Microbial drivers of ammonium accumulation in Holocene sediments of the Pearl River Delta.

Elevated ammonium concentrations in deltaic groundwater pose a widespread environmental challenge, yet the microbial mechanisms linking depositional history to ammonium dynamics remain poorly understood. The Pearl River Delta, with the highest naturally occurring groundwater ammonium concentrations documented globally, provides a unique natural system to investigate these processes. Here, by integrating geochemical and metagenomic data, we show that fermentation-related genes are the most prevalent across all depositional zones, suggesting fermentation as the potential primary pathway for ammonium production, with the functional potential declining as sedimentary organic matter becomes increasingly recalcitrant with sediment age. Secondary mechanisms shift from nitrate reduction to nitrite ammonification across terrestrial-to-marine-dominated zones, reflecting salinity-driven metabolic partitioning. Notably, the marine-derived genus Brevirhabdus emerges as a key taxon linking depositional history to present-day biogeochemistry, demonstrating remarkable metabolic versatility. These findings demonstrate that paleo-depositional and hydrogeological evolution fundamentally shape microbial landscapes and dictate groundwater quality in deltaic systems worldwide.