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.

中文翻译：

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在前瑞士高山牧场植树造林 130 年后，土壤有机碳储量没有变化

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年的森林 = 11.0 ± 0.3 kg m ⁻²，SOC 牧场 = 11.5 ± 0.5 kg m ⁻² )。然而，包括森林中有机层的额外碳，高达 1.7 kg m ⁻²在 55 年树龄的森林中，造林后总体 SOC 储量略有增加。我们发现，随着林龄的增加，表土（0-5 厘米）中矿质土壤的土壤碳氮比增加，从草地的 11.9 ± 1.3 到 130 年树龄森林的 14.3 ± 1.8。反过来，我们观察到土壤 C:N 比随着所有林分年龄的深度增加而降低。这表明，凋落物衍生的有机物质（C:N 从 35.1 ± 1.9 到 42.4 ± 10.8）很可能从有机地层并入并转移到剖面的矿物表层土（0-10 厘米）。由于根具有非常高的 C:N 比率（牧场 63.5 ± 2.8，森林在 54.7 ± 3.9 和 61.2 ± 2.9 之间），颗粒状根系衍生的有机质似乎对森林土壤 C:N 比率以及 SOC 储量影响较小在矿质土壤中积累。这些结果表明，虽然造林仅对 SOC 存量产生适度影响，但通过植被转移引起的凋落物组成变化，SOC 动态发生明显变化。我们得出结论，在以前的亚高山牧场上进行云杉造林并不一定会改变总 SOC 存量，因此在十年到百年的尺度上没有 SOC 封存。