Vertical Variation of Methane Fluxes and Methanogenic Communities in Riparian Buffer of Urban Wetland Park at a Sponge City Park

Background. Wetland soil is one of the largest natural contributors to methane emissions, which is a prevalent greenhouse gas. The vertical emission pattern of methane in soils is unclear. To investigate the relationship between methane vertical emissions, soil total organic carbon and methanogen, methane fluxes, TOC and methanogen community dynamics were monitored in the riparian buffer of the sponge city wetland park from August 2018 to January 2020. Methods. The study area is the Living Water Garden (LWG), a wetland park in Chengdu, Sichuan Province, western China (3040 N, 10405 E). Methane flux was measured once per month (in mid-month) using a portable greenhouse gas flux measurement system (WS-L1820, WEST Ltd., Italy). The sampling frequency is one time on a selected day and the sampling time was between 11:00 am and 12:00 am. There are 6 selected monitoring sites with 4 monitoring depths including surface, 5cm, 10cm and 15cm. Soil sampling and methane flux measurements are performed on the same days. After methane flux monitoring, soil samples were collected. Results. During the study period, the average surface methane flux was 81.86 mg m-2 h-1 and ranged from 20.42 mg m-2 h-1 to 190.75 mg m-2 h-1. Cumulative methane emissions from studied area was 7.26 kg CO2eq m2 year1 and the global warming potential (GWP) was at a moderate level. The results reveal that the Methanobacteriales, Methanosarcinales, and Methanomirobiales were the major methanogenic microorganisms in riparian buffer of urban wetland at a sponge city park. The mathematical regression of methane flux (z, mg m-2 h-1) with soil depth (x, cm) and TOC (y, g kg-1) was as follows: z = 52860.66 + 54.44x - 2.96x2 - 26788.64y + 4487.80y2 - 249.34y3 (R2=0.82). It indicates that the relationship between methane flux, soil depth and TOC is non-linear and relatively high.