Nitrous oxide (N₂O) and methane (CH₄) are key radiatively active greenhouse trace gases which have been recognized to contribute global warming by 5% and 25%, respectively. Quantification of the trace gases is a subject of great interest because accurate information is required to determine the contribution of these gases to global greenhouse gas fluxes. Coastal marsh ecosystem is characterized by high temporal and spatial variations involved with topographic feature, environmental factors and astronomic tidal fluctuation. Above all, the intertidal zone between terrestrial and aquatic coastal ecosystems may represent a high dynamic interface of intense material processing and transport, with potentially high decomposition and associated greenhouse gases emission.

In the past two decades, considerable efforts have been made to quantify the N₂O and CH₄ fluxes in different coastal ecosystems. In China, the studies on N₂O and CH₄ emissions from coastal marshes started quite late (in the 2000s), and the related research primarily focused on the coastal tundra marshes in Antarctica and the coastal marshes in the Yangtze River estuary and the Min River estuary.

However, few studies have reported on coastal marshes in northern estuaries, such as Liao River estuary and Yellow River estuary. In order to quantify the N₂O and CH₄ fluxes from the intertidal zone, evaluate the potential influences of exogenous nitrogen (N) loading on emissions, and estimate the annual N₂O and CH₄ inventories, systematic studies have been done by SUN Zhigao’s group and the latest research results have been reported in the journals of Chemosphere, Plant and Soil and Ecological engineering.

This study identified the spatial and temporal variations of N₂O and CH₄ emissions and determined the key affecting factors based on two years in situ data. Dissimilar to previously published results, seasonal variations of N₂O fluxes were not dependent on temperatures, but were related to the limited carbon (C) and mineral N in sediments during summer and autumn and the frequent freeze/thaw cycles in surface sediments during spring and winter. Seasonal variations of CH₄ fluxes were also not dependent on temperatures. CH₄ emissions were related to the interactions of abiotic factors (sediment moisture and salinity) and the variations of limited C and mineral N in sediments. For another, they were closely correlated with sulfur (S) biological cycling.

The total sulfur (TS) and sulfate (SO₄^2-) were abundant in sediments and the S biological cycle coefficients were high (0.8625~0.9014), indicating that both SO₄^2- dissimilatory reduction process (performed by sulfate reducing bacteria, SRB) and microbially mediated AOM (anaerobic oxidation of methane) process (performed by a consortium of anaerobic methanotrophic archaea (ANME) and SRB) would be enhanced under anoxic condition, which further inhibited CH₄ emissions. From the perspective of C and S coupling, the mechanism could better explain why the CH₄ emissions across the intertidal zone were very low.

It also evaluated the global warming potential (GWP) of N₂O and CH₄ emissions and the effects of N loading on emissions. Dissimilar to previous results, large spatial variations of N₂O and CH₄ fluxes (especially CH₄) across the intertidal zone were observed (158.3% and 7856.3%, respectively). Also, the importance of seasonal N₂O or CH₄ contributions (particularly during non-growing season) to the estimation of local N₂O or CH₄ inventory was highlighted. This study also showed that N₂O and CH₄ emissions from the intertidal zone would be enhanced with increasing N loading to the Yellow River estuary.

Professor Bodelier (Institute of Ecology of the Royal Netherlands Academy of Arts and Sciences) considered this as “an important innovation research to accurately estimate the C emission from intertidal zone”, and it also put forward some important advices for estimating N₂O and CH₄ inventories in other estuaries. Next step, the related studies should emphasize the importance of seasonal N₂O and CH₄ contributions (particularly during non-growing season) to the estimation of local inventories, and synthetically consider both the large spatial variations of N₂O and CH₄ fluxes across the intertidal zone and the potential priming effects of exogenous N loading to the Yellow River estuary on N₂O and CH₄ emissions.