Sensitivity of soil organic carbon stocks and fractions to different land-use changes across Europe

Land-use changes (LUC) influence the balance of soil organic carbon (SOC) and hence may cause CO2 emissions or sequestration. In Europe there is a side by side of LUC types that lead to SOC loss or SOC accumulation. However, there is a lack of studies covering all major LUC types to investigate qualitative and quantitative LUC effects on SOC. In this study we sampled 24 paired sites in Europe to a depth of 80 cm, covering a wide range of pedo-climatic conditions and comprising the major European LUC types cropland to grassland, grassland to cropland, cropland to forest and grassland to forest. To assess qualitative changes and the sensitivity of different functional SOC pools with distinct turnover times, we conducted a fractionation to isolate five different fractions of SOC. The mean SOC stock changes after LUC were 18±11 Mg ha-1 (cropland to grassland), 21±13 Mg ha-1 (cropland to forest), -19±7 Mg ha-1 (grassland to cropland) and -10±7 Mg ha-1 (grassland to forest) with the main changes occurring in the topsoil (0–30 cm depth). However, subsoil carbon stocks (>30 cm depth) were also affected by LUC, at 19 out of 24 sites in the same direction as the topsoil. LUC promoting subsoil SOC accumulation might be a sustainable C sink. Particulate organic matter (POM) was found to be most sensitive to LUC. After cropland afforestation, POM accounted for 50% (9.1±2.3 Mg ha-1) of the sequestered carbon in 0–30 cm: after grassland afforestation POMincreased on average by 5±2.3 Mg ha-1, while all other fractions depleted. Thus, afforestations shift SOC fromstable to labile pools. The resistant fraction comprising the so-called inert carbon was found to be only slightly less sensitive than the total SOC pool, suggesting that an inert carbon pool was not chemically extracted with NaOCl oxidation, if there is any inert carbon