Abstract Short-term metabolic activities, including ammonincation, nitrification, denitrification and the release of CO 2, with and without added substrate, and most probable numbers of ammonifiers, nitrifiers and denitrifiers were measured on stored topsoil from pasture over 140 days in the absence of growing plants. Parallel samples of irradiated and untreated soil were examined. Release of mineral-N, chiefly as NH + 4N, was greater from the irradiated soil. In the untreated soil there was a slight increase in NO 3 −N. Microorganisms (estimated by MPN method) and microbial respiration in the untreated soil increased, and then diminished with time. The release of 13N 2O and 13N 2 was measured from intact soil cores amended with 13NO 3 −N and 13NH + 4N. The principal product from the treated soil when 13NO 3 −N was added was 13N 2, with little 13N 2O, whereas the irradiated soil evolved both 13N 2 and 13N 2O. Similarly, the irradiated soil continuously evolved 13N 2O from 13NH + 4N in contrast to the untreated soil.