Modelling the evolution of regional carbon stocks in Belgian cropland soils

In long-term experiments, it has been demonstrated that management of cropland influences the evolution of soil organic carbon (SOC) stocks. National inventories of SOC stocks in Belgium have recently been compiled, and show an evolution of SOC stocks of arable land from 1960 to 1990 and 2000. In order to analyse the driving forces of these changes, we concentrate on the SOC evolution in the soil associations of three of the 13 Belgian agricultural regions (Dunes-Polders, Loam belt and Condroz). The small confidence limits around the mean SOC values within some soil associations (0.4-12.7 t C ha(-1)) allow us to compare the observed values with the results of the RothC soil carbon model and hence quantify the most important driving forces. After estimating the local parameters by fitting the model to SOC values from a long-term experiment in central Belgium, the model was run from 1960 to 2000 for typical soil profiles of soil associations in the three agricultural regions. The main factors inducing changes in SOC stocks are'the increase in plough depth as a result of continued mechanisation in the 1960s and the sustained input of organic amendments in the form of farmyard manure and slurry. In contrast to earlier publications on CO2 emissions from agricultural soils, the model did not predict a decrease in SOC stocks for the period 1990-2000. A slight increase Was observed, although this increase is not significant for most soil associations. The comparison between modelled and observed SOC data at two time slices allows the uncertainty of the model results to be estimated. This uncertainty ranges from 7.5 to 14.4% of the SOC stock and is in the same order of magnitude as the uncertainty around SOC modelling for the long-term experiments both in Belgium and elsewhere in Europe. The organic matter concentration in the topsoil, an indicator for soil quality, was in the range of 1.5 to 3.3%. Organic matter content increased in the Dunes-Polders and decreased in the Loam belt and the Condroz from 1960 to 2000. Many soils in the Loam belt are now close to the critical level of 2% under which the soils are vulnerable to compaction and erosion.