Nitrogen pollution alters bacterial carbonate mineralization potential in karst river

Karst systems represent critical carbon sinks where microbial-mediated carbonate precipitation is influenced by anthropogenic nitrogen pollution. This study investigated nitrogen pollution impacts on microbial mineralization in karst rivers using physicochemical and metagenomic analyses. Proteobacteria and Actinobacteria dominated carbonic anhydrase-producing bacterial communities, with β-carbonic anhydrases being most abundant (84.51%). Nitrogen pollution significantly reduced the diversity and relative abundance of these bacteria and drove variations in their community structure. This further triggered a cascade of changes in carbonic anhydrase activity, bicarbonate concentration, and total alkalinity. Co-occurrence network analysis showed that increased nitrogen pollution weakened interactions between carbonic anhydrase-producing and other non-producing bacteria. Functional analysis revealed that nitrogen pollution significantly impaired the potentials of alkalinity engine metabolism (particularly the Calvin-Benson-Bassham cycle and fatty acid catabolism), extracellular polysaccharides biosynthesis, and Mycobacterium cell wall formation. Furthermore, carbonate mineralization degenerates markedly beyond a critical threshold of ∼22 mg/L total inorganic nitrogen. These findings provide guidance for water resource management and establish a foundation for future carbon budget assessments in karst systems under anthropogenic influence.