Greenhouse gas emission characteristics from urban rivers in Shanghai

The floating chamber and diffusive models were used to investigate the differences of greenhouse gas (N₂O, CH₄ and CO₂) fluxes between an urban river (Suzhou River) and a suburban river (Dianpu River) in Shanghai during April, July and October in 2013 and January in 2014.The influence of environmental variables on greenhouse gas emissions under the disturbance of anthropogenic activities was also discussed. The results demonstrated that the two rivers were main sources of N₂O, CH₄ and CO₂. N₂O and CH₄ fluxes from the urban river were one to two magnitudes higher than those from the suburban river. CO₂ fluxes from the two rivers were at the same magnitude. Estimated annual N₂O, CO₂ and CH₄ fluxes by the diffusive model were 15.88, 6748.27 and 84.98 μmol/(m²ï¿½h) in Suzhou River, and 0.61, 2978.98 and 9.61 μmol/(m²ï¿½h) in Dianpu River. Estimated annual N₂O, CO₂ and CH₄ fluxes by the floating chamber method were 15.77, 4041.61 and 6721.08 μmol/(m²ï¿½h) in Suzhou River, and 0.60, 1214.77 and 59.58 μmol/(m²ï¿½h) in Dianpu River. Significant correlations between dissolved oxygen, nitrate, ammonium and N₂O fluxes indicated that the enrichment of dissolved inorganic nitrogen and depletion of oxygen might enhance microbial denitrification, resulting in high N₂O fluxes from the urban river. At the same time, high emissions of CH₄ occurred in hypoxia conditions in the urban river. Water temperature greatly controlled CO₂ fluxes, probably through stimulating the microbial activities of CO₂ production. The difference between floating chamber and diffusive model fluxes of CH₄ indicated that the nitrogen-rich and oxygen-deficit conditions in urban river were favorable for bubble events, which could enhance CH₄ emission.