The present study quantifies changes in soil organic carbon (SOC) stocks in Belgium between 1960Belgium between , 1990 and 2000 for 289 spatially explicit land units with unique soil association and land-use type, termed landscape units (LSU). The SOC stocks are derived from multiple nonstandardized sets of field measurements up to a depth of 30 cm.Approximately half of the LSU show an increase in SOC between 1960 and 2000. The significant increases occur mainly in soils of grassland LSU in northern Belgium. Significant decreases are observed on loamy cropland soils. Although the largest SOC gains are observed for LSU under forest (22 t C ha À1 for coniferous and 29 t C ha À1 for broadleaf and mixed forest in the upper 30 cm of soil), significant changes are rare because of large variability. Because the number of available measurements is very high for agricultural land, most significant changes occur under cropland and grassland, but the corresponding average SOC change is smaller than for forests (9 t C ha À1 increase for grassland and 1 t C ha À1 decrease for cropland).The 1990 data for agricultural LSU show that the SOC changes between 1960 and 2000 are not linear. Most agricultural LSU show a higher SOC stock in 1990 than in 2000, especially in northern Belgium. The observed temporal and spatial patterns can be explained by a change in manure application intensity. SOC stock changes caused by land-use change are estimated. The SOC change over time is derived from observed differences between SOC stocks in space. Because SOC stocks are continuously influenced by a number of external factors, mainly land-use history and current land management and climate, this approach gives only an approximate estimate whose validity is limited to these conditions.

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Organic carbon levels of 542 soil samples from temperate lowland forest were determined by the original and modified Walkley–Black (WB) dichromate methods and total organic carbon (TOC) analysis. The performance and the lower and upper quantification limits of the WB method were assessed. Variable recovery rates were related to laboratory and field conditions and to the sample composition. The percentage carbon recovered by the original WB method was found to be systematically lower than commonly accepted, and the correction factor was estimated at 1.58 instead of 1.30–1.35. However, a good linear relationship with TOC enabled acceptable prediction of soil organic carbon which was most precise when using the original WB method. Texture class and pedogenetic horizon showed significant differences in recovery. Depending on the modifications of the WB method, recoveries varied significantly between laboratories, explaining up to 29% of the total variation of the topsoil carbon assessment of a site. Low recovery from samples was partly attributed to charcoal and resistant elementary carbon particles. No interference was found to be caused by iron or manganese compounds. In order to use WB carbon data of forest soils, laboratory‐ and method‐specific determination of the recovery rate using a total analyser is strongly recommended. The original WB method was unable to predict reliably forest soil carbon contents higher than 8% TOC.

Abstract. Belgium's soil survey data collected between 1950 and 1970 (pre‐Kyoto Protocol) contain more than 13 000 geo‐referenced soil profile descriptions, which allow the computation of a spatially distributed baseline carbon content for incremental soil depths, based on soil/land‐use combinations (landscape units) and multiple matching soil profile observations. The results show that the soil organic carbon (SOC) and soil inorganic carbon (SIOC) contents of many landscape units do not differ significantly. However, landscape units under forest and grassland tend to contain more carbon. The same is true for landscape units on poorly drained and/or clayey soils, podzols or anthropogenic soils. The change of the SOC in the upper 100 cm of mineral soil follows a logarithmic decline with increasing depth, useful for the extrapolation of SOC of surface layers to deeper layers. SIOC values are strongly related to the geological soil characteristics and increase linearly with depth. Integrating the mean SOC and SIOC content of landscape units over the Belgian territory results in a total soil carbon stock of 303 Mt C in the upper 100 cm layer. Ectorganic horizons contain 35 Mt C and mineral soil contains 245 Mt C in organic form and 23 Mt C in inorganic form. These results are shown to be consistent with an independent set of SOC measurements on 3134 surface samples.