Microbial fermentation produces ammonium beneath Pearl River Delta

A study led by Professor Jimmy Jiujiu Jiao from the University of Hong Kong and Professor Meng Li from Shenzhen University has found that microbial fermentation is the main driver of ammonium production in sediments beneath the Pearl River Delta. Their research, published in *Nature Communications*, shows that natural fermentation by microbes in the deep underground turns organic nitrogen into ammonium, explaining why groundwater in the region is unusually rich in ammonia.

The Pearl River Delta is one of the world’s most densely populated and industrialized regions, but its groundwater also contains high levels of ammonium—up to 100 times higher than in most other areas. Previous explanations blamed industrial pollution or fertilizer runoff, but this study points to a natural biological process instead. The team analyzed sediment cores up to 150 meters deep and found ammonium concentrations increasing with depth, alongside signs of microbial fermentation: high levels of short-chain fatty acids, methane, and low levels of sulfate. These are classic markers of fermentation in oxygen-poor environments.

The findings shift attention from human activity to natural geochemical cycles. Ammonium in groundwater can be harmful if it seeps into drinking supplies, where it can convert to nitrite and threaten human health. The study also suggests that similar fermentation processes may be happening in other large river deltas around the world, where organic-rich sediments accumulate and microbes break them down in the absence of oxygen. The researchers now want to expand their work to other delta systems to see if the pattern holds.

Why this matters is simple: if ammonium is being produced naturally at depth, managing groundwater quality becomes more complex. Traditional cleanup methods, like pumping and treating water, won’t work if the ammonium is coming from deep underground. The team plans further experiments to model how groundwater pumping and urban development might be disturbing these natural processes, potentially releasing more ammonium into usable water supplies.