In semiarid areas, traditional, intensive tillage has led to the depletion of soil organic matter, which has resulted in reduced soil fertility. The aim of the present work was to evaluate the effects of different soil management systems, practised over 12 years, on soil organic carbon (SOC), nitrogen (SN) and biologically active organic matter (particulate organic matter [POM]; potentially mineralisable nitrogen [PMN]; microbial biomass [MB]). A Mediterranean Alfisol, located in central Spain, was managed using combinations of conventional tillage (CT), minimum tillage (MT) or no-tillage (NT), plus a cropping background of either continuous wheat (WW) or a fallow/wheat/pea/barley rotation (FW). Soil was sampled at two depths on four occasions during 2006-2007. The results showed the sampling date and the cropping background to significantly affect the SOC (p<0.0057 and p<0.0001 respectively). Tillage practice, however, had no effect on SOC or SN. The C-and N-POM contents were significantly influenced by the date, tillage and rotation. These variables were significantly higher under NT than CT and under WW than FW. The PMN was influenced by date, tillage and rotation, while C-MB was significantly affected by tillage (p< 0.0063), but not by rotation. The NT plots accumulated 66% C-POM, 60% N-POM, 39% PMN and 84% C-MB more than the CT plots. After more than 12 years, the benefits of conservation practices were found in the considered soil properties, mainly under no tillage. In order to obtain a consistent data set to predict soil biological status, it is necessary further study over time.