Changes in soil organic carbon and nitrogen following forest expansion on grassland in the Southern Alps

The dominant land-use change in the Southern Alps is grassland abandonment, followed by forest expansion. Forest expansion is commonly thought to increase ecosystem C sequestration, but contradictory results have been reported. Our objective was to investigate soil organic carbon (SOC) and nitrogen (N) changes after forest expansion on abandoned grasslands. A land use and management gradient was identified in a subalpine area of Trentino (Italy) comprising four successional stages: (i) managed grassland (manG), mown and manured annually for more than 100 years; (ii) grassland abandoned 10 years ago (abanG), with shrubs and Picea abies saplings; (iii) early-stage forest (earlyF), dominated by P. abies established on grassland abandoned around 1970; and (iv) old forest (oldF), dominated by Fagus sylvatica and P. abies, representing a reference for long-term forest land use. Organic and mineral soil layers were collected down to 30 cm at eight points within three plots for each successional stage. Significantly more SOC was stored in organic layers of oldF compared with other successional stages and the organic layer N stocks were higher in oldF compared with manG and abanG. The SOC and N concentrations were higher in the top 10 cm mineral soil of grassland plots compared with forest. The earlyF and oldF stored, on average, 28–35% less SOC in top 10 cm mineral soil compared with manG and abanG when stocks were calculated using equivalent soil mass (ESM) and equivalent soil depth (ESD) approaches respectively. SOC stocks in mineral soil (0–30 cm) calculated with ESD decreased from manG to oldF; when accounting for total stone content, SOC stocks in earlyF and oldF were 40% and 50% lower than in manG, respectively. The inclusion of organic layers offset mineral SOC stock differences when only stones <5 cm were accounted for, but when considering the total stone content, SOC stocks in earlyF remained significantly lower (−31%) than in manG. Tree biomass contributed to higher ecosystem C stocks in oldF, which stored 82–93 Mg ha−1 of C more than grasslands. Our study revealed a decrease in total SOC stocks after forest expansion on abandoned subalpine grasslands. Tree biomass development contributed to increasing C storage, especially in a long-term forest land use. Forest management should consider that tree biomass and organic layer development require a minimum time period following grassland abandonment to compensate for the decrease in mineral SOC stocks and contribute positively to climate change mitigation