Soil organic carbon (SOC) is the carbon that remains in the soil after partial decomposition of any material produced by living organisms. Depending on local geology, climatic conditions and land use and management (amongst other factors), soils hold different SOC amounts.
The largest amounts of SOC are stored in the northern permafrost region, mostly in peat soils, where carbon accumulates in soils in huge quantities due to the low temperatures leading to low biological activity and slow decomposition of soil organic matter. In contrast, in dry and hot regions, plant growth is naturally scarce and only very little carbon enters the soil, leading to low SOC content. Climate change can also alter SOC levels, in contrasting magnitudes and directions depending on the considered regions.
Land cover and land use also have an important influence on SOC. Conversion of natural vegetation to cropland can cause large decreases in SOC levels. Unsustainable agricultural management practices such as excessive irrigation or leaving the soil bare are also drivers of important SOC losses, while the opposite is true for practices associated to sustainable soil management, such as mulching, planting cover crops, reduced or no tillage, moderate irrigation and judicious fertilization (Scharlemann et al., 2014; FAO and ITPS, 2018a, 2018b) . The SOCI provides useful information about the soil condition in protected areas, particularly when compared with other unprotected areas with similar environmental conditions, such as the unprotected buffers around protected areas. This information can contribute to identify potentially degraded areas, evaluate the conservation performance of protected areas, set restoration targets, and assess the contribution of protected areas to reduce net global carbon emissions.