Geogenic Ammonium Enrichment in Alluvial-Lacustrine Aquifer Systems: Coupled Controls of Microbial Pathways and Organic Matter Composition

Elevated geogenic ammonium (NH4+) reported
globally in alluvial-lacustrine aquifer systems is generally attributed
to the mineralization of natural organic matter (OM). However, the
contribution of microbial nitrogen (N) metabolism remains unclear.
This study integrated hydrogeochemical profiling, metagenomic sequencing,
and molecular-level OM characterization to elucidate how microbial
nitrogen transformation and organic N mineralization could drive geogenic
NH4+ accumulation in groundwater. A distinct
shift in microbial N metabolism pathways was identified along the
NH4+ enrichment gradient. Under N-limited and
strongly reducing conditions, N fixation and dissimilatory reduction
of nitrate to ammonium provided additional NH4+ sources, whereas nitrification served as the primary sink by oxidizing
NH4+ to nitrate. At low NH4+ levels, NH4+ was mainly produced via deamination
of simple mono-N compounds (CHO+1N) found in highly unsaturated low-oxygen
(O) compounds and low-O polyphenols. In contrast, at high NH4+ levels, urease-mediated hydrolysis of multi-N compounds
(CHO+nN) in highly unsaturated high-O compounds became dominant. Co-occurrence
network analysis revealed tight, pathway-specific linkages between
functional genes and NH4+-associated OM compounds,
highlighting the substrate-dependent nature of geogenic NH4+ production. These findings could advance our understanding
of microbially mediated NH4+ enrichment mechanisms
and offer implications for the management of NH4+ and other geogenic contaminants in organic-rich aquifers.