Saltmarsh sediments with wastewater input emit more carbon greenhouse gases but less N2O than mangrove sediments

Wastewater discharge and salt marsh invasion are two main disturbances in global mangrove wetland ecosystems, but their combined effects on greenhouse gas (GHG) emissions remain unclear. Here, we performed a laboratory incubation experiment by adding wastewater (aquaculture and domestic wastewater) to mangrove and salt marsh sediments. Dissolved GHGs (CO2, CH4 and N2O) and environmental parameters were measured over a time series of 50 days. The nitrification and denitrification rates and corresponding functional gene abundances (AOA, AOB, nirS, and narG) were also quantified. The results show that wastewater addition significantly increased GHG emissions and global warming potential (GWP) in wetland sediments (p < 0.05 for both). Compared with mangroves, salt marsh sediments produced more carbon GHGs (CO2 and CH4) but less N2O after wastewater input. These different stimulatory effects could be explained by the nitrogen and carbon substrate availability, organic matter quality, and functional gene abundances in sediments. Assessing GHG emissions as CO2 equivalents, future scenarios of increasing salt marsh invasion with wastewater addition would increase the GWP caused by CO2 and CH4, decrease blue carbon sequestration potential in wetlands, and decrease the GWP caused by N2O and total GWP, indicating a great impact on global climate change and important implications for managing coastal wetlands.