Seasonal and diurnal variations of greenhouse gas emissions from a saline mangrove constructed wetland by using an in situ continuous GHG monitoring system

Wetland systems play important roles in the issues of global warming. That is because wetlands can not only intake carbon and nitrogen in the plants and sediments, but could also release carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) to the atmosphere during the microbial decomposition processes in the water. In the past, greenhouse gases (GHGs) were measured mostly by using the manual sampling technique; however, it is difficult to measure the diurnal variation of greenhouse gas emissions. In this study, a floating chamber used to collect GHGs, which were then in-site monitored with a continuous GHG analyzer, through a Teflon tube connected to the top of the chamber was designed. The data for seasonal and diurnal variations of GHGs, emission fluxes of GHGs, and equivalent carbon dioxide emissions (CO2-e) were measured to explore the extent of a saline mangrove constructed wetland contributing to global warming. In addition, the correlation and regression analysis among greenhouse gas emissions, water quality, and other environmental factors were analyzed statistically. The results of continuous monitoring of GHGs showed that the concentrations of CO2 emitted from a saline mangrove constructed wetland ranged from 383.5 +/- 25.9 to 476.8 +/- 24.2 ppm. The diurnal variation of GHG concentrations was significant, which showed that the GHG concentrations in the daytime were generally lower than those at nighttime. The emissions of methane (CH4) from the wetland were monitored in a range between 3.7 +/- 1.4 and 28.6 +/- 7.6 ppm, while the concentrations of nitrous oxide (N2O) emitted from the wetlands ranged from 3.1 +/- 2.6 to 0.9 +/- 0.6 ppm. Hence, the diurnal variations of CH4 and N2O concentrations showed higher values in the daytime than those at nighttime. The correlation analytical results among GHG emissions, water quality, and other environmental factors presented that the emission fluxes of CO2 and BOD5 were positively correlated, but a negative correlation was shown for global solar radiation (GSR). Moreover, the concentrations of CH4 and N2O had moderately positive correlation with both air temperature and water temperature.