There is limited knowledge about the consumption and interaction of methane (CH4) and ethylene (C2H4) in forest soils under disturbances of temperature and acidification. Temperate volcanic forest topsoils (0-5 cm) sampled under different tree species (e.g. Pinus sylvestris, Cryptomeria japonica and Quercus serrata) were used to study the capacities for CH4 and C2H4 consumption and their sensitivity to temperature and pH. We also studied the responses of soil nitrogen (N) transformations to temperature and relationships to consumption of both CH4 and C2H4. The C2H4 consumption rates increased with temperature up to 35oC, whereas the optimum temperature for CH4 consumption rates was approximately 25oC. Both Q10 values and activation energies for CH4 consumption rates over the range 5 to 25oC were larger than corresponding values for C2H4 consumption rates. The rates of nitrous oxide (N2O) and nitric oxide (NO) evolution and net N mineralization in the soils increased exponentially with temperature up to 35oC, with relatively large Q10 values and activation energies for NO evolution. In these forest topsoils, rates of CH4 and C2H4 consumption at pH < 4.0 were negligible, and the pH optimum for both consumptions varied from 5.5 to 6.2. Most of the tested forest soils had an optimum pH for CH4 and C2H4 consumption that was above natural pH values, which indicated that soil acidification would inhibit CH4 and C2H4 consumption in situ. There was a high rate of net C2H4 evolution from forest soils acidified experimentally to pH < 4.0, particularly from Cryptomeria japonica forest soil, and 67% of the variation in C2H4 evolution rates could be accounted for by the increase in soil water-soluble organic carbon concentrations. Previous studies have shown that addition of C2H4 in headspace gases can inhibit atmospheric CH4 consumption in such forest soils. Hence, the evolution of C2H4 from temperate volcanic forest soils at decreasing pH can exacerbate inhibition of the soil atmospheric CH4 consumption in situ.