Extending CO2 and CH4 footprints of created habitats through drainage ditches in a subtropical coastal region of southeastern China.

Man-made habitats such as sewage outfalls, farmlands and aquaculture ponds are known as hotspots for greenhouse gas production and emissions, but their climate influence extends out to neighboring areas through a network of drainage ditches, which are often overlooked in greenhouse gas assessments. This study analyzed the concentrations and diffusive fluxes of CO2 and CH4 in urban sewage ditches, agricultural ditches, and aquaculture ditches within a subtropical estuary in southeastern China. Urban ditches had higher combined concentration (μmol L-1) of CO2 (30.7) and CH4 (8.8) than agricultural and aquaculture ditches, likely because of their high dissolved organic carbon levels and mcrA gene abundances. The estimated diffusive flux was 0.2-0.5 mmol m-2 h-1 for CO2 and 30.1-258 μmol m-2 h-1 for CH4 among all three ditch types. Emissions tended to be highest in summer and lowest in winter. On average, dissolved CO2 and CH4 concentrations in ditch waters were equivalent to 18.9 % and 11 % of the respective dissolved gas concentrations in the sources they drained from (i.e. farmlands, sewage outfalls and aquaculture ponds), suggesting strong outgassing as the water entered the ditches. Overall, drainage ditches are stronger emission sources than natural rivers and streams.