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

Abstract

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.

Source or decomposition of soil organic matter: what is more important with increasing forest age in a subalpine setting?

“…We also found a high concentration of n-C 31 alkane in the mineral soil of all forest stand ages which is rather indicative for grass and shrub plants (Bush and McInerney, 2013). However, it was most prominent only in the 40-year-old forest and indicates additional input of understorey vegetation (Rowland et al, 2009) or remains of OM derived from the grass vegetation of the previous pasture (Speckert et al, 2023). Conclusively, among our Norway spruce afforestation chrono-sequence we observed a shift in the OM input with increasing forest age of mainly grass leaf-derived OM in the pasture (0-year-old) to OM input of grass remains, mosses and spruce needles in younger forest stand ages (40-and 55-year-old) towards mainly needle-derived OM in old growth forest stands (130year-old), which confirms our expectation of more needle-derived OM with increasing forest age.…”

“…130 years of afforestation with Norway spruce on a former pasture not only resulted in a change of the aboveground biomass but also of the belowground biomass quality and quantity (Hiltbrunner et al, 2013;Speckert et al, 2023). This thereby caused an alteration of the composition and decomposition of SOM in the soils, which we traced at a molecular level.…”

“…The sampling campaign was conducted in July 2020. For each forest stand age, five individual plots were placed in a line over the entire area of the respective forest age and pasture, respectively (see Speckert et al, 2023). Spruce needles and moss samples were collected in forests of all stand ages [n = 3 (n = number of replicates)] and most dominant grass species (F. arundinacea and P. lanceolata) were collected (n = 1) in the pasture area.…”

“…The second objective is to assess the decomposition of OM with increasing soil depth and forest age. With increasing forest age, we expect less decomposition, specifically in organic horizons and surface mineral soils as the organic horizons in old forest stand ages are characterized by higher C:N ratios compared to younger forest stand ages (Speckert et al, 2023). Furthermore, a decrease in pH with increasing forest age (Hiltbrunner et al, 2013) hampers decomposition of OM in soils.…”

“…The current study aims to investigate whether OM input or decomposition is more important in soils of an afforestation sequence on a former pasture -0 to 130-years -with Norway spruce (Picea abies L.) in a subalpine setting at Jaun (Switzerland; Hiltbrunner et al, 2013;Speckert et al, 2023). A major objective was to determine the potential sources of OM (above-vs. belowground plant litter) and how the sources vary with increasing soil depth and from pasture to forest with increasing forest age.…”