Soil Physical and Chemical Responses Along a Pasture-to-Forest Succession in the Alps

Forest expansion following agro-pastoral abandonment is widespread in European mountains and has been further accelerated by recent climate warming (Piccinelli et al., 2020). Previous studies show that woody encroachment does not necessarily increase soil carbon, as accumulation often occurs mainly in woody biomass while soil responses remain variable and context-dependent (Guidi et al., 2014). Recent evidence also indicates that forest expansion alters soil aggregation, reducing microaggregate (Ortiz et al., 2022). Climate, local environmental conditions, and land management strongly modulate these responses, highlighting the lack of a universal pattern and the need for site-specific studies in mountain ecosystems. In this study, we investigated how soil physical (bulk density, aggregate stability) and chemical (total C and N, inorganic N forms, available P) properties change along a gradient of a pasture recolonization by larch, from an actively managed pasture to a mature forest, through a case study in the Northwestern Italian Alps (LTER research site Tellinod, Aosta Valley Region, NW Italian Alps; ~2100 m a.s.l.). Topsoil (0–15 cm) was sampled in five plots representing successive recolonization stages (started around 2008), for a total of 50 samples. Our results show that soil physical and chemical properties varied clearly along the gradient. Bulk density was highest in the grazed pasture, while aggregate stability was lower and soil erodibility (K) higher in the mature forest. pH showed no significant differences, whereas total C and N were slightly lower in the pasture and significantly reduced in the mature forest. Ammonium, nitrate, and dissolved organic carbon (DOC) were lowest in the pasture and mature forest, indicating that successional stages differ in their activity within the carbon and nitrogen cycles, with nutrient availability highest in intermediate stages. Reduced total C in the mature forest likely contributes to lower aggregate stability and higher erodibility. These findings highlight the tight coupling between soil structure and fertility during woody encroachment and underscore the importance of local studies for sustainable land management and biodiversity conservation in alpine environments.