Nitrogen removal processes coupled with nitrification in coastal sediments off the north East China Sea

Purpose: Denitrification and anaerobic ammonia oxidation (anammox) play key roles in nitrogen (N) loss, and nitrification can supply substrates of NO₂^– and NO₃^– for denitrification and anammox. Coupled nitrification-denitrification/anammox processes are thus crucial for N removal in coastal ecosystems. This study aims to examine the spatial-temporal variations of ambient, coupled, and uncoupled N removal rates in the coastal sediments off the north East China Sea, and to clarify the controlling factors and microbial mechanisms of coupled nitrification-denitrification/anammox. Materials and methods: The rates of ambient, coupled, and uncoupled denitrification and anammox in coastal sediments off the north East China Sea were quantified using the continuous-flow experiments combined with ¹⁵N isotope pairing technique. The quantitative polymerase chain reaction method was used to determine the abundances of nitrifiers, denitrifiers, and anammox bacteria, with the functional genes of amoA and nirS, and 16S rRNA gene, respectively. Results and discussion: Ambient denitrification rates varied between 0.43 and 7.39 μmol N m⁻² h⁻¹, and ambient anammox rates ranged from 0.05 to 0.62 μmol N m⁻² h⁻¹. Coupled nitrification-denitrification was the dominant N removal pathway. The rates and coupling of N removal processes with nitrification varied distinctly between nearshore and offshore sites, which were driven by diverse environmental factors. Redundancy analysis suggested that nitrate and sulfide were important factors controlling the coupled and uncoupled N removal rates, and nitrate was proved to be the key factor influencing the ratio between coupled and uncoupled N removal via an integrated analysis. Abundances of ammonia oxidizing bacteria (AOB) correlated significantly with coupled denitrification rates and abundances of denitrifiers, suggesting the importance of AOB in coupled nitrification-denitrification. Conclusions: This study investigated the ambient, coupled, and uncoupled denitrification and anammox rates in coastal sediments off the north East China Sea. Nitrate was proved to be the critical factor influencing the ratio between coupled and uncoupled N removal, and AOB may play important role in coupled nitrification-denitrification. These results emphasized that nitrification is crucial for N removal with important implications on N loss in coastal ecosystems.