Seasonal dynamics of geogenic phosphorus in alluvial-lacustrine aquifers: Coupling of phosphorus-containing dissolved organic matter and microbes as a key driver.

Geogenic phosphorus (P) enrichment in groundwater driven by dissolved organic matter (DOM) degradation has been well documented worldwide but remains underexplored regarding temporal dynamics. As microbial metabolism regulates P-containing DOM mineralization, resolving seasonal shifts in microbial strategies is essential for clarifying geogenic P mobilization. In this study, seasonal field monitoring was integrated with molecular DOM characterization and metagenomic analyses to elucidate the coupling among geogenic P, P-containing DOM, and microbial functional pathways. Dissolved inorganic phosphorus (DIP) fluctuations were controlled by a degradation gradient of P-containing DOM and concurrent adjustments in microbial metabolism. The seasonal oxidant influx from dry to wet seasons modified redox conditions and promoted the transformation of high-molecular-weight humic substances into low-molecular-weight polar compounds, thereby weakening the microbial degradation of P-containing DOM and reducing DIP enrichment. Meanwhile, microbial P metabolism shifted from multi-pathway degradation involving dephosphorylation and C-P bond cleavage to a single dominant dephosphorylation pathway. These findings indicate that seasonal redox dynamics restructure microbial metabolic strategies and ultimately regulate temporal DIP enrichment in groundwater system.