Microbial community structure as influenced by season and stand age in a Douglas-fir (Pseudotsuga menziesii) ecosystem

Forest harvest can have significant impacts on forest ecosystems that may influence the capacity of soils to sequester carbon (C). The microbial community controls decomposition, which is a critical process in partitioning litter- and root-C between CO2 and storage in semi-permanent soil-C pools. The objectives of this study were to determine the effect of clear-cutting and stand age on: (1) temporal dynamics of soil microbial community (SMC) structure and physiological status; and (2) shifts among microbial functional groups in taking up 13C-labeled plant materials during decomposition. The experiment was conducted in Douglas-fir ecosystems within the Gifford Pinchot National Forest, Washington. We chose stands of three different ages: old-growth where trees are between 300 and 500 years old; an 8-year old stand; and a 25-year old stand. Phospholipid fatty acid (PLFA) profiling and 13C-PLFA labeling techniques along with the ratio of saturated to monounsaturated PLFAs and the ratios of cyclopropyl PLFAs to their monoenoic precursors as microbial physiological stress markers were utilized.; Microbial PLFA profiles showed that SMC structure and physiological status was most affected by season and secondarily by time since clear-cutting. Total microbial biomass and bacterial and fungal biomass were significantly reduced in CC8 but not in CC25 sites relative to old-growth sites. Total microbial biomass concentration was lowest and the stress indicators were highest in August, which corresponded to low soil moisture and high temperatures.; The relative amount of 13C incorporated into PLFAs was also influenced by stand age and 13C source (13C-labeled litter vs. 13C-labeled root material). A significantly greater amount of 13C was incorporated in CC8 samples compared to OG1 samples in five out of the seven sample dates. Additionally, a significantly greater proportion of 13C was incorporated into soil samples containing the 13C-labeled litter material relative to samples containing 13C-labeled root material in four out of the seven dates. In general, 18:1o9 and 18:2o6,9 (common fungal biomarkers) had the greatest amount of 13C incorporation throughout the study period in both clear-cut and old-growth sites, indicating the important role of fungi in the decomposition of litter and root material and translocation of C within soil layers.