Temporal changes in the microbial nitrogen cycling communities in intertidal estuarine seagrass ecosystems

Coastal vegetated habitats have recently begun to gain attention for their potential to act as greenhouse gas sinks. However, so far, this research has largely focused on the microbial driven carbon dioxide (CO2) flux in saltmarshes, whilst the more potent greenhouse gas (GHG) nitrous oxide (N2O) has often been overlooked. The overarching aim of this study was to measure the temporal changes in the microbial communities driving the cycling of nitrogen (N) in estuarine seagrass (Zostera noltei) meadows on the coast of East Anglia, United Kingdom, and relate to sediment nutrient concentrations and N2O flux. This study found that seagrass meadows harboured higher abundances of archaeal and bacterial 16S rRNA genes compared to non-vegetated sediments. Ammonia-oxidation was largely dominated by ammonia-oxidising bacteria (AOB) rather than archaea (AOA) throughout the year. Specifically, greater AOA abundance was found in the rhizosphere in spring, whereas greater AOB abundance occurred in non-vegetated anoxic sediments in winter. Nitrite (NO2-) reduction was potentially being driven by nirS rather than nirK gene associated communities. Additionally, the nosZ gene was significantly higher than either nirS or nirK regardless of seagrass presence, but significantly lower than the two genes combined. N2O concentration was negligible in winter, but drastically increased in the spring. Furthermore, non-vegetated habitats sunk N2O at a higher rate than seagrass habitats. However, more importantly, there was a decrease in N2O in both habitats over time. Eutrophicating nutrients (e.g. nitrates, ammonium) were also found at higher concentrations in the seagrass sediments. These findings suggest that Zostera noltei seagrass meadows can act as a sink for N2O due to the overall reduction in N2O emissions and may work as a nature-based solution to reduce GHG emissions and estuarine eutrophication as part of the wider estuarine ecosystem function.