Regional assessment of soil organic carbon changes under agriculture in Southern Belgium (1955–2005)

The evolution of SOC stock over time is difficult to assess at a regional scale due to the small magnitude of the changes, to the important spatial variability of SOC and the lack of detailed information on present and past management practices. This paper aimed to detect changes in SOC stocks of agricultural soils of southern Belgium over a long time period (1955–2005), and to determine the driving forces of SOC evolution. The stratification of the study area into homogeneous units (based on land use, soil type, climate and agricultural region) and the re-sampling of soil profiles from the 1950s allowed detection of significant changes in SOC stocks. The use of equivalent masses for SOC stock comparisons based on the plough depth of 2005 allowed excluding dilution effect from changes in plough depth or in bulk density. For units under cropland, an average decrease of 5.8 t C ha− 1 was measured in the plough layer (from an initial equivalent SOC stock of 46.4 t C ha− 1), while for units under grassland, an average increase of 21.9 t C ha− 1 was observed in the 0–30 cm depth (from an initial equivalent SOC stock of 61.2 t C ha− 1). Explanatory factors include human driving forces (land management) and inherent soil properties. The decrease in mass of farmyard manure and slurry applied on cropland along with the change in the types of crops cultivated (progressive replacement of cereals by root crops and fodder) could explain the decrease in SOC stocks observed for cropland, while the increase in livestock density per grazing area has lead to an increase in the SOC stocks of grassland. The increase in plough depth for cropland (+ 1.5 cm) was slightly correlated to the silt content (r = − 0.14) but not to the decrease in SOC content. The impact of erosion or climate on SOC changes remained uncertain. Soil texture was not highlighted as a driving force in the SOC changes, while a strong negative relationship existed between the initial SOC content and the change in SOC content. Therefore, any attempt to increase SOC content in agricultural soils should mainly focus on farming practices through adapted regulations and policies.