Tillage effects on soil aggregation and soil organic carbon profile distribution under Mediterranean semi-arid conditions

In rainfed semi-arid agroecosystems, soil organic carbon (SOC) may increase with the adoption of alternative tillage systems (e.g. no-tillage, NT). This study evaluated the effect of two tillage systems (conventional tillage, CT vs. NT) on total SOC content, SOC concentration, water stable aggregate-size distribution and aggregate carbon concentration from 0 to 40 cm soil depth. Three tillage experiments were chosen, all located in northeast Spain and using contrasting tillage types but with different lengths of time since their establishment (20, 17, and 1-yr). In the two fields with mouldboard ploughing as CT, NT sequestered more SOC in the 0-5 cm layer compared with CT. However, despite there being no significant differences, SOC tended to accumulate under CT compared with NT in the 20-30 and 30-40 cm depths in the AG-17 field with 25-50% higher SOC content in CT compared with NT. Greater amounts of large and small macroaggregates under NT compared with CT were measured at 0-5 cm depth in AG-17 and at 5-10 cm in both AG-1 and AG-17. Differences in macroaggregate C concentration between tillage treatments were only found in the AG-17 field at the soil surface with 19.5 and 11.6 g C/kg macroaggregates in NT and CT, respectively. After 17 yr of experiment, CT with mouldboard ploughing resulted in a greater total SOC concentration and macroaggregate C concentration below 20 cm depth, but similar macroaggregate content compared with NT. This study emphasizes the need for adopting whole-soil profile approaches when studying the suitability of NT versus CT for SOC sequestration and CO2 offsetting.