The addition of small or trace amounts of carbon to soils can result in the release of 2-5 times more C as CO2 than was added in the original solution. The identity of the microorganisms responsible for these so-called trigger effects remains largely unknown. This paper reports on the response of individual bacterial taxa to the addition of a range of 14C-glucose concentrations (150, 50 and 15 and 0 [mu]g C g-1 soil) similar to the low levels of labile C found in soil. Taxon-specific responses were identified using a modification of the stable isotope probing (SIP) protocol and the recovery of [14C] labelled ribosomal RNA using equilibrium density gradient centrifugation. This provided good resolution of the [`]heavy' fractions ([14C] labelled RNA) from the [`]light' fractions ([12C] unlabelled RNA). The extent of the separation was verified using autoradiography. The addition of [14C] glucose at all concentrations was characterised by changes in the relative intensity of particular bands. Canonical correspondence analysis (CCA) showed that the rRNA response in both the [`]heavy' and [`]light' fractions differed according to the concentration of glucose added but was most pronounced in soils amended with 150 [mu]g C g-1 soil. In the [`]heavy RNA' fractions there was a clear separation between soils amended with 150 [mu]g C g-1 soil and those receiving 50 and 15 [mu]g C g-1 soil indicating that at low C inputs the microbial community response is quite distinct from that seen at higher concentrations. To investigate these differences further, bands that changed in relative intensity following amendment were excised from the DGGE gels, reamplified and sequenced. Sequence analysis identified 8 taxa that responded to glucose amendment (Bacillus, Pseudomonas, Burkholderia, Bradyrhizobium, Actinobacteria, Nitrosomonas, Acidobacteria and an uncultured [beta]-proteobacteria). These results show that radioisotope probing (RNA-RIP) can be used successfully to study the fate of labile C substrates, such as glucose, in soil.