How do acid or alkaline soil environments affect soil organic carbon stocks in a post-abandonment secondary succession process in Mediterranean mountain areas?

Farmland abandonment has become one of the most important land use and land cover (LULC) changes threatening mountain areas in the Mediterranean. The concomitant natural revegetation process brings with it a series of ecosystem services and disservices for these marginal areas. Secondary succession can improve soil environment properties; however, its effects can be influenced by the specific characteristics of the site (topography, climate, vegetation, etc.). Thus, the main objective of this research was to study how soil quality and carbon sequestration are affected by post-abandonment secondary succession depending on the environment (delimited from the bedrock and the typical vegetation communities of the area. Soil sampling was carried out in a representative area of the Mediterranean mid-mountains (Leza Valley, Spain) for two soil environments (acid and alkaline), at different depths (0-40 cm) and in 5 stages of the abandonment process: cropland (CRL); shrubland (SH); bushland (BS); young forest (YF); and old forest (OF). Soil physico-chemical parameters were analysed and data analysis carried out with the following results: (i) significant differences were detected between environments for most LULCs; (ii) LULC, depth and soil environment significantly affected most of the soil variables studied; (iii) alkaline soil environments showed a progressive increase of soil organic carbon (SOC) and total nitrogen (N) stocks as the revegetation process and age of abandonment advanced (from 101.16 Mg ha-1 SOC (CRL) to 181.68 Mg ha-1 SOC (OF); (iv) in acid soil environments, the highest stocks were observed in the YF (161.56 Mg ha-1 SOC) with an irregular evolution pattern. Our study showed that secondary succession can improve soil quality and SOC sequestration. However, factors such as soil environment may be of great importance in selecting the best post-abandonment management strategy in a bid to gain recovery of ecosystem services and mitigate the effects of Global Change.