Estimates of carbon stock changes in Belgian cropland

Article 3.4 of the Kyoto Protocol allows carbon emissions to be offset by demonstrable removal of carbon from the atmosphere by improved management of agricultural soils. To make use of this possibility, a good estimate of soil organic carbon (SOC) stocks and baseline emissions (in 1990) are crucial factors. Over 210 000 topsoil (0-24 cm) measurements have been made in Belgian cropland in the period 1989-1999, which are available for seven different agro-pedological regions and for three periods (1989-91, 1992-95, 1996-99). We used this extensive SOC data set to estimate SOC stocks and fluxes in Belgian cropland. The measurements of SOC were extrapolated to 1 m depth using an exponential SOC depth distribution model, based on another large data set of over 5184 fully described soil profiles on Belgian cropland made during the National Soil Survey. The SOC data were combined with cropland area figures to calculate SOC stocks to 1 m depth. The 1990 baseline SOC flux of Belgian cropland was then obtained using two different calculation methods, which each yielded similar results and showed that SOC stocks were decreasing in the 1990s at a mean rate of 608 kton OC yr(-1). Consequently, a large part of the Belgian cropland acted as a net source of CO2 emissions during the period 1989-1999.