Aspen harvest intensity decreases microbial biomass, extracellular enzyme activity, and soil nitrogen cycling

Forest harvesting alters plant litter inputs to soil and modifies the soil environment, which could alter the composition and function of soil microbial communities. Harvest-induced reductions in microbial activity could eventually feed back to modify soil N availability and forest productivity. We reasoned that increasing harvest intensity should decrease microbial community biomass and function via reduced litter input to soil. We further expected microbial communities to differ in response to harvest intensity in aspen-dominated (Populus tremuloides Michx. and P. grandidentata Michx.) forests located in climatically and edaphically different conditions. To test these ideas, we quantified microbial community composition and function 8 to 10 yr following harvest in two climatically and edaphically distinct aspen-dominated forests in Michigan. Harvest treatments included control (no harvest), merchantable bole harvest (MBH), total tree harvest (TTH). and total tree harvest + forest floor removal (FFR). Microbial community composition was quantified using phospholipid fatty acid (PLFA) analysis, and microbial community function was assayed using extracellular enzyme activity and N pool dilution. All harvest methods reduced microbial biomass (-24%), the activity of extracellular enzymes involved with litter decomposition (-10 to -30%), gross N mineralization (-36%), and microbial N immobilization (-38%), regardless of climatic and edaphic differences between stand locations. Microbial community composition was not affected by harvest treatment, nor did it differ between locations. Lower rates of extracellular enzyme activity and gross N transformation in harvested aspen stands corresponded with a reduction in microbial biomass, which in turn may be driven by reduced litter input and changes in soil microclimate following clear-cut harvest.