No-tillage (NT) has been considered an agronomic tool to sequester soil organic carbon (SOC) and match the 4p1000 initiative requirements of conservative soil management. Recently, some doubts have emerged about the NT effect on SOC sequestration, often because observations and experimental data vary widely depending on climate and geographic characteristics. Therefore, a suitable SOC accounting method is needed that considers climate and morphology interactions. In this study, the yearly ratio between SOC in NT and conventional tillage (CT) (RRNT/CT) collected in a previous study for flat (96 samples) and sloping (44 samples) paired sites was used to map the overestimation of SOC sequestration. It was assumed that there would be an overestimation of NT capacity in sloping fields due to lower erosion processes with respect to CT. Towards this aim, Geographical Information System (GIS) techniques and an extensive input database of high spatial resolution maps were used in a simplified procedure to assess the overestimation of SOC stocks due to the sloping conditions and spatial variability of the Aridity Index (AI). Moreover, this also made it possible to quantify the effects of adopting NT practices on soil carbon sequestration compared to CT practices. The method was applied to the arable lands of five Mediterranean countries (France, Greece, Italy, Portugal and Spain) ranging between the 35° and 46° latitude. The results showed an overestimation of SOC sequestration, when the AI and soil erosion were considered. The average overestimation rate in the studied Mediterranean areas was 0.11 Mg ha−1 yr−1. Carbon stock overestimation ranged from 34 to 1417 Gg for Portugal and Italy, respectively. Even if overestimation is considered, 4p1000 goals are often reached, especially in the more arid areas. The findings of this research allowed us to map the areas suitable to meet the 4p1000 that could be achieved by adopting conservative practices such as NT.
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