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In alpine areas of the European Alps, many of the pastures are no longer economically profitable and are converted into forests (Bolli et al., 2007). Afforestation on former pastures affects soil organic matter (SOM) dynamics through alteration of quality and quantity of root and aboveground biomass litter input. Compared with pasture OM, forest OM is less decomposable and characterized by increased C:N ratio as well as increased lignin concentration (Hiltbrunner et al., 2013). Therefore, it could be expected that long-term afforestation on a centennial scale may have a severe impact on SOM dynamics, an aspect that remains so far unknown as most of the earlier studies focused on successions between 30 and 50 years (Vesterdal et al., 2002).In the current study, we aimed to identify the major sources of SOM in a subalpine afforestation sequence (40-130 years) with Norway spruce (Picea abies L.) on a former pasture in Jaun, Switzerland. Therefore, we combined plant- and microorganism-derived molecular proxies from several compound classes such as free-extractable fatty acids and phospholipid fatty acids.We observed a decline in soil organic carbon (SOC) stock (9.6 ± 1.1 kg m-2) after 55 years and a recovering of the SOC stock 130 years (12.7 ± 0.9 kg m-2) after afforestation. Overall, there is no alteration of the SOC stock in the mineral soil following afforestation of former pasture (13.3 ± 0.9kg m-2) after 130 years. But if we consider the additional SOC stock accumulated in the organic horizons (between 0.8 and 2 kg m-2), the total SOC stock slightly increased, although OM in organic horizons is less stabilized than mineral-bound OM. An increase of the C:N ratio in the Oi-horizon with increasing forest age (40yr: 36.9 ± 2.6; 55yr: 40.9 ± 4.1; 130yr: 42.4 ± 6.6) reflects the alteration in litter quality towards poorly decomposable compounds in older forests. In addition, preliminary results show an increase in the abundance of Gram+ (+3%) and Gram- bacteria (+6%), especially in the young (40yr) forest. Thus, the bacterial community seems to proliferate in the early succession before the fungal-dominated community takes over. Thus, the change in SOM source and quality following afforestation may not result in considerable stock changes, but results in better stability of SOM in the mineral soil.ReferencesBolli, J. C., Rigling, A., 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.Hiltbrunner, D., Zimmermann, S., Hagedorn, F. (2013). Afforestation with Norway spruce on a subalpine pasture alters carbon dynamics but only moderately affects soil carbon storage. Biogeochemistry, 115, 251-266.Vesterdal, L., Ritter, E., Gundersen, P. (2002). Change in soil organic carbon following afforestation of former arable land. Forest Ecology and Management, 169, 137-147.

Soil organic matter (SOM) plays an important role in the global carbon cycle, especially in alpine ecosystems. However, ongoing forest expansion in high elevation systems potentially alters SOM storage through changes in organic matter (OM) inputs and microclimate. In this study we investigated the effects of an Picea abies L. afforestation chrono-sequence (40–130 years) of a former subalpine pasture in Switzerland on soil organic carbon (SOC) stocks and SOM dynamics. We found that SOC stocks remained relatively constant throughout the chrono-sequence, with comparable SOC stocks in the mineral soils after afforestation and previous pasture (SOC40-year-old forest = 11.6 ± 1.1 kg m−2, SOC130-year-old forest = 11.0 ± 0.3 kg m−2, and SOC pasture = 11.5 ± 0.5 kg m−2). However, including the additional carbon of the organic horizons in the forest, reaching up to 1.7 kg m−2 in the 55-year-old forest, resulted in a slight in-crease in overall SOC stocks following afforestation. We found that the soil C:N ratio in the mineral soil increased in the topsoil (0–5cm) with increasing forest stand age, from 11.9 ± 1.3 in the grassland to 14.3 ± 1.8 in the 130-year-old forest. In turn, we observed a decrease in soil C:N ratio with increasing depth in all forest stand ages. This suggests that litter-derived organic matter (C:N from 35.1 ± 1.9 to 42.4 ± 10.8) is likely incorporated and translocated from the organic horizon to the mineral topsoil (0–10 cm) of the profiles. As roots had very high C:N ratios (pasture 63.5 ± 2.8 and forests between 54.7 ± 3.9 and 61.2 ± 2.9), particulate root-derived organic matter seems to have a minor influence on forest soil C:N ratio and thereby on SOC stock accumulation in the mineral soil. These results suggest that, although the afforestation only moderately affected the SOC stock, there is an apparent alteration in the SOC dynamics through changes of the litter composition caused by the vegetation shift. We conclude that spruce afforestation on a former subalpine pasture does not necessarily change the total SOC stock and that consequently there is no SOC sequestration on a decadal to centennial scale.

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-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 (pReferencesBolli, 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

Following the Kyoto Protocol, afforestation has been acknowledged as a promising strategy for soil organic matter (SOM) conservation and to mitigate anthropogenic CO2 emissions (Huang et al., 2011). However, the effect of carbon sequestration in soils depends on ecosystem properties, the former land use and on type of trees planted. Some studies showed a decline in SOM (Hiltbrunner et al., 2013) while others reported an increase in SOM 30 to 40 years after afforestation of former pastures (Thuille and Schulze, 2006). Thus, there is a need for well-designed and site-specific long-term experiments on a decadal scale to investigate changes in SOM dynamics following afforestation to predict the behaviour of carbon sequestration under changing environmental conditions. One approach to trace the sources of SOM is the application of molecular proxies like n-alkanes or fatty acids. Though, focusing only on one compound class may lead to flawed conclusions due to missing information offered by other compound classes. One way to obtain a more solid conclusion on the SOM dynamic in soils is the combination of multiple compound classes (Li et al., 2018). The aim of this project is to identify possible sources of OM in soils in a subalpine afforestation sequence (40-130 years) with Norway spruce (Picea abies L.) on a former pasture in Jaun, Switzerland, by combining molecular proxies from several compound classes originating from various plant and microbial sources.A higher (+70%) number of fine roots (-1) compared to fine roots of spruce (310 mg g-1). In our project we expect a decline in the SOC stocks, specifically in the younger (40 to 55yr) forest stands and a change in SOM quality following afforestation.ReferencesHiltbrunner, D., Zimmermann, S., and Hagedorn, F. (2013). Afforestation with Norway spruce on a subalpine pasture alters carbon dynamics but only moderately affects soil carbon storage. Biogeochemistry, 115, 251-266.Huang, Z., Davis, M. R., Condron, L. M., and Clinton, P. W. (2011). Soil carbon pools, plant biomarkers and mean carbon residence time after afforestation of grassland with three tree species. Soil Biology and Biochemistry, 43, 1341-1349.Li, X., Anderson, B. J., Vogeler, I., and Schwendenmann, L. (2018). Long-chain n-alkane and n-fatty acid characteristics in plants and soil-potential to separate plant growth forms, primary and secondary grasslands? Science of the Total Environment, 645, 1567-1578.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