Further increasing nitrogen concentration does not enhance the denitrification and DNRA potentials in eutrophic reefs: Insights from the differences in benthic categories

Low nitrogen (N) availability is critical to coral health and resilience. Dissimilatory nitrate reduction processes, such as denitrification, anammox, and dissimilatory nitrate reduction to ammonium (DNRA), play a vital role in regulating seawater N content. Thus, quantifying the rates of these processes is essential for assessing the overall health and recovery potential of N-enriched coral reefs. However, the diversity and complexity of benthic categories in coral reefs severely limit our understanding of the overall nitrate reduction capacity of these ecosystems. To address this, we utilized the ¹⁵N isotope tracing technique to assess denitrification, anammox, and DNRA for various benthic categories, including live corals, turf algae, sediments, biogenic rock, and coral rubble in the coral reefs of Weizhou Island. Our findings indicate that, except for sediments, the denitrification potential (0.1–5.6 mmol N m⁻² d⁻¹) significantly exceeded that of DNRA (0.1–1.8 mmol N m⁻² d⁻¹), while anammox (0.01–0.10 mmol N m⁻² d⁻¹) was negligible. There was a significant difference in denitrification rate among benthic categories, with massive corals Galaxea fascicularis and biogenic rock exhibiting the highest rate. Extrapolated to the respective reef area, both live corals and biogenic rock contributed the most to the integrated rates of denitrification and DNRA (27–41%). However, no significant change in the rate was observed when the nitrate concentration increased from 10 to 20 μM. Consequently, it is crucial to strictly control N emissions in N-enriched reefs impacted by human activities, while also prioritizing the protection of benthic categories that significantly contribute to denitrification.