Gas exchange between soils and the atmosphere is an important contributory factor to global climate change due to increasing release of greenhouse gases (N₂O, CO₂, CH₄, etc.). CO₂ and CH₄ are the principal gases. Wetland is one of the important ecosystems for carbon cycle, especially for the CO₂ and CH₄ exchange. Wetland ecosystem emits CO₂ and CH₄ through aerobic and anaerobic respiration and fixes carbon (CO₂) through photosynthesis. So the ratio of CO₂ to CH₄ is important to reveal the greenhouse gas emissions of carbon sources and to mediate their emissions. An amount of approximately 18,000 km of coastline in China is covered by an estimated 12,000 km² of tidal estuary wetlands. These tidal wetlands are generally rich in animal and plant diversity and appear to have important biogeochemical roles within the entire estuary ecosystem. One of these important tidal wetland ecosystems is found within the Minjiang River estuary in southeast China, which is located at the transition of mid- and southern subtropical climate zones in China.
Vegetation and tide are two important factors influencing the biochemistry process of estuarine wetland and thereby controlling CO₂ and CH₄ emissions. Most of the previous studies have been showed that plants act as a key control on the spatial variability of CO₂ and CH₄ emissions due to their influence on the formation and transportation. Tidal systems are likely to be affected by tidal processes including time of inundation and salinity. Flooding is known to reduce CO₂ and CH₄ emissions, probably by blocking the pathway of CO₂ and CH₄ release. However, the effects of vegetation and tide on CO₂∶CH₄ values are not clear.
This study was conducted in the Shanyutan wetland (119°34'12″-119°40'40″E, 26°00'36″ -26°03'426″N), which is the largest tidal wetland (nearly 3120 hm²) in the Minjiang River estuary. The climate is warm and wet, with a mean annual temperature of 19.6℃ and a mean annual precipitation of 1346 mm. The Shanyutan wetland is belt-shaped. The sediment surface at the study site is submerged for 3-3.5 hours during each tidal inundation. The vegetation mainly occupies two zones: a 150-200 m wide Scirpus lacustris zone close to the sea, and a 150 m wide Phragmites australis and Cyperus malaccensis var. brevifolius zone which extends from the intertidal zone to near the bank.
Here we examined the stoichiometry characteristics of carbon-source greenhouse gas emissions in Minjiang River estuarine tidal wetland. The results showed that the emission fluxes of CO₂ in both Phragmites australis wetland and Cyperus malaccensis var. brevifolius wetland were positively correlated with the emission fluxes of CH₄. The monthly average CO₂∶CH₄ values in P. australis wetland and C. malaccensis wetland were 55.4 and 185.0, 96.3 and 305.5, and 68.7 and 648.6 in the stages of before high tide, during tidal fluctuation, and after ebb, respectively. But the monthly average CO₂∶CH₄ values were not significantly different in the three different stages for both P. australis wetland and C. malaccensis wetland (P>0.05). The lowest CO₂∶CH₄ values were found before high tide in both wetlands but with different responses to tide. The monthly average CO₂∶CH₄ values in the three stages for P. australis wetland were lower than those for C. malaccensis wetland (P<0.05). Our results indicated that the spatial variation of CO₂∶CH₄ values was greater than the temporal one in estuarine tidal wetland, and tide, vegetation type and temperature were the key factors controlling the variation of CO₂∶CH₄.