Microorganisms play a central role in litter decomposition and partitioning C between CO2 evolution and sequestration of C into semi-permanent pools in soils. At the ecosystem level, forest stand age influences rates of litter accumulation and quality and micro-climatology, which could affect the microbial community structure and C sequestration processes. To investigate this we combined stable-isotopic tracking with PhosphoLipids Fatty Acid (PLFA) analysis to study time-course 13C incorporation into microbial community members under field conditions at the southern Cascade Range of Washington State in the Wind River Experimental Forest. This was done during plant decomposition in situ in forest soil of old-growth Douglas-fir (Pseudotsuga menziessi) stands (>500 yrs) compared to stands clear-cut in either 1994 or 1977. PLFAs are biomarkers for major functional microbial groups such as fungi, actinomycetes, gram positive and negative bacteria, and protozoa with typically 30 to 45 lipid biomarkers per soil sample. We used in situ microcosms that contained either 13C-labelled litter or root material and analyzed the soil for the relative amount of 13C incorporated (13C %) into PLFAs (using a gas chromatograph linked to a mass spectrometer). Microcosms were collected seven times over a 22-month incubation period. When a significant effect of stand age was found, the 13C % incorporation was always greater in clear-cut samples compared to old-growth samples (Fig. 1). When ‘source' was significant, PLFAs from samples containing the 13C-labeled litter had greater amounts of 13C incorporated than PLFAs from samples containing 13C-labeled roots. In general, 18:1ω9 and 18:2ω6,9 (common fungal biomarkers) had the greatest amount of 13C incorporation throughout the study period in both clear-cut and old-growth sites (e.g. Fig. 1). This relatively high amount of 13C incorporated in the fungal biomarkers, especially in the samples containing 13C-labeled litter, suggests that fungi have grown up into the litterbag and translocated C into the soil layers. Our results confirm that 13C-labeled materials may be applied in the field and tracked into the microbial community using 13C-PLFA analysis. This novel approach enabled us to connect organisms with specific decomposition activities, which has been a long-standing, disconnect between linking microbial organisms with biogeochemical processes.