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Changes in the agricultural management can potentially increase the accumulation rate of soil organic carbon (SOC), thereby sequestering CO₂ from the atmosphere. In a long-term experiment (1992-2008) we examined the effects of various tillage intensities: no-tillage (NT), minimum tillage with chisel plow (MT), and conventional tillage with mouldboard plow (CT), on the topsoil profile distribution (0-30cm) of SOC, on a semi-arid loamy soil from Central Spain. The crop sequence established was cheap pea (Cicer arietinun L.) cv. Inmaculada/barley (Hordeum vulgare L.) cv. Volley. Soil organic carbon in the various tillage treatments was expressed on a content bases and the equivalent soil mass approach. Measurements made at the end of 17 years showed that in the 0-30cm depth, stocks of SOC had increased under NT compared with MT and CT. Most dramatic changes occurred within the 0-5cm layer where plots under NT had 5.8 and 7.6Mgha⁻¹ more SOC than under MT or CT respectively. No-tillage plots, however, exhibited strong vertical gradients of SOC with concentrations decreasing from 0-5 to 20-30cm. Stratification ratios of SOC in 1992 showed no significant differences between tillage systems. On the contrary, from 1993 onwards all stratification ratios were significantly higher in NT than in the other two tillage systems. In addition, since 2003 stratification ratios of SOC obtained under NT were systematically >2 and more than 2-fold those obtained under MT and CT. Stratification ratios >2 are uncommon under degraded conditions and could suggest that NT management system may have the most benefits to soil quality in semi arid regions with low native soil organic matter.
