130 Years of afforestation does not result in changes in soil organic carbon stocks in the Swiss Alps

In alpine areas of the European Alps, many of the pastures that are no longer economically profitable are being converted into forests (Bolli et al., 2007). Afforestation of former pasture has been acknowledged as a contribution to mitigate CO₂ emissions by an increased storage of soil carbon in soil and biomass (Smal et al., 2019). So far, several studies indicated that afforestation of former pastures does not always lead to an increase or decrease of soil organic carbon stocks after 30 to 40 years of afforestation. The loss of soil organic carbon in the mineral soil, however, can be rebalanced by the increased accumulation of soil organic carbon in the organic forest floor (Thuille and Schulze, 2006). Nevertheless, studies concerning the changes as well as restoration of SOM following afforestation are limited.

In this study, we aimed to trace the source and transformation of SOM in a subalpine afforestation sequence (0-130 years) with Norway spruce (Picea abies L.) on a former pasture in Jaun, Switzerland. Soil and root samples were taken with volumetry cylinders to a depth of 45cm at 5cm increments. To trace the source and transformation of SOM, soil samples were analysed for plant-and microorganism-derived SOM by combining multiple compound classes as free extractable lipids, such as n-alkanes and free fatty acids.

Preliminary results show a higher (p<0.05) fine roots biomass in pasture (8.2±4.4gm^-2) compared to forested areas. The highest (p=0.92) fine root biomass was observed in the youngest forest (40yr; 2.3±0.7gm^-2), followed by the 130yr (0.7±0.2gm^-2) and 55yr (0.6±0.2gm^-2) old forest. Highest carbon stocks (14.0±0.8 kgm^-2) were observed in the youngest forest followed by the 130yr (11.0±0.3 kgm^-2) old and 55yr (9.6±1.1kgm^-2) old forest. In summary, afforestation of former pasture (11.2±0.0 kgm^-2) does not result in changes (p=0.37) in the total (0-45cm) organic carbon stock over a period of decades. However, there is a significant (p<0.001) higher C concentration in the organic forest floor in all forested areas compared to the mineral soil of both pasture and forest. To conclude, the change in the SOM sources and quality following afforestation may not lead to stock changes, but the stability of SOM might be modified by this change. The changes in SOM dynamics following afforestation are further analysed by the use of phospholipid fatty acids as well as free extractable fatty acids and alkanes to improve our understanding of aboveground and belowground litter incorporation and cycling.

References

Bolli, J. C., Rigling, A., and Bugmann, H. (2007). The influence of changes in climate and land-use on regeneration dynamics of Norway spruce at the treeline in the Swiss Alps. Silva Fennica, 41, 55.

Smal, H., Ligęza, S., Pranagal, J., Urban, D., and Pietruczyk-Popławska, D. (2019). Changes in the stocks of soil organic carbon, total nitrogen and phosphorus following afforestation of post-arable soils: A chronosequence study. Forest Ecology and Management, 451, 117536.

Thuille, A., and Schulze, E. D. (2006). Carbon dynamics in successional and afforested spruce stands in Thuringia and the Alps. Global Change Biology, 12, 325-342