Decoding the role of organic matter in groundwater Feammox processes: Insights from the Yangtze River paleochannel

Abstract

Groundwater nitrogen (N) contamination is becoming increasingly severe worldwide. Anaerobic ammonia oxidation coupled with iron-reduction processes (Feammox) has great potential as an effective method for N removal in groundwater systems. However, previous studies on nitrogen removal by Feammox have generally focused on surface sediment soils, and the quantification of this process in groundwater remains inadequate. Moreover, the impact of native organic matter (OM) within the groundwater system on the Feammox process remains uncertain. The paleochannel of the middle reaches of the Yangtze River was selected as a representative study area for this research. The occurrence of Feammox and other N cycle (non-Feammox) processes in regional groundwater was identified and differentiated through the analysis of δ¹⁵N/δ⁵⁶Fe isotopes and 16S rRNA functional gene quantification, along with hydrochemical characteristics. These findings indicate that the groundwater in the study area is characterized by anoxic conditions and slight acidity. The occurrence of Feammox is substantiated by an increase in δ¹⁵N_NH4, which coincides with the concurrent increase of Fe(II) concentrations and δ⁵⁶Fe values in the groundwater, alongside the predominance of Acidimicrobiaceae bacterium A6. ¹⁵N isotope-labeled incubation experiments demonstrated that the potential rate of N removal via the Feammox process in the groundwater system ranged from 0.09 to 0.16 mg N kg^-1d^-1. Correlation results suggested that the functional microorganisms facilitating the Feammox process are closely linked to environmental factors associated with organic matter activity. Terrestrial humic substances present in groundwater, characterized by a high degree of unsaturation, aromaticity, humification, elevated biological activity, and nitrogen-rich composition, may act as pivotal drivers of the Feammox process.