Consequences of defoliation on C partitioning and C release by roots are contradictory. (Mikola and Kytöviita 2002, Miller and Roses 1992, Paterson and Sim 1999, 2000, Todorovic et al 1999). In this context, this work aimed to quantify the effects of defoliation of ryegrass shoots on C partitioning in the plant-soil-micro-organisms system and on the short-term below ground carbon allocation under two N treatments (0 and 100U). After two days, rhizodeposition is not affected by defoliation. Four days after defoliation, a decrease of shoots C content could be explained by a decrease of nutrient assimilation by roots, resulting from root senescence, and reduced photosynthesis. Senescent tissues involve an increase of C availability for micro-organisms, especially under high N. This explain the enhance of the number of culturable cells (as colony forming units CFU), and the non significant increase of micro-organisms activity (test based on the short term utilization of 14C glucose by rhizosphere micro-organisms). Thus under high N treatment, defoliation involve, after 4 days, an increase of rhizodeposition mainly in the form of sloughed-off cells. Finally, the allocation of current assimilates to plant roots is not affected by defoliation, but roots senescence increase the root released organic C.