Fermentation emerges as key pathway for ammonium production beneath the Pearl River Delta

A recent study led by Professors Jimmy Jiao from the University of Hong Kong and Meng Li from Shenzhen University has revealed that microbial fermentation is the primary pathway for ammonium production beneath the Pearl River Delta. This discovery explains the region's unusually high natural groundwater ammonium levels, making the groundwater unsuitable for drinking without treatment.

The researchers analyzed sediment samples from 36 boreholes across the delta, spanning approximately 13,000 years of geological history and representing various depositional environments. By examining the microbial genetic material in the sediments, they reconstructed 770 metagenome-assembled genomes (MAGs) to understand the microorganisms present and their metabolic functions.

The study found that fermentation-related genes were most abundant across all sediment zones, indicating that fermentation is likely the dominant microbial pathway for ammonium production. However, the abundance of these genes decreased with sediment depth and age as readily degradable organic matter became scarce, limiting substrates for microbial fermentation and ammonium production.

This process was particularly pronounced in the marine-dominated zone, where fine-grained, organic-rich sediments encourage microbial production while limiting groundwater flow, trapping ammonium for thousands of years.

The researchers also noted that nitrogen processing pathways varied across the land-sea gradient. In the terrestrial-dominated zone, genes associated with nitrate reduction were more abundant, while in the saline transitional and marine-dominated zones, genes related to nitrite conversion into ammonium became more prominent. The presence of the marine-associated genus Brevirhabdus, which carries genes for fermentation and nitrite-to-ammonium conversion, suggests that ancient depositional conditions may still influence current groundwater chemistry.

Professor Jiao explained that the findings move beyond the conventional understanding that buried organic matter is the sole source of ammonium. The study highlights the specific microbial pathways and organisms responsible for ammonium production and demonstrates how these processes change across sediments formed under different environmental conditions.

By integrating depositional history, hydrogeochemistry, and microbial function, the research offers a framework for understanding ammonium accumulation in delta regions worldwide, providing insights into identifying vulnerable areas and improving groundwater assessment, monitoring, and treatment planning in densely populated delta regions.