Influence of elevated CO₂ on nitrification and denitrification in water bodies: A review

Elevated atmospheric CO₂ has imperceptible impacts on carbon cycle in aquatic ecosystems. However, it remains a question how this process will impact nitrogen cycle that is naturally coupled with carbon cycle. The nitrification and denitrification are two critical processes in the nitrogen cycle. It is reasonable to expect that elevated atmospheric CO₂ will influence both processes. We reviewed the previous literatures concerning the effects of elevated atmospheric CO₂ on the physico-chemical properties, nitrification, denitrification and nitrogen transformation in water bodies. The published results revealed that the elevated CO₂ would reduce the water pH, increase CO₂ and HCO₃^- concentrations, but with different effects on the nitrification and denitrification between eutrophic and oligotrophic water. Elevated atmospheric CO₂ could inhibit nitrification and denitrification in oligotrophic water, thereby reduce N₂O flux from water. The nitrification process in the eutrophic water bodies was also inhibited, but its denitrification might be promoted by the elevated CO₂. In the eutrophic water bodies, there could be an increase of N₂O flux when pH was maintained in the range of 7-9. These might eventually result in the accumulation of NH₄⁺ and reduction of NO₃^- in water, producing an impact on the microbial diversity. Based on these reviews, we proposed some research gaps related to the relevant research fields as well as some scientific questions that is worth to be further explored. This review would be helpful to better understanding on how the greenhouse effect caused by the elevated atmospheric CO₂ would affect nitrogen cycle in aquatic ecosystem.