REGIONAL CHARACTERIZATION OF THE LONG-TERM CHANGE IN SOIL ORGANIC-CARBON UNDER INTENSIVE AGRICULTURE

To study the change in soil organic carbon (SOC) since it was recorded during the Belgian National Soil Survey some 40 years ago, we recentl y revisited 939 locations still under use as arable land. The study ar ea comprised almost the entire province of West Flanders (about 3000 k m(2)) characterized by profound changes in its arable land management. Taking the increased ploughing depth (by 9.8 cm on average) into acco unt, a significant (P = 0.001) increase of the SOC content by 0.2% on average was found, Expressed as an amount, the SOC in the topsoil rose by 9.3 t/ha on average, representing an increase of 25%. This is comp arable with the conversion of arable land into grassland for 2 to 3 de cades. Geostatistical tools were used to map the SOC at the two times of observation. These showed that most of the spatial variation occurr ed within about 4 km. Since the community level is the smallest spatia l resolution on which agricultural statistics are gathered officially, a detailed modelling of the change in SOC was;as impossible. However, by selecting communities with extreme changes in SOC, we found indica tions that the major source of increase in SOC was due to the large in crease in pig breeding.