<p>In alpine areas of the European Alps, many of the pastures are no longer economically profitable and are converted into forests (Bolli <em>et al.,</em> 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 <em>et al.,</em> 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 <em>et al.,</em> 2002).</p> <p>In the current study, we aimed to identify the major sources of SOM in a subalpine afforestation sequence (40-130 years) with Norway spruce (<em>Picea abies</em> 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.</p> <p>We observed a decline in soil organic carbon (SOC) stock (9.6 ± 1.1 kg m<sup>-2</sup>) after 55 years and a recovering of the SOC stock 130 years (12.7 ± 0.9 kg m<sup>-2</sup>) 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<sup>-2</sup>) after 130 years. But if we consider the additional SOC stock accumulated in the organic horizons (between 0.8 and 2 kg m<sup>-2</sup>), 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 O<sub>i</sub>-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<em><sup>+</sup></em> (+3%) and Gram<em><sup>-</sup></em> 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.</p> <p>References</p> <p>Bolli, 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. <em>Silva Fennica</em>, 41, 55.</p> <p>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. <em>Biogeochemistry</em>, 115, 251-266.</p> <p>Vesterdal, L., Ritter, E., Gundersen, P. (2002). Change in soil organic carbon following afforestation of former arable land. <em>Forest Ecology and Management</em>, 169, 137-147.</p>

<p>Alpine and sub-alpine areas react very sensitive to global climate change and carbon cycling therein has been understudied, so far. A major component of plant litter that is commonly regarded as hardly decomposable is lignin. Consequently, the improved knowledge on degradation of lignin and soil organic carbon in alpine areas is of great importance to better understand their response to climate change. Therefore, we conducted a closed-jar incubation experiment under controlled conditions. <sup>13</sup>C labelled plant litter (above ground litter from <em>Lolium perenne</em>) was added to two different soils from a sub-alpine area, one pasture soil and one forest soil originating from Jaun, Switzerland. To investigate the effect of increasing temperatures, the incubation was conducted under three different temperature regimes (average growing season temperature of 12.5°C, +4°C (16.5°C) and +8°C (20.5°C)) for the period of one year with five consecutive destructive samplings.</p><p>Lignin phenols were extracted using the CuO oxidation method, subsequent sample clean-up and quantification by GC-FID. Compound-specific stable carbon (δ<sup>13</sup>C) isotope composition of the lignin phenols was measured by GC-IRMS.</p><p>For all treatment groups, lignin concentrations decreased over the period of one year. The average decrease across all treatment groups was -22.7%. The decrease was slightly higher for the forest soil (-24.9%) than for the pasture site (-20.5%). No significant difference was observed between the control soil with and without added labelled litter. Average lignin decrease for the pasture soil was highest for the lowest temperature (-27.1%). For the two higher temperature treatments the decreases were identical with -17.1% and -17.3%. For the forest soil, the decrease was highest for a temperature of 16.5 °C (26.9%) and slightly lower for 12.5°C (25.7%). Surprisingly, the lowest decrease was observed for 20.5°C (22.1%).</p><p>The evolution of the <sup>13</sup>C labelled litter signal enables the assessment of the degradation of fresh litter in the soils. For all different soils and incubation temperatures, the amount of litter-derived lignin phenols decreased by more than 50% already within two weeks after litter addition. In the further course of time, the <sup>13</sup>C signal decreased much more slowly but remained considerably different from control soils. A possible explanation for this is a high availability of easily degradable carbon within the litter, providing enough energy to produce enzymes for lignin degradation.</p><p>Over the course of a year, also older lignin in the control samples degraded in a similar range as in the samples with litter addition, with a strong decrease in the initial phase and a slower decomposition in the later phase. This can be explained by the better availability of carbon at the beginning of the experiment and missing fresh litter during the later course.</p><p>Contrary to expectations, the degradation of lignin did not increase with rising temperature. This could be due to a lower temperature optimum of the current microbial community which is adapted to the current sub-alpine temperature regime. A complementary field incubation will show whether and how the laboratory results can be transferred to field conditions.</p><p> </p><p> </p>

Abstract. 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.

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