This study examined the seasonal partitioning of nitrogen (N) between plants and soil microorganisms on a mountain plateaux (Carex bigelowii - Racomitrium lanuginosum heath) in the Scottish Highlands. Specifically, we determined whether there is a temporal relationship between the abundance of dominant soil fauna, soil nitrogen availability, and the partitioning of N between microbes and plants. We found that soil microorganisms and plant tissue of Carex contain similar portions of the total N pool, but that this N partitioning varies greatly over the growing season. At the onset of plant growth in May, the microbial N pool was at its lowest (1.8% of total N), and available N was sequestered by the severely N-limited microbial biomass. At this time, plant N requirement appears to have been met by the use of internal reserves of N in roots. Significant net mineralisation of N was not detected until microbial demands for N had been satisfied in June. Peak rates of N mineralisation corresponded to maximal plant biomass and shoot N content of Carex in July, whereas microbial sequestration of N was found to be most intense in late season when plant demands had subsided after senescence. Although microbial biomass was lowest in early season, we find no convincing evidence to support the notion that microbial competition for N is limited at this time. The patterns of N partitioning that we describe are likely to have implications for the retention of N in these high mountain ecosystems. © 2002 Elsevier Science B.V. All rights reserved.