Our objective was to determine the effect of tillage system, crop rotation, and N fertilization on soil organic carbon (SOC) storage in the 0- to 90-cm profile in a long-term (20-yr) experiment established in 1986 on a rainfed Mediterranean Vertisol in southern Spain. The treatments studied were: conventional tillage (CT) vs. no-tillage (NT); five crop rotations: wheat (Triticum aestivum L.)-chickpea (Cicer arietinum L.) (WC), wheat-sunflower (Helianthus annuus L.) (WS), wheat-bare fallow (WF), wheat-faba-bean (Vicia faba L.) (WFB), and continuous wheat (WW); and N fertilizer applied at four rates (0, 50, 100, and 150 kg N ha(-1)). The SOC content of soil samples was determined in 1995,1997,2000,2003, and 2006 for four different soil layers (0-15, 15-30, 30-60, and 60-90 cm). The application of N fertilizer did not influence SOC sequestration. The other treatments showed a gradual increase of total SOC content over time, although there were no differences between some consecutive years. The SOC accumulation was higher for 30- to 60- and 60- to 90-cm depths than other depths due to characteristic cracks of Vertisol. Over the 20 yr of the study, WW and WFB sequestered 21 and 15 Mg C ha(-1) more under NT than under CT, respectively. The other crop rotations did not show any difference in C sequestration between NT and CT. Under CT, WS sequestered more SOC than other rotations, while under NT, WW and WFB sequestered more SOC. In general, the crop rotation intensification and NT had a positive effect over time on SOC sequestration in this rainfed Mediterranean Vertisol.
