The ecosystem carbon sink implications of mountain forest expansion into abandoned grazing land: The role of subsoil and climatic factors

Woody encroachment is a widespread phenomenon resulting from the abandonment of mountain agricultural andpastoral practices during the last century. As a result, forests have expanded, increasing biomass and necromass car-bon(C) pools. However,the impact on soil organic carbon(SOC) isless clear.The main aim of this study wasto in-vestigate the effect of woody encroachment on SOC stocks and ecosystem C pools in six chronosequences located along the Italian peninsula, three in the Alps and three in the Apennines. Five stages along the chronosequences were identified in each site. Considering the topsoil (0–30 cm), subsoil (30 cm-bedrock) and whole soil profile, the temporal trend in SOC stocks was similar in all sites, with an initial increment and subsequent decrement inthe intermediate phase. However, thefinal phase of the woody encroachment differed significantly between the Alps (mainly conifers) and the Apennines (broadleaf forests) sites, with a much more pronounced increment inthe latter case. Compared to the previous pastures, after mature forest (N62 years old) establishment, SOC stocksincreased by: 2.1(mean) ± 18.1(sd) and 50.1 ± 25.2 Mg C·ha−1in the topsoil, 7.3 ± 17.4 and 93.2 ±29.7 Mg C·ha−1in the subsoil, and 9.4 ± 24.4 and 143.3 ± 51.0 Mg C·ha−1in the whole soil profile in Alps and Apennines, respectively. Changes in SOC stocks increased with mean annual air temperature and average minimum winter temperature, and were negatively correlated with the sum of summer precipitation. At the same time, all other C pools (biomass and necromass) increased by 179.1 ± 51.3 and 304.2 ± 67.6 Mg C·ha−1in the Alps and theApennines sites, respectively. This study highlights the importance of considering both the subsoil, since deep soillayers contributed 38% to the observed variations in carbon stocks after land use change, and the possible repercussions for the carbon balance of large areas where forestsare expanding, especially under pressing global warming scenarios.The project of this work is part of the research activities of the PhD in science, technology and biotechnology for sustainability. The first author received a fully founded scholarship partially by the University of Tuscia (Viterbo - Italy) and partially by the University of Molise. Tommaso Chiti participated in the project by conducting his work with the funding obtained through the LIFE MediNet project (grant number LIFE15 PRE IT/732295). Jorge Curiel Yuste was financed in part by the Basque Government through the BERC 2018-2021 (grant code) program and by Spanish Ministry of Economy and Competitiveness (MINECO) through BC3 María de Maeztu excellence accreditation MDM-2017-0714. I.C (grant code)