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Total soil organiccarbon SOC stocks for grassland soils in Flanders N Belgium were determined for the Kyoto Protocol reference year 1990 and 2000 in order to investigate whether these soils have been CO2sinks or sources during that period. The stocks were calculated by means of detailed SOC datasets, which were available at the community scale for the whole of Flanders. The total SOC stocks for Flemish grassland soils 1 m depth were estimated at 38 Mt SOC in 1990 and 34 Mt SOC in 2000. The loss of SOC resulted from a decrease in the SOC content of grassland soils 71 and could also partly 29 be explained by a decline in grassland area. Significant decreases in SOC for the 0–6 cm depth layer were found for the 1990s for the coarsertextured soils with SOC losses ranging between –0.3 and –0.5 over the 10 y period. Specific management practices that disturb the SOC balance such as conversion to temporary grassland and a reduction of animalmanure application are hypothesized to have contributed to the observed loss of SOC stocks. We furthermore conducted an analysis of uncertainty of the 1990 and 2000 grassland SOC–stocks calculation using Monte Carlo analysis. Probabilitydistribution functions were determined for each of the inputs of the SOCstock calculation, enabling us to assess the uncertainty on the 1990 and 2000 SOC stocks. The frequency distributions of these simulated stocks both closely approached lognormal distributions, and their 95confidence intervals ranged between 150 and 50 of the calculated mean SOC stock. The standard error on the measured decrease in SOC stocks in Flemish grassland soils during the 1990s was calculated to be 7–8 Tg SOC, which is equivalent to twice this decrease. This clearly shows that largescale changes in SOC stocks are uncertaintyridden, even when they are based on detailed datasets.