Stable isotopes: A tool for detecting nitrogen cycle processes in laboratory microcosm experiments

Salt marsh estuaries are valuable habitats that perform important functions such as providing habitat and nursery areas for commercially important species, mitigating the effects of storms on coastal communities, and filtering nutrients and other contaminants from water entering the coast. Anthropogenic nitrogen (N) inputs have the potential to degrade the quality of salt marsh estuaries and affect their ability to perform these valuable functions. 15N has been used in field studies at natural abundance and in laboratory studies using an enriched tracer to study N cycle processes occurring in coastal ecosystems. The purpose of this study was to adapt N stable isotope natural abundance techniques to laboratory denitrification assays to examine other N cycle processes that may be occurring. This technique was then used to examine the effects of anthropogenic N loading on N cycle processes in salt marsh estuaries by comparing the results from two study sites, one receiving anthropogenic N inputs from a nearby golf course and one in an undeveloped area. Potential denitrification rates at both sites were directly related to the amount of nitrate (NO3-) added to the overlying water column and potential denitrification rates were higher at the impacted site than the unimpacted site regardless of water column NO3- concentration (0, 700 or 1400 microg NO3--N). Potential dissimilatory NO3- reduction to ammonium (NH4+) (DNRA) rates measured through mass balance did not appear to vary by either NO3- addition or study site. In most instances measured NH4+ accumulation in microcosms was small, but the delta15N of NH4 + changed significantly with time suggesting that microbial assimilation and sediment organic matter (SOM) mineralization were occurring even when N species concentrations were stable. However quantifying the exact proportions of microbial assimilation and SOM mineralization using the stable isotope data was not possible because the fractionation factors of these processes were unknown.