Feammox in alluvial-lacustrine aquifer system: Nitrogen/iron isotopic and biogeochemical evidences

Groundwater nitrogen contamination is becoming increasingly serious worldwide, and natural nitrogen attenuation processes such as anaerobic ammonium oxidation coupled to iron reduction ("Feammox") play an important role in mitigating contamination. Although there has been intensive study of Feammox in soils and sediments, still lacks research on this process in groundwater. This study makes effort to demonstrate the occurrence of Feammox in groundwater by combining information from Fe/N isotope composition, the quantitative polymerase chain reaction (qPCR) assay, and 16S rRNA gene sequencing. Poyang Lake Plain of Yangtze River in central China was selected as the case study area. The critical evidences that indicate Feammox in groundwater include favorable hydrogeochemical conditions of the alluvia-lacustrine aquifer systems, the simultaneous enrichment of ¹⁵N in ammonium and ⁵⁶Fe, the relative high abundance of Acidimicrobiaceae bacterium A6, and the joint elevation of the abundance of the Feammox bacteria and the concentration of Fe(III). Redundancy analysis (RDA) indicated that Geothrix and Rhodobacter may participate directly or cooperatively in the Feammox process. Ammonium-oxidizing archaea (AOA) involved in ammonium-oxidizing or Feammox process may be stimulated by Fe(III) under a low oxygen concentration and weakly acidic condition. Anammox may be indirectly enhanced by products of the nitrogen transformation processes involving Feammox bacteria and AOA. Fe(III) concentration is an important environmental factor affecting the abundance of functional microorganisms related to nitrogen cycling and the composition of ammonium-oxidizing and iron-reducing microbes. Specific geological background (such as the widespread red soils) and anthropogenic input of ammonium, iron, and acidic substances may jointly promote Feammox in groundwater.