Organic carbon characteristics in Swedish forest soil trace post-depositional carbon dynamics

Summary We investigated soil organic carbon (SOC) characteristics in three forests along a north–south transect in Sweden where these forest types cover about 69% of the landscape. There was variation in 14C ages, and the median ages ranged from about 25 to > 2500 cal BP in SOC. Although total SOC and nitrogen (N) contents decreased, stable carbon isotope and humification indices increased with depth. These progressive changes with depth and age were related to degradation. The δ13C values and specific biomarkers indicated that organic carbon was primarily from C3 plants. Biomarkers were effective in distinguishing OC input from specific sources (i.e. angiosperms, gymnosperms and grasses). A sharp decrease in biomarkers with depth indicated degradation of OC in the upper soil horizon, and limited contribution in the subsoil towards the stabilization of SOC. The sharp decrease in carbon stocks and 14C age in the soil OC pool with increasing soil depth, and quite large values for the percentage of modern carbon, suggested a decrease in SOC pools. Overall, these results showed that carbon sequestration in high latitude forests was small, and their role as potential carbon sinks needs to be reassessed. Highlights We studied soil organic carbon (SOC) characteristics in Swedish boreal and temperate forests. The decline of SOC with depth contributes little to carbon sequestration in deeper soil horizons. Soil degradation causes a decline in total carbon, nitrogen, alkanes and lignin with depth and age. The role of these forests as carbon sinks needs to be reassessed because of degradation of SOC.