What is the magnitude of belowground carbon flux? What are the pathways for transfer of C to soil (role of mycorrhiza)? How long does it takes from leaves to the soil and what are the residence times in the different compartments? Do different patterns of carbon allocation exist across species? Answering these questions are a prerequisite for improving the capacity of models to reproduce forest carbon budget and to accurately predict the effects of global change on carbon cycling. The fate of carbon in the plant -soil system was followed by using pulse-labelling of 10 m tall trees in the field with 13CO2 for a short period of time. The study was carried on three major European tree species (beech, oak and pine). Trees were labelled at distinct phenological stages between Sept 2008 and Oct 2009. The assimilated 13C by plants during the pulse labelling was then tracked in soil CO2 efflux at high temporal resolution using tuneable diode laser absorption spectrometry (Plain et al. 2009), and in the microbial biomass extracted from soil cores (root + mycorrhiza transfer) and in mesh cores that permit the ingrowth of mycorrhiza hyphae but exclude roots (no-root cores). There is a rapid transfer of 13C belowground with a maximum occurring within 2 to 4 days after labelling and it was observed both in roots and microbial biomass. Maximum recovery occurred earlier in beech and oak, while it happened later in Pine. Plain C, Gérant D, Maillard P, Dannoura M, Dong Y, Zeller B, Priault P, Parent F, Epron D. 2009. Tracing of recently assimilated carbon in respiration at high temporal resolution in the field with a tuneable diode laser absorption spectrometer after in situ 13CO2 pulse labelling of 20-year-old beech trees. Tree Physiology doi:10.1093/treephys/tpp072