Unravelling CH4 and N2O dynamics in tidal wetlands using natural abundance isotopes and functional genes

Abstract

Tidal wetlands are one of the major sources of CH4 and N2O in natural systems to the atmosphere; yet we still lack insights into the impact of their biogeochemical dynamics on the emissions of these greenhouse gases (GHGs). Here, we investigated the CH4 and N2O sources in four tidal wetlands ranging from freshwater to polyhaline with a focus on their production pathways. By using natural abundance isotopes and functional marker genes, we found that salinity level, sediment moisture content, quantity and quality of organic carbon (OC) and nutrients were major drivers of the wetland CH4 and N2O emissions. As the salinity levels decreased in the tidal wetlands, both the labile nature and concentration of nutrients and OC increased. These conditions favored methanogenesis as indicated in the abundance and expression of mcrA and the CH4 emissions from the freshwater wetland. Conversely, higher salinity depressed organic matter decomposition rates and microbial activities, causing much lower CH4 production in the saline wetlands. Isotope mapping revealed that denitrification contributed mainly to wetland N2O emissions (80–90%), reflected in the strong expression of denitrifier marker genes nirS, nirK, and nosZ and low nosZ:nir ratio. Nitrification played an important role in wetland N2O emissions at high NH4+ and low salinity levels. This condition was the case in the freshwater wetland – the strongest N2O emitter – where we found the highest NH4+ concentrations and the most abundant and expressed nitrifier marker genes amoA AOA and amoA AOB. Methanogen and denitrifier marker genes were more abundant and expressed at the surface layer compared to the subsurface layer, implying the presence of methane and denitrification paradoxes in the tidal wetlands. This study paves a way for the coupling of isotope and functional gene analyses to deeply explore GHG formation pathways and responsible microbial activities in dynamic wetland systems.