Revegetation is key for soil organic carbon sequestration on abandoned and degraded land in northern Spain

Agricultural land abandonment is a major land use change in the Mediterranean region, especially affecting marginal areas. The fields of the abandoned village Sierra Estronad (Aragón, Spain), experienced heavy impact treatments (bulldozing) after which half of the fields were kept open and tilled without planting any crop and the other half of the fields were left fallow. From these two treatments and the surrounding natural forest 483 soil samples were collected in addition to corresponding vegetation data at 162 GPS referenced sampling points. Soil samples were analyzed using predictive models based on visible and near-infrared spectroscopy for Soil Organic Carbon (SOC), total Nitrogen, and Permanganate Oxidizable Carbon. Comparing the fallow fields, which have had a 15-year recovery period to the tilled fields, a SOC sequestration rate of 0.64 Mg ha−1 y−1 was found. On tilled fields however, even after a recovery period of 5 years, very few plants were able to colonize the area, resulting in a sparse soil cover and significantly lower SOC and total N stocks. These results show the interdependence of soil fertility proxies (SOC and /total Nitrogen) and the degree of vegetation cover, and how practices of preventing former agricultural fields from revegetating have a long-lasting impact of soil degradation, even after their termination. However, if left fallow, abandoned fields do have the potential to support a secondary succession and serve as a carbon sink thus contributing to soil fertility and climate change mitigation.