Nitrogen removal through denitrification in China's aquatic system.

Area of Science:

• Environmental Science
• Ecology
• Biogeochemistry

Background:

• Understanding aquatic denitrification is crucial for nitrogen (N) removal in ecosystems, particularly in China, a major global N fertilizer consumer.
• Benthic denitrification rates (DNR) are key indicators of ecosystem health and nitrogen cycling.
• China's aquatic ecosystems face significant nitrogen loading due to agricultural and industrial activities.

Purpose of the Study:

• To analyze long-term trends and spatial/system differences in benthic denitrification rates (DNR) across China's aquatic ecosystems.
• To compare China's DNR with global averages and identify factors influencing these rates.
• To estimate total nitrogen removal via denitrification in China's aquatic systems.

Main Methods:

• Compilation and analysis of 989 data points on benthic denitrification rates (DNR) over two decades.
• Comparison of DNR across different aquatic systems (rivers, lakes, estuaries & coasts, continental shelves).
• Spatio-temporal analysis of DNR trends and correlation with environmental factors like nitrogen fertilization and population density.

Main Results:

• Rivers exhibit the highest DNR, attributed to hyporheic exchange, nutrient supply, and suspended particles.
• China's average DNR is significantly higher than the global average, indicating elevated nitrogen inputs and lower nitrogen use efficiency.
• DNR shows a slight temporal decline, linked to national water quality recovery, but hotspots persist in coastal and downstream riverine areas.
• Nitrogen fertilization intensity and population density strongly correlate with increased DNR.

Conclusions:

• Aquatic denitrification plays a vital role in nitrogen removal in China, with significant regional variations.
• While water quality improvements have led to a slight decline in DNR, human activities remain a dominant driver.
• Future research should focus on larger spatial scales and long-term monitoring to understand nitrogen removal mechanisms under climate change.