Contrasting impact of elevated atmospheric CO2 on nitrogen cycle in eutrophic water with or without Eichhornia crassipes (Mart.) Solms

Abstract The elevation of atmospheric CO 2 is an inevitable trend that would lead to significant impact on the interrelated carbon and nitrogen cycles through microbial activities in the aquatic ecosystem. Eutrophication has become a common trophic state of inland waters throughout the world, but how the elevated CO 2 affects N cycles in such eutrophic water with algal bloom, and how vegetative restoration helps to mitigate N 2 O emission remains unknown. We conducted the experiments to investigate the effects of ambient and elevated atmospheric CO 2 (a[CO 2 ], e[CO 2 ]; 400, 800 μmol﹒mol −1 ) with and without the floating aquatic plant, Eichhornia crassipes (Mart.) Solms, on N-transformation in eutrophic water using the 15 N tracer method. The nitrification could be slightly inhibited by e[CO 2 ], due mainly to the competition for dissolved inorganic carbon between algae and nitrifiers. The e[CO 2 ] promoted denitrification and N 2 O emissions from eutrophic water without growth of plants, leading to aggravation of greenhouse effect and forming a vicious cycle. However, growth of the aquatic plant, Eichhornia crassipes , slightly promoted nitrification, but reduced N 2 O emissions from eutrophic water under e[CO 2 ] conditions, thereby attenuating the negative effect of e[CO 2 ] on N 2 O emissions. In the experiment, the N transformation was influenced by many factors such as pH, DO and algae density, except e[CO 2 ] and plant presence. The pH could be regulated through diurnal photosynthesis and respiration of algae and mitigated the acidification of water caused by e[CO 2 ], leading to an appropriate pH range for both nitrifying and denitrifying microbes. Algal respiration at night could consume DO and enhance abundance of denitrifying functional genes ( nirK , nosZ ) in water, which was also supposed to be a critical factor affecting denitrification and N 2 O emissions. This study clarifies how the greenhouse effect caused by e[CO 2 ] mediates N biogeochemical cycle in the aquatic ecosystem, and how vegetative restoration mitigates greenhouse gas emission.