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The separate and combined effects of moderate levels of acid load and Cu-Ni deposition on humus (F/H layer) microbial community structure were examined six growing seasons after the start of an artificial irrigation experiment. A 2²-factorial design with acid load (pH 3.1) and Cu-Ni addition was used. The acid application resulted in acidification as defined by humus chemistry, including a decrease in the concentration of base cations and an increase in the concentration of H⁺, Al and Fe, leading to a decrease in humus pH from 4.10 to 3.95. The metal additions resulted in a 2–3-fold increase in humus total Cu and Ni concentrations. The treatments did not affect the total microbial biomass, measured by substrate-induced respiration, or the basal respiration rate. However, the microbial community structure determined by phospholipid fatty acid (PLFA) analysis showed that the acid load affected the bacterial …