We used an acetylene reduction assay to measure rates of nitrogen fixation on a 38-year-old Alnus hirsuta plantation in central Korea. The diurnal pattern of acetylene reduction changed significantly during May, August, and October, typically varying by 3-fold throughout the course of the day. Maximum rates occurred at 3 p.m. in May and October, but at 6 p.m. in August. Increasing trends were evident during the early growing season, with sustained high rates from mid-May through late September; July had the highest rates, averaging $7.2mu mole g^{-1}$ dry nodule $h^{-1}$. The average nodule biomass for this plantation was $220kg ha^{-1}$. Rates of acetylene reduction were related to soil temperature, but not to soil moisture content. Combining these nodule biomass calculations with seasonal average acetylene reduction rates yielded an estimate of current annual nitrogen fixation of $60kg N ha^{-1}$ for the plantation. This rate of annual nitrogen addition was very large in relation to the yearly nitrogen requirements of coniferous and deciduous forests in central Korea.