Nitrous Oxide And Methane Emission And Influence Of Sediment Microbial Communities From Rivers In Shanghai

Nitrous oxide (N2O) and methane (CH4) are important greenhouse gases. The increasing concentration of N2O and CH4 may have a huge impact on climatic change. The rapid urbanization process leads to an increasing loading of nitrogen and carbon in river ecosystem, which has been a potential strong source of N2O and CH4. Nitrification, denitrification, methanogenesis and methane-oxidation are important microbial process involving the biogeochemical cycle of N2O and CH4. Urbanization could affect the organic matter, nutrition and other inorganic pollutants, which could lead to the alteration of microbial community diversity, composition and function. Large amounts of studies focusing on the response of microbes to the environmental alteration, but few connects the microbial diversity and composition with N2O and CH4 emissions to investigate the influence of microbial communities on N2O and CH4 emissions with different carbon and nitrogen loadings in river ecosystem.To fulfill the aim above, this paper launched a research in several representative rivers in Shanghai river network were studied. Using diffusion model, floating chamber method and in lab simulated incubation, N2O and CH4 emissions at water-air and sediment-water interface were measured. Microbial community diversities and structures were analyzed by 454 pyro-sequencing. Corresponding environmental variables were also measured. Main conclusions were as followings.(1) Rivers in Shanghai are important sources of N2O and CH4. During April of 2013 and January of 2014, the annual fluxes of N2O and CH4 at water-air interfaces were 683.94±354.32～13247.82±11849.66nmol/m2/h and 5.91±7.43～148.94 ± 171.43μmol/m2/h. N2O and CH4 fluxes were high in nitrogen-rich and oxygen-deficit rivers like FX, SJ, JS and PT, and relatively low in nitrogen-low and oxygen-rich rivers like CM, QP, JD and PD. N2O and CH4 annual fluxes in PT site were higher than those in QP site from 10 to 100 times.(2) Nitrate and dissolved oxygen are important factors affecting the spatial differences of N2O and CH4 fluxes at water-air interfaces. The enrichment of carbon and nitrogen, and hypoxia could result in high fluxes of N2O and CH4. N2O emissions correlate with nitrate, temperature and dissolved oxygen significantly, while CH4 emissions show weak relationship with the environmental variables. In the meantime, the nutrient-rich and oxygen-deficit river is favorable to the production and emission of bubbles, which will enhance the emission of N2O and CH4. The random ebullition events may cover the seasonal patterns of diffusion fluxes, and show weak correlation between floating chamber fluxes and diffusive fluxes. Otherwise, the weak correlations between N2O floating chamber fluxes and nitrate, and CH4 floating chamber fluxes and dissolved oxygen, ammonium indicate that ebullition events could weaken the impact of environmental factors on N2O and CH4 emission at air-water interfaces.(3) Rivers sediments are sources of N2O and CH4. During April of 2013 and January of 2014, the annual fluxes of N2O and CH4 at sediment-water interfaces were 533.32±719.91～1512.17±1444.72 nmol/m2/h and 22.82±13.33～334.77±134.233 μmol/m2/h. N2Ofluxes at sediment-water interfaces were high in rivers with high nitrogen and carbon loadings like FX, SJ, JS and PT (895.61～1512.17 nmol/m2/h), and relatively low in rivers with low nitrogen and carbon loadings like CM, QP, JD and PD (537.96-690.06 nmol/m2/h). CH4 fluxes at sediment-water interfaces were high in ammonium-rich rivers like PT(334.77μmol/m2/h), relatively low in rivers with moderate concentration of ammonium like JS, SJ, JD, PD and QP (123.39-170.66μmol/m2/h) and low in rivers with low-ammonium like CM and FX (22.82 and 61.87μmol/m2/h).(4) In this study, the spatial differences of N2O flux at sediment-water interfaces are mainly controlled by organic carbon, but not nitrate. Nitrate and organic carbon are both substrates of microbial denitrification, which is an important pathway for N2O production. In nitrate-rich rivers, nitrate are over-supplied, thus the availablility of dissolved organic carbon become the limiting factor for denitrification, exhibiting as a stronger consumption of nitrate with increasing concentration of dissolved organic carbon. CH4flux at sediment-water interfaces has a strong relevance to ammonium. The hypoxia may be the link of the two phenomena. The nitrogen-rich rivers may have strong mineralization, resulting in accumulation of ammonium in the sediment surface and consumption of oxygen. Such conditions would be in favor of ammonium diffusion and microbial production and emission of CH4.(5) In river with high nitrogen and carbon loadings, the diversity of denitrifiers, nitrifiers and methanogenesis communities, and the richness of denitrifies and nitrifies communities were high in sediments. The dominant denitrifiers (Dechloromonas) and methanogenesis (Methanospirillum) in genus level were both abundant in the sediment surface, especially in the heavy-polluted river. The dominant methane-oxidizing bacteria (Methylocystis) in genus level was accumulated in the deeper layer of river sediments. The dominant genus of nitrifying bacteria were different in rivers with different nitrogen and carbon loadings, but accumulated in the surface layer of sediments.(6)In river with high nitrogen and carbon loadings, the accumulation and emission of N2O and CH4 in river sediments have a relevance to the abundance of denitrifies, nitrifiers and methanogenesis, in particular of the dominant genus, which indicates that the nitrogen and carbon loadings may enhance N2O and CH4 emission at sediment-water interfaces through stimulating the growth of dominant species.