First published online April 2008. DOI:10.1111/j.1574-6968.2008.01133.x

ammonium; natural abundance stable isotope; nitrogen mineralization; nitrogen assimilation; 15N; E. coli. Escherichia coli cells were forced to mineralize or assimilate nitrogen in vitro by manipulating substrate carbon and nitrogen availability. When grown on an organic nitrogen source, E. coli cells released NH4 1 and were enriched in 15N relative to the nitrogen source (1.6–3.1%). However, when cells were grown on an inorganic nitrogen source, the biomass was depleted (6.1–9.1%) relative to the source. By measuring 15N enrichment of microorganisms relative to nitrogen pools, ecosystem ecologists may be able to determine if microorganisms are assimilating or mineralizing nitrogen