Carbon dioxide and methane fluxes from different surface types in a created urban wetland

[摘要] Many wetlands have been drained due to urbanization, agriculture, forestry or other purposes, which has resulted in a loss of their ecosystem services. To protect receiving waters and to achieve services such as flood control and storm water quality mitigation, new wetlands are created in urbanized areas. However, our knowledge of greenhouse gas exchange in newly created wetlands in urban areas is currently limited. In this paper we present measurements carried out at a created urban wetland in Southern Finland in the boreal climate. We conducted measurements of ecosystem CO 2 flux and CH 4 flux ( F CH 4 ) at the created storm water wetland Gateway in Nummela, Vihti, Southern Finland, using the eddy covariance (EC) technique. The measurements were commenced the fourth year after construction and lasted for 1 full year and two subsequent growing seasons. Besides ecosystem-scale fluxes measured by the EC tower, the diffusive CO 2 and CH 4 fluxes from the open-water areas ( F w CO 2 and F w CH 4 , respectively) were modelled based on measurements of CO 2 and CH 4 concentration in the water. Fluxes from the vegetated areas were estimated by applying a simple mixing model using the above-mentioned fluxes and the footprint-weighted fractional area. The half-hourly footprint-weighted contribution of diffusive fluxes from open water ranged from 0 % to 25.5 % in 2013. The annual net ecosystem exchange (NEE) of the studied wetland was 8.0 g C- CO 2  m −2  yr −1 , with the 95 % confidence interval between −18.9 and 34.9 g C- CO 2  m −2  yr −1 , and F CH 4 was 3.9 g C- CH 4  m −2  yr −1 , with the 95 % confidence interval between 3.75 and 4.07 g C- CH 4  m −2  yr −1 . The ecosystem sequestered CO 2 during summer months (June–August), while the rest of the year it was a CO 2 source. CH 4 displayed strong seasonal dynamics, higher in summer and lower in winter, with a sporadic emission episode in the end of May 2013. Both CH 4 and CO 2 fluxes, especially those obtained from vegetated areas, exhibited strong diurnal cycles during summer with synchronized peaks around noon. The annual F w CO 2 was 297.5 g C- CO 2  m −2  yr −1 and F w CH 4 was 1.73 g C- CH 4  m −2  yr −1 . The peak diffusive CH 4 flux was 137.6 nmol C- CH 4  m −2  s −1 , which was synchronized with the F CH 4 . Overall, during the monitored time period, the established storm water wetland had a climate-warming effect with 0.263 kg  CO 2 -eq m −2  yr −1 of which 89 % was contributed by CH 4 . The radiative forcing of the open-water areas exceeded that of the vegetation areas (1.194 and 0.111 kg  CO 2 -eq m −2  yr −1 , respectively), which implies that, when considering solely the climate impact of a created wetland over a 100-year horizon, it would be more beneficial to design and establish wetlands with large patches of emergent vegetation and to limit the areas of open water to the minimum necessitated by other desired ecosystem services.

[学科分类] 大气科学