Labile and Stable Soil Organic Carbon in abandoned lands with different management

Abstract

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-20th 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. 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 “Salvador de Madariaga” programme. Melani Cortijos-López 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 “Juan de la Cierva Formación” postdoctoral contract (FJC2020-043890-I/AEI/ 10.13039/501100011033) from the Spanish Ministry of Science and Innovation.