How acidic or alkaline soils affect SOC stock in a post-abandonment secondary succession process: a case study in th Mediterranean mid-mountains.

Mediterranean mountains have been affected by an intense process of cropland abandonment since the middle of the last century, as a result of the rural exodus. This has led to the activation of natural revegetation processes in marginal areas that have not been managed. Literature has recorded different soil responses to secondary succession depending on factors such as climate, altitude, soil depth and type, but still very little is known about the influence of soil pH. Thus, the main objective of this work is to identify how soil quality and carbon sequestration are affected by secondary succession after abandonment for two types of soil lithologies (acidic and alkaline). For this purpose, the Leza Valley (La Rioja, Spain) was selected as the representative study area. Soil samples were collected for each lithology, at different depths (0-40 cm), for 5 stages of succession (cropland (CRL); shrubland (SH); bushland (BS); young forest (YF); and old forest (OF)), and their physicochemical properties were analysed in the laboratory. Data analysis was carried out and these are the most relevant results: i) there are significant differences between acidic and alkaline organic carbon stocks; ii) the alkaline soils increase their SOC stock with the advance of succession, and significant differences were observed between the first stages of abandonment and BS, YF and OF; iii) while in the acidic soils no significant differences were observed, and the highest values were recorded in YF; iv) these results may be the combination of interactions between pH, soil properties and plant and microbiological communities that establish in these areas. Our work has shown the relevance of considering the lithology of our soils in order to determine which post-abandonment management practices may be the most appropriate for our study area. Therefore, it is necessary that policies and management strategies include this type of analysis to achieve the best results of soil carbon sequestration.

Acknowledgement: This research project was supported by the MANMOUNT (PID2019-105983RB-100/AEI/ 10.13039/501100011033) project funded by the MICINN-FEDER. Melani Cortijos-López is working with an FPI contract (PRE2020-094509) from the Spanish Ministry of Economy and Competitiveness associated to the MANMOUNT project

Keywords: abandoned croplands, natural revegetation, carbon sequestration, soil pH, Iberian System (Spain)