Relationship between denitrification and anammox rates and N 2 production with substrate consumption and pH in a riparian zone

Denitrification and anaerobic ammonium oxidation (anammox) are the key processes to quantitatively remove nitrate (NO₃^-) and balance the nitrogen (N) budget of the ecosystem. In this paper, a slurry-based ¹⁵N tracer approach was used to study the correlation and quantitative relation of substrate consumption and pH with rates of denitrification and anammox in a riparian zone. The results showed that the fastest rates of 0.93 µg N h^-1 and 0.32 µg N h^-1 for denitrification (Denitrif-N₂) and anammox (Denitrif-N₂), respectively. N₂ produced by denitrification occupied 74.04% and produced by anammox occupied 25.96% of the total N₂, proving denitrification is the dominant process to remove NO₃^-. The substrate content (NO₃^-, NH₄⁺ and TOC) and pH varied during incubation and were significantly correlated with Dentrif-N₂ and Anammox-N₂. Nitrate and TOC as the substrates of denitrification demonstrated a significant correlation with Anammox-N₂, which was associated with the products of denitrification involved in the anammox process. This proved a coupling of denitrification and anammox. A quantitative relationship was observed between Dentrif-N₂ and Anammox-N₂ in the range of 2.75-2.90 when TOC, NH₄⁺ and NO₃^- consumption per unit mass or pH changed per unit. Nitrogen mass balance analysis showed that 1 mg N substrate (NO₃^-+NH₄⁺) consumption in the denitrification and anammox can produce 1.05 mg N₂ with a good linear relationship (r² = 0.9334). This could be related to other processes that produced extra N₂ in denitrification and anammox system.