Our understanding of forest biosphere-atmosphere interactions is fundamental for predicting forest ecosystem responses to climatic changes. Currently, however, our knowledge is incomplete partly due to inability to separate the major components of soil CO2 effluxes, viz. root respiration, microbial decomposition of soil organic matter and microbial decomposition of litter material. In this study we examined whether the delta(13)C characteristics of solid organic matter and respired CO2 from different soil-C components and root respiration in a Danish beech forest were useful to provide information on the root respiration contribution to total CO2 effluxes. The delta(13)C isotopic analyses Of CO2 were performed using a FinniganMAT Delta(PLUS) isotope-ratio mass spectrometer coupled in continuous flow mode to a trace gas preparation-concentration unit (PreCon). Gas samples in 2-mL crimp seal vials were analysed in a fully automatic mode with an experimental standard error +/- 0.11parts per thousand. We observed that the CO2 derived from root-free mineral soil horizons (A, B-W) was more enriched in C-13 (delta(13)C range -21.6 to -21.2parts per thousand) compared with CO2 derived from root-free humus layers (delta(13)C range -23.6 to -23.4parts per thousand). The CO2 evolved from root respiration in isolated young beech plants revealed a value intermediate between those for the soil humus and mineral horizons, delta(13)C(root) = -22.2parts per thousand, but was associated with great variability (SE +/- 1.0parts per thousand) due to plant-specific differences. delta(13)C Of CO2 from in situ below-ground respiration averaged -22.8parts per thousand, intermediate between the values for the humus layer and root respiration, but variability was great (SE +/- 0.4parts per thousand) due to pronounced spatial patterns. Overall, we were unable to statistically separate the CO2 of root respiration vs. soil organic matter decomposition based solely on delta(13)C signatures, yet the trend in the data suggests that root respiration contributed similar to43% to total respiration. The vertical gradient in delta(13)C, however, might be a useful toot in partitioning respiration in different soil layers. The experiment also showed an unexpected C-13-enrichment Of CO2 (> 3.5parts per thousand) compared with the total-C signatures in the individual soil-C components. This may suggest that analyses of bulk samples are not representative for the C-pools actively undergoing decomposition. Copyright (C) 2004 John Wiley Sons, Ltd.