The availability to the plant of minerals derived from organic fertilisers depends to a large extent on the activity of soil microorganisms, which is commonly stimulated by the release of organic C from roots. Therefore, we investigated the rhizosphere of maize plants cultivated on experimental plots located in west France and that have received annually composted pig manure vs ammonium nitrate for 7 years or pig slurry vs ammonium nitrate for 10 years. The year of the experiment, all plots received 80 kg of mineral N ha-1 as unique fertilisation. Samples of bare soil and of soil adhering to roots were collected (0-15 cm) at 9-13 unfolded leaves stage. Compared to the mineral plots, plants of the organic plots produced significant greater shoot biomass and in case of manure plot, roots were more branched and had more soluble C in their tissues. The type of fertilisation had no effect on microbial C (fumigation-extraction), which was twice as high in rhizosphere soil as in bare soil. Whatever the fertilisation, in situ soil respiration and microbial activity (in vitro microbial utilisation of 14C-glucose [1]) were greater in planted soil than in bare soil, the difference being significantly more pronounced in manure plot compared to the mineral plot. In manure plot, solely, there were more soil soluble C and N in the rhizosphere soil by comparison to the bare soil. In manure plot, the higher microbial activity and the greater soil soluble C content might originated at least partially, from a greater rhizodeposition as suggested by the root characteristics. Indeed, the root soluble C and the density of root branches, which were greater for the plants of the manure plot, are important variables determining the passive diffusion of root exudates and the production of root mucilage. [1] Nguyen C., Henry F. 2002. A carbon-14-C-glucose assay to compare microbial activity between rhizosphere samples. Biology and Fertility of Soils, 35, 270-276.