Nitrogen Isotope Dynamics of Anammox and Denitrification in Coastal Groundwater

Anaerobic ammonium oxidation (anammox) is an autotrophic microbial process that converts NO2- and NH4+ into nitrogen gas (N2), and may be important for attenuating fixed nitrogen in groundwater prior to discharge into coastal systems. Isotope enrichment factors are useful in identifying dominant processes in various environments. The influence of anammox on the nitrogen isotope dynamics of DIN species and N2 was assessed through laboratory incubations using groundwater and sediment from a nitrogen-contaminated groundwater plume. These were conducted under conditions of varied anammox contribution to total N2 production. Experimentally observed enrichment factors for nitrate (NO3-) and nitrite (NO2-) reduction ranged from -17 to -25‰ regardless of treatment. A finite time stepping model was then used to determine a set of enrichment factors representing “best fits” for concentrations and isotope evolution of DIN species, N2, and N2O concentration. Modeled isotopic effects in the NO2- and NO3- pools were on similar scale to that of denitrification. This was consistent with results from separate 15N tracer experiments suggesting anammox accounts for up to 8 or 28% of N2 production, depending on denitrification weighting within treatments. NH4+ fractionations could not be clearly discerned likely because of low rates, a large NH4+ pool, and isotopic exchange between aqueous and sediment NH4+ pools. Nitrogen isotope systematics appeared dominated by denitrification, and good modeled fits could be attained within the range of published denitrification enrichment factors with or without anammox. This work highlights challenges in interpreting in situ patterns of δ15N as unique indicators of anammox