Quantifying soil organic carbon stocks above the treeline in the Swiss Alps

Abstract. More than 90 % of the total carbon (C) in alpine ecosystems is stored belowground, yet spatial estimates of soil organic carbon (SOC) stocks remain scarce due to limited accessibility and the demanding nature of SOC stock estimates in rocky alpine terrain. By combining new measurements at 144 sites across the Swiss Alps with data from existing inventories, we compiled a comprehensive dataset on SOC stocks totalling 307 sites from treeline to the permafrost region (1750 m – 3100 m a.s.l.). We predicted the spatial distribution of SOC by linking stock measurements to environmental covariates using Quantile Regression Forests (QRF) and produced a SOC stock map at 25 m resolution illustrating the spatial SOC variability in alpine terrain. Our results show that SOC stocks average 7.3 ± 3.3 kg m⁻² in alpine grasslands and 1.8 ± 1.7 kg m⁻² in partly vegetated areas around and above the vegetation line. Overall, the alpine region of Switzerland, which covers one-third of the total country area, stores an estimated amount of 47.6 Mt SOC, representing a non-negligible share of the Swiss greenhouse gas inventory. Vegetation productivity, represented by the Normalized Difference Vegetation Index (NDVI) and topo-climatic covariables, together with vegetation-derived indicators of humus content and soil pH, were highly informative for spatial predictions. This study identifies hotspot regions of SOC storage and influential spatial predictors of its distribution, providing a quantitative baseline for assessing the status-quo and future changes in alpine SOC stocks under continued climate and land-use change. The observed increase in SOC stocks with increasing NDVI suggests that climate change-driven greening at high elevations, where vegetation cover is currently sparse, may enhance SOC storage, although the rates and magnitude of these changes require further investigation.