HMN 2025: How to Examine microbes that clean up nitrogen pollution across China

A new study has revealed how tiny microbes in rivers and wetlands across China help clean up excess nitrogen pollution, offering fresh insights into the health of freshwater ecosystems and the global nitrogen cycle.

Nitrogen is essential for life, but too much of it—often from fertilizer use, fossil fuel burning, and agriculture—ends up in rivers and lakes. This overload can trigger harmful algal blooms, oxygen loss, fish die-offs, and long-term damage to ecosystems. Scientists have long known that microbes play a critical role in removing nitrogen from water, but how these processes vary across landscapes has remained unclear.

In the study, published in Nitrogen Cycling, researchers examined 30 major riverine wetlands spanning a 3,500-kilometer transect from southern to northern China.

Using advanced isotope tracing and genetic tools, they measured two key microbial processes: denitrification, which converts nitrate into nitrogen gas, and anammox (anaerobic ammonium oxidation), which turns ammonium and nitrite into nitrogen gas without producing greenhouse gases. Both processes permanently remove nitrogen from ecosystems.

The results revealed striking spatial patterns. Denitrification rates were higher in northern rivers than in southern ones, showing a strong latitudinal trend. In contrast, anammox was less tied to geography but emerged as especially important in deeper riparian soils, where it often dominated nitrogen removal.

“Denitrification has long been considered the main pathway for nitrogen removal, but our findings show that anammox plays an equally crucial role—particularly in sandy soils along riverbanks,” said senior author Wenzhi Liu of the Wuhan Botanical Garden, Chinese Academy of Sciences.

Across river sediments and soils, the team found that denitrification was responsible for most nitrogen removal in sediments and root-associated soils, contributing 56–64% of nitrogen gas production. Meanwhile, anammox dominated in bulk riparian soils, accounting for up to 58%. Environmental conditions such as soil carbon, iron content, and nitrate availability were key drivers of these processes.

“These findings suggest that both denitrification and anammox must be included in models of river nitrogen cycling,” Liu explained. “By better understanding how microbes work in different habitats, we can improve predictions of water quality and design more effective conservation strategies.”

The research also highlights the overlooked role of natural river wetlands in buffering human impacts on water quality. As agriculture and urbanization continue to add nitrogen to rivers, knowing how and where microbes remove it will be vital for managing pollution and protecting biodiversity.

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