Reactive nitrogen enrichment has become a key pressure for freshwater ecosystems. A new study published in the Journal of Hazardous Materials reveals that rare-earth-element-rich, high-nitrogen mining drainage (REEs-HNMD) weakens river nitrogen-sink function by suppressing sediment denitrification, causing mining-impacted rivers to shift from nitrogen pollution buffers into nitrogen transport corridors.

Led by Associate Researcher XUAN Yingxue from the South China Botanical Garden (SCBG) of the Chinese Academy of Sciences (CAS), under the guidance of Professor LU Xiankai, the research provides key insights into how combined rare-earth enrichment and high nitrogen loading alter natural nitrogen removal processes in mining watersheds. The findings highlight an urgent need to protect and restore sediment nitrogen-sink functions in rare-earth mining watersheds.

The work was funded by the National Natural Science Foundation of China and the National Key R&D Program of China.

Rivers normally serve as important nitrogen sinks by permanently removing reactive nitrogen and mitigating watershed-scale pollution. Previous studies have confirmed that coupled nitrification, denitrification and algal assimilation can sustain nitrogen-sink capacity in high-nitrogen urban rivers, though with potential N₂O emission risks.

Different from ordinary urban pollution, ion-adsorption rare-earth mining generates distinctive REEs-HNMD, driven by residual ammonium from in-situ leaching, rare-earth element mobilization, water acidification and sediment disturbance. It remained unclear whether rivers under such combined stress could still maintain their natural nitrogen-sink function.

To solve this problem, the team investigated a typical rare-earth mining watershed by integrating ¹⁵N isotope tracing, nitrate dual isotopes, microbial community analysis, denitrification functional genes and structural equation modeling.The results showed that denitrification dominates permanent nitrogen removal, while anammox plays only a minor role.

Sediment denitrification responds nonlinearly to REEs-HNMD: moderate pollution stimulates denitrification by increasing nitrogen substrate availability, whereas heavy pollution strongly inhibits denitrification under the combined pressure of rare-earth enrichment, excessive nitrogen input and sediment disturbance. The study further demonstrates that rare-earth mining drainage not only acts as a nitrogen source but also impairs microbial nitrogen removal in sediments, lowering rivers’ natural pollution buffering capacity.