Effects Of Cyanobacterial Decay And Decomposition On N₂O Release Flux And Production Pathway

Nitrous oxide(N₂O),as an important greenhouse gas,has attracted wide attention.One of its main emission sources is freshwater lakes.However,most freshwater lakes in China are affected by cyanobacteria blooms,and the effects of cyanobacteria blooms decay and decomposition on the release of N₂O from lakes and their mechanisms still have not been systematically studied.In this study,Taihu Lake was selected to reveal the spatiotemporal N₂O emission flux in non-and cyanobacteria accumulation areas.The main influence factors of N₂O production and release were also analyzed through field investigation.Further,the N₂O production pathway was discerned during the cyanobacteria decay and decomposition by using the methods of stable isotopes in microcosm simulation.The main conclusions are as follows:(1)The N₂O emission flux and dissolved N₂O concentration in the water in cyanobacteria accumulation area of Taihu Lake were higher than those in non-cyanobacteria accumulation area.The average N₂O emission fluxes in the Zhushan Bay and the artificial reed ditch were 59.65±7.81?mol·m^-2·d^-1 and 27.85±3.91?mol·m^-2·d^-1,respectively.The average N₂O emission fluxes in the Xukou Bay and the lake central area were 3.36±0.59?mol·m^-2·d^-1 and 10.77±2.97?mol·m^-2·d^-1,respectively.The average concentrations of dissolved N₂O in water of Zhushan Bay and the artificial reed ditch were 0.049±0.012?mol·L^-1 and 0.051±0.010?mol·L^-1,respectively,which were about twice as high as that in the surface water of Xukou Bay and 9 times as high as that in the lake center area.(2)The decay and decomposition of cyanobacteria had a significant effect on the dissolved N₂O concentration in the water.The concentration and saturation of N₂O in the cyanobacteria accumulation area of Taihu Lake were higher than those in Xukou Bay and the lake central area without algae,indicating that the cyanobacteria accumulation area has higher N₂O release potential.The release flux of N₂O in the decay stage of cyanobacteria was significantly higher than that in the initial and outbreak stage,indicating that the relative increase of N₂O release flux was mainly caused by the decay and decomposition of cyanobacteria,while the release flux of N₂O in the outbreak stage of cyanobacteria was inhibited.(3)The decomposition process of cyanobacteria blooms had a significant impact on the basic physical and chemical properties of water bodies,releasing a large amount of nutrients into water bodies and promoting the decrease of DO and pH.The release flux of N₂O increased with the increase of the added cyanobacteria biomass,and the release rate of N₂O increased from 3.15±0.05?g·m^-2·h^-1 to 8.81±0.86?g·m^-2·h^-1 when the added cyanobacteria biomass increased by two times.The results showed that the high concentration of cyanobacteria blooms could promote the N₂O production rate higher than the consumption rate in the lake.(4)The decay and decomposition of cyanobacteria blooms influenced the production pathway of N₂O through the competitive effect of microbial nitrification and denitrification on nitrogen and the regulation effect on DO.During the decay and decomposition of cyanobacteria in Taihu Lake,the proportion of N₂O produced by nitrification increased from 42.9%to 53.4%.In the simulation experiment,the addition of cyanobacteria increased the proportion of the denitrification process from 51.5% to 63.2%.