Novel insights into the DOM-driven processes affecting nitrogen cycling in shallow aquifers of the West Liao River Plain: Evidence from FT-ICR MS and isotope analyses

The significant influence of hydro(bio)geochemical processes induced by organic matter (OM) on groundwater nitrogen contamination has been widely reported. However, the mechanisms controlling the heterogeneous distribution of this contamination as well as the impact of the biodegradability of OM on nitrogen species in sedimentary aquifers are not yet well understood. In the West Liao River Plain (WLRP), northeastern China, shallow groundwater is severely affected by ammonia nitrogen (NH₄⁺-N), accompanied by high Fe²⁺ concentrations. This study used high-resolution organic matter spectroscopy (FT-ICR MS) and stable isotope techniques to investigate the contrasting sources of DOM and their effect on nitrogen cycling in two adjacent reaches with different hydrogeological settings. The disparities in controlling nitrogen occurrence and transformation by coupled processes involving OM and Fe oxides in the two reaches were identified. High levels of NH₄⁺-N (average 1.94 mg/L) in groundwater were observed in the reaches of XKR, where groundwater flow is sluggish and sedimentary aquifers are rich in OM derived from terrestrial humic components. The strong humification and degradation of DOM as well as the preferential utilization of terrestrial humic-like components with high NOSC values mediate the reductive dissolution of Fe oxides to produce a large amount of Fe²⁺, which provides sufficient electron donors for DNRA and promotes the presence of high ammonia groundwater. In contrast, in groundwater from the WLR reaches, DOM is mainly biogenic and less degraded. The degradation of DOM and N-containing DOM is dominated by aliphatic and highly unsaturated compounds, respectively, resulting in the mineralization of N-containing molecules and relatively high NH₄⁺-N concentrations (average 0.73 mg/L). Significant accumulation of heteroatom(S)-containing DOM is evident in WLR groundwater, highlighting the importance of sulfate reduction in shaping the composition of DOM, dissolution of Fe oxides, and DNF process. These findings provide new evidence and perspectives for understanding the hydro(bio)geochemical processes controlling nitrogen cycling in alluvial aquifer systems.