<p>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.</p> <p>Acknowledgement: This research project was supported by the MANMOUNT (PID2019-105983RB-100/AEI/ 10.13039/501100011033) project funded by the MICINN-FEDER. Melani Cortijos-L&#243;pez is working with an FPI contract (PRE2020-094509) from the Spanish Ministry of Economy and Competitiveness associated to the MANMOUNT project</p> <p>Keywords: abandoned croplands, natural revegetation, carbon sequestration, soil pH, Iberian System (Spain)</p>
<p>Soil is a key element in the Earth system, and changes in land management can affect soil organic carbon (SOC) dynamics and the provision of ecosystem services related to soil. The content and stock of SOC has been adopted as the measure to be used for land degradation assessment. Nevertheless, SOC dynamics linked to land use and land cover changes after land abandonment is still a controversial issue. From the mid-20<sup>th</sup> century, Mediterranean mountains have undergone intense transformations due to climate change and severe socio-economic marginalisation, linked to depopulation and cropland and pasture abandonment. These processes led to a very different landscape, with a massive invasion of shrubs, secondary succession to forest and afforested areas. The objective of this study is to analyse the effects of post-land abandonment management practices on soil organic carbon dynamics (stocks and fractions). We selected three land abandonment scenarios in the Central Pyrenees: (i) natural revegetation with broadleaf vegetation; (ii) natural revegetation versus afforestation with coniferous forest; and (iii) abandonment of grasslands and woody encroachment. The total SOC stocks and fractions (through density fractionation) were analysed: Free light Fraction (FLF), Occluded Light Fraction (OLF), and Heavy Fraction (HF). Results showed that: (i) revegetation resulted in a constant and slow SOC accumulation processes; (ii) grassland enhanced accumulation of SOC, and the woody encroachment of these areas produced a significant loss of SOC during the first revegetation stages; (iii) density fractions were significantly affected by land use and land cover changes; (iv) the labile fractions (FLF and OLF) were significantly higher under coniferous forest than under grassland and shrubland; and (v) differences were also observed between coniferous and broadleaf forest, suggesting that broadleaf litter is incorporated faster in the mineral soil. The present study showed that land abandonment in Mediterranean mountains does not only affect SOC stocks, but also triggers the addition of labile carbon into the soil. It also changes the dynamics and sequestration of stable SOC, making this information essential in the design of future post-land abandonment strategies.</p><p>&#160;</p><p>Acknowledgements: This research project was supported by the MANMOUNT (PID2019-105983RB-100/AEI/ 10.13039/501100011033) project funded by the MICINN-FEDER and the PRX21/00375 project funded by the Ministry of Universities of Spain from the &#8220;Salvador de Madariaga&#8221; programme. Melani Cortijos-L&#243;pez is working with an FPI contract (PRE2020-094509) from the Spanish Ministry of Economy and Competitiveness associated to the MANMOUNT project. Manel Llena has a &#8220;Juan de la Cierva Formaci&#243;n&#8221; postdoctoral contract (FJC2020-043890-I/AEI/ 10.13039/501100011033) from the Spanish Ministry of Science and Innovation.</p>
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