INVESTIGATING NOVEL PATHWAYS FOR N REMOVAL IN THE SUBTERRANEAN ESTUARY

 

Nitrogen cycling in anoxic environments is fundamentally important to local and global fixed nitrogen budgets but remains poorly understood with respect to underlying biogeochemical mechanisms and controls. Following the discovery of anammox, evidence of additional novel N-cycling pathways rooted in the anaerobic oxidation of ammonium has been accumulating and investigations have expanded to focus on the potential interaction between nitrogen transformations and the geochemical cycles of common metals, notably iron and manganese. This study aimed to evaluate whether an iron-mediated anoxic ammonium oxidation process, or “feammox,” occurs in the coastal subterranean estuary, which is characterized by complex and highly interactive biogeochemical cycles. Iron-rich sediments from the subterranean estuary in Gloucester Point (VA, USA) were collected and incubated under anoxic conditions with ammonium-rich groundwater. Chemical concentrations were monitored over time, and isotope ratio mass spectrometry was used to determine production of enriched dinitrogen gas from ¹⁵NH₄⁺ added to incubation bottles. Results of four sediment incubation experiments consistently indicated consumption of ¹⁵NH₄⁺ in ammonium-spiked treatments and subsequent production of ²⁹N₂ and ³⁰N₂, which strongly suggests the occurrence of the hypothesized iron-mediated ammonium oxidation reaction. However, data on ammonium and ferrous iron concentrations revealed a number of competing biogeochemical processes that make it difficult to evaluate the quantitative importance of this reaction in situ. Importance of the feammox reaction appears to depend on availability of specific chemical substrates and active microbial populations, both of which may vary with biogeochemical conditions across different environmental settings (e.g. pH, seasonal temperatures, oxygen saturation, magnitude and pattern of groundwater advection, redox cycle interactions).