An accidental water purifier
Downstream from the ancient city of Bukhara, the landscape is dominated by a web of irrigation canals. These channels divert water from the Amu Darya river to vast fields of cotton and wheat, the backbone of the region's agriculture. This intensive farming requires significant fertilizer input, and the excess nitrogen inevitably washes from the fields into drainage canals. These canals, choked with the common reed Phragmites australis, feed a series of extensive wetlands. What happens in these reed beds is an example of natural biogeochemical engineering.
The combination of nitrogen-rich water, warm summer temperatures of 26°C or higher, and the unique environment created by the reeds has turned these wetlands into highly efficient water purifiers. The dense reed beds function as enormous bioreactors. The plant roots and submerged stems provide a massive surface area for microbial biofilms to colonize. It is within these slimy layers and the oxygen-depleted sediment that specialized bacteria perform the task of denitrification.
Record-breaking bacterial activity
Scientific investigations into these reed beds have revealed some of the highest rates of denitrification ever measured in a natural system. In controlled experiments mimicking the conditions, sediments colonized by Phragmites australis can convert nitrate to harmless nitrogen gas (N2) at rates between 600 and 18,000 micromoles of nitrogen per square meter per hour. For comparison, nearby unvegetated sediments show much lower rates, typically between 300 and 3,300 micromoles per square meter per hour.
This process is entirely microbial. Anaerobic bacteria in the sediment use the nitrate (NO3−), a potent pollutant that causes eutrophication in waterways, as a substitute for oxygen in their respiration. The end product is dinitrogen gas (N2), the same inert gas that makes up 78% of Earth's atmosphere. Studies show planting reeds can increase total nitrogen removal from sediments by a factor of 7 to 20 compared to unplanted areas. This is because the plants themselves create ideal conditions, releasing small amounts of oxygen from their roots which fuels a coupled cycle of nitrification and denitrification, while also absorbing between 56% and 69% of the nitrogen themselves. The abundance of important denitrifying bacterial genes, such as nirK, increases significantly in the presence of the reeds, confirming the plants' role in supporting these microbial communities.