Vegetation alters the effects of salinity on greenhouse gas emissions and carbon sequestration in a newly created wetland

• Vegetation altered effects of salinity on carbon equivalency in created wetlands.
• Low salinity wetland had high carbon emissions and high carbon absorption rate.
• High salinity inhibited greenhouse gas emissions, but also inhibited plant growth.

Wetland creation or restoration in degraded areas has become a new type of disturbance worldwide. Coastal wetlands serve a vital role in global carbon cycles; thus, it is important to understand the impacts of wetland creation on carbon storage functions. Carbon emissions and accumulation in wetlands are reported to be highly site-specific depending on factors such as salinity, plant type and productivity, and water table. This study investigated the effects of different salinities (<2‰, ∼5‰ and >10‰) on greenhouse gas emissions and carbon sequestration of created wetlands in the Yangtze River estuary. CH4 emissions significantly declined with increasing salinity, likely because of the higher sediment sulfate content at higher salinities. CO2 emissions were highest at intermediate salinities (∼5‰). In unvegetated sites, the absolute CO2 emission equivalent was 0.178 kg m−2 y−1 in the <2‰ salinity treatment, which was 8.09 times higher than the >10‰ salinity treatment. In vegetated sites, the <2‰ salinity treatment had the highest annual net flux of carbon. Thus, despite the high carbon emission of low salinity wetland, enhanced plant productivity resulted in a high carbon absorption rate. Overall, these results demonstrate that the presence of vegetation altered the effects of salinity on carbon equivalency in created wetlands. This study suggests that to conserve the wetland carbon sink function, landscape design for wetland restoration in estuarine regions should consider creating open water wetland in high salinity regions and restoring vegetation in low salinity regions to facilitate the growth of macrophytes such as Phragmites australis.