 5–10 cm soil layers divided by that in the 25– 40 cm. Stratification ratio of soil organic C under the CT (>2) was significantly greater than under TT (<2); values >2 indicating better soil quality. Our results show a loss of CaCO 3 under both tillage systems. However, the loss of CaCO 3 was significantly higher under TT than under CT. Also, P and K accumulated in the soil surface and stratification ratio for both nutrients was greater in CT than in TT. # 2005 Elsevier B.V. All rights reserved. Keywords: Tillage systems; Organic matter; Soil nutrients and carbonates; Semi-arid climate 1. Introduction The efficiency of conservation tillage (CT) for reducing soil erosion and improving water storage is universally recognised. This is particularly important in arid and semi-arid zones, where water is the limiting factor for crop development under rainfed conditions. In these areas, management of crop residues is also of prime importance for obtaining sustainable crop production (Du Preez et al., 2001). Greater retention of water in the soil profile under conservation tillage has been reported for the Mediterranean conditions in Southern Spain (Pelegrı ´n et al., 1990; Moreno et al., 1997). The contents of www.elsevier.com/locate/still Soil & Tillage Research 85 (2006) 86–93 * Corresponding author. Tel.: +34 95 4624711; fax: +34 95 4624002. E-mail address: fmoreno@irnase.csic.es (F. Moreno). 0167-1987/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2004.12.001