Soil microorganisms regulate the supply of nitrogen to plants and so are important controllers of plant productivity and ecosystem carbon sequestration. Johnson et al.1 report that exposure of a subarctic heath ecosystem to increased ultraviolet-B (UV-B) irradiation causes a drastic decline in the mass ratio of C:N in soil microorganisms, which would increase the amount of nitrogen stored in the microbial biomass and possibly alter the availability of nitrogen to plants. However, we argue that some of the authors' microbial C:N data are unrealistic, possibly because of an artefact of the technique used to measure microbial carbon and nitrogen concentrations. As a result, there is little reason to suppose that increased exposure of ecosystems to UV-B radiation will influence microbial nitrogen storage, plant nitrogen availability or rates of carbon sequestration.