The transformation of organic N into mineral forms, that are readily available for plant uptake, is a process carried out by soil microorganisms.  The mobilization of N by microbes is crucial as N often limits plant growth in natural and agricultural systems.  We propose that when the soil microbial biomass is actively mineralizing N it becomes enriched in ¹⁵N compared to the soil substrate in a predictable manner. We evaluated the temporal variability of microbial ¹⁵N enrichment and net N mineralization rates at five sites along an elevational gradient in Northern Arizona.  Sites range from precipitation limited semi-arid grasslands to temperature limited temperate grasslands.  Results indicate that the ¹⁵N signature of the microbial biomass relative to that of the soil N is correlated to C/N ratios and rates of net N mineralization. Traditional measurements of net N mineralization are highly variable in time and space due, in part, to small variations in soil temperature and moisture.  However, because the microbial ¹⁵N enrichment is a more time-integrated measure of the C-to-N balance of substrates and net N mineralization, it should provide a useful measure of longer-term N availability to plants.