Soil carbon sequestration and stratification in a cereal/leguminous crop rotation with three tillage systems in semiarid conditions

The stratification of soil organic matter at different depths is common under conservation tillage and especially under no-tillage. The degree of stratification, or stratification ratio (SR), can be used as an indicator of soil quality because surface organic matter is essential to erosion control, water infiltration, and the conservation of nutrients. In the semiarid regions of the Mediterranean which are devoted to rain-fed crop production, soil has low organic carbon content because of the high mineralization rates of soil organic matter and the lack of crop residue after periods of drought. Twenty-year effects (1985-2006) of tillage systems on soil organic carbon (SOC) stratification were studied in a Vertic Luvisol with a loam texture. SOC was expressed in carbon concentration (Cc) and in equivalent soil mass (esm). The tillage treatments used were conventional tillage (CT), minimum tillage (MT), and no-tillage (NT). These treatments were under winter wheat (Triticum aestivum L.), vetch (Vicia sativa L.) and pea (Pisum sativum L.) rotation (W-VP). Similarly, several stratification ratios (SRs) were assessed as indicators of SOC's time evolution. Before the start of the experiment the crop rotation was cereal-fallow (C-F). In each treatment and replication four soil layers of the same thickness (10cm) were sampled to obtain soil bulk density (BD) and Cc. After 20 years the study revealed that the adoption of a W-VP rotation was at least as important as the shift from CT to NT in the increase of stocked SOC in the soil profile. This last treatment was the tillage system with the highest SOC, whereas no significant differences were encountered between MT and CT. The average SOC was 14% higher in NT than in MT and CT. This trend has been systematically observed practically since 1996 to the present in all treatments. The steady state of SOC sequestration was reached after 11 years of starting the experiment in NT and 12 years in CT and MT. SOC, expressed as Cc and esm, showed the highest stratification in NT, second highest in MT and lowest in CT. In NT, stocked SOC increased from 1996 to 2005 in the top layer but it declined systematically in the bottom layer.