08 May 2023
 | 08 May 2023
Status: this preprint is open for discussion.

Soil organic carbon stocks did not change after 130 years of afforestation on a former Swiss Alpine pasture

Tatjana Carina Speckert, Jeannine Suremann, Konstantin Gavazov, Maria Joao Santos, Frank Hagedorn, and Guido Lars Bruno Wiesenberg

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.

Tatjana Carina Speckert et al.

Status: open (until 09 Jul 2023)

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Tatjana Carina Speckert et al.

Tatjana Carina Speckert et al.


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Short summary
Afforestation on former pastures affects soil organic carbon (SOC) by alteration of quality and quantity of root and aboveground biomass litter input. Compared with pasture organic matter (OM), forest OM is less decomposable and characterized by increased C:N ratios. It could be expected that long-term afforestation on a centennial scale may have a severe impact on SOC dynamics, an aspect that remains so far unknown as most of the earlier studies focused on successions between 30 and 50 years.