Stocks and fluxes of soil organic carbon for landscape units in Belgium derived from heterogeneous data sets for 1990 and 2000

The global carbon cycle has an important influence on global change. Soil organic carbon (SOC), the largest component of the terrestrial carbon pool, plays a vital role in the terrestrial carbon cycle and therefore knowledge of soil organic carbon pools and fluxes is required. This paper assesses typical values and measures of variation for soil organic carbon contents of soil-land use combinations in Belgium, further termed landscape units (LSU). Data from several heterogeneous data sets collected around the years 1990 and 2000 are used. Their heterogeneity is related to the purpose for which the soil carbon data were collected, the measurement technique, the nature and total depth of the sampled layers and the level of spatial aggregation with which the data were made available. In order to make integrated use of the data and present a spatial and temporal uncertainty assessment of the SOC contents of LSU, we exploit all available information on variability using simple statistical rules. Two thirds of the LSU under arable or grassland have lost SOC between 1990 and 2000 for the upper 20 cm. Approximately one third of these LSU show significantly decreased SOC stocks. The decreases are more pronounced in northern Belgium (region of Flanders), which is possibly due to specific manure regulations. The average SOC content of cropland in the upper 100 cm decreased from 8.8 kg C m−2 in 1990 to 8.4 kg C m−2 in 2000. A comparable decrease is observed under grassland, namely from 13.9 to 13.0 kg C m−2. For soils, the highest SOC losses occur in the sandy soil associations of northern Belgium. LSU under forest could only be assessed for the year 2000. Forest soils store on average between 14.8 and 15.5 kg C m−2 in the upper 100 cm of mineral soil, depending on forest type. These values are comparable to grassland but significantly higher than cropland.