Contemporary carbon stocks of mineral forest soils in the Swiss Alps

Soil organic carbon (SOC) has been identified as the main global terrestrial carbon reservoir, but considerable uncertainty remains as to regional SOC variability and the distribution of C between vegetation and soil. We used gridded forest soil data (8-km x 8-km) representative of Swiss forests in terms of climate and forest type distribution to analyse spatial patterns of mineral SOC stocks along gradients in the European Alps for the year 1993. At stand level, mean SOC stocks of 98 t C ha(-1) (N = 168, coefficient of variation: 70%) were obtained for the entire mineral soil profile, 76 t C ha(-1) (N = 137, CV: 50%) in 0-30 cm topsoil, and 62 t C ha(-1) (N = 156, CV: 46%) in 0-20 cm topsoil. Extrapolating to national scale, we calculated contemporary SOC stocks of 110 Tg C (entire mineral soil, standard error: 6 Tg C), 87 Tg C (0-30 cm topsoil, standard error: 3.5 Tg C) and 70 Tg C (0-20 cm topsoil, standard error: 2.5 Tg C) for mineral soils of accessible Swiss forests (1.1399 Mha). According to our estimate, the 0-20 cm layers of mineral forest soils in Switzerland store about half of the C sequestered by forest trees (136 Tg C) and more than five times more than organic horizons (13.2 Tg C). At stand level, regression analyses on the entire data set yielded no strong climatic or topographic signature for forest SOC stocks in top (0-20 cm) and entire mineral soils across the Alps, despite the wide range of values of site parameters. Similarly, geostatistical analyses revealed no clear spatial trends for SOC in Switzerland at the scale of sampling. Using subsets, biotic, abiotic controls and categorial variables (forest type, region) explained nearly 60% of the SOC variability in topsoil mineral layers (0-20 cm) for broadleaf stands (N = 56), but only little of the variability in needleleaf stands (N = 91, R-2 = 0.23 for topsoil layers). Considerable uncertainties remain in assessments of SOC stocks, due to unquantified errors in soil density and rock fraction, lack of data on within-site SOC variability and missing or poorly quantified environmental control parameters. Considering further spatial SOC variability, replicate pointwise soil sampling at 8-km x 8-km resolution without organic horizons will thus hardly allow to detect changes in SOC stocks in strongly heterogeneous mountain landscapes.