2, A. de Varennes 3

Crop management, including rotation, tillage and mineral fertilization, generally affect the active organic matter (OM) pool. Cereal straw left in the soil after cropping may increase its OM content and this is likely to have beneficial effects in soils chemical and physical properties, such as increased water retention in sandy soils and an increase in plant nutrients. However, it can also have detrimental effects on the growth of the subsequent crop. Soil disturbance promotes the decomposition of residues and nitrogen (N) mineralisation. Mineralised N in surface residues is easily immobilized being less subject to leaching, whereas most of the mineralised N from incorporated residues is not immobilized being subject to leaching. The quantification of mineralisation under field conditions is difficult because several processes are involved and the spatial variation of inorganic N is large. Measurements unde