It remains a challenge to quantify and assess the importance of the direct plant below-ground flux of photosynthate carbon (C) to soil microorganisms, especially in forests because of the size of the plants and the great spatial heterogeneity of soils. We studied the importance of labile C inputs from trees on the respiratory activity of soil microorganisms by comparing the response of plots with and without girdled pine trees (Pinus sylvestris L.) to additions of C₄-sucrose, thus enabling us to differentiate between utilization of endogenous C₃-soil C sources and exogenous C₄-sucrose. In both girdled and non-girdled plots the respiration rate after sucrose application, i. e. substrate induced respiration measured in the field, was on average ca. double that of basal respiration rate measured in the field. However, the C₄-sucrose-induced increase in respiration of endogenous C₃-C was significantly higher in non-girdled plots. Expression of C₃-respiration as a percentage of induced respiration in the field showed that in girdled plots, C₃-respiration decreased after sucrose addition and, consequently, the induced respiration in the field was totally C₄-C based. A previous laboratory experiment found no increase in total respiration rate when C₃-sucrose was added to the soil substrate of non-mycorrhizal and ectomycorrhizal pine plants. Hence, we see no reason to attribute the increased respiration to (mycorrhizal) roots. Thus, our results indicate that despite the alleged C limitation of the soil microorganisms there is a fraction of SOM, or C within the microbial biomass that is available to microbial metabolism if their C limitation is relieved by the supply of labile C. This fraction corresponds to roughly 10 - 20% of biomass C of the heterotrophic organisms and seems to become exhausted in the long-term absence of supply of photosynthate to roots.