Forest floor microbial community response to tree species and fertilization of regenerating coniferous forests

We studied the effect of tree species and fertilization on the forest floor microbial community of 15-year-old regenerating forests. We sampled F and H forest floor layers of plots planted to Thuja plicata (Donn ex D. Don.) or Tsuga heterophylla (Raf.) Sarg. on N-poor and N-rich sites, with and without fertilizer treatments. Microbial community composition was assessed using phospholipid fatty acid analysis and by enumerating populations of culturable bacteria and fungi. Potential microbial functioning was assessed using community-level physiological profiling. There was little differentiation of communitylevel physiological profiles of F and H layers and few differences among the treatments. Total microbial biomass was greater in the F than H layer, and the two layers had distinct phospholipid fatty acid profiles. Site effects were detected mainly in the residual H layer, and tree species effects were seen mainly in the F layer, which has developed since harvesting. The effect of fertilization depended on site and tree species, with very little response in cedar plots, and the greatest effects in hemlock plots, coinciding with the greater growth response of hemlock. These results indicate that differences in plant growth rates, rather than direct effects of fertilization, influenced the microbial communities.