Investigation Of Greenhouse Gases Emission Pattern And The Main Influential Factors In Coastal Wetland In Yancheng, Jiangsu Province

Coastal wetlands in the parallel zone affected by the ocean and land are vulnerable ecologically sensitive regions, and it plays an important role in the process of global climate change. For the coastal protection and siltation, Spartina alterniflora has been introduced into China from North America since 1980s, and after that, it has became the invasive species in China’s coastal line, and consequently, introduced deep consequences on structure and function of local ecosystem. The invasion probably significantly increases the soil carbon content in coastal wetland, and alleviates the contribution to global warming. Therefore, further understanding about the greenhouse gases emission pattern and its influential factors between terrestrial ecosystem and atmosphere, and the difference in soil carbon or nitrogen between invasive S. alterniflora and native species (S. salsa and P. australis) is extremely important for understanding the greenhouse gases budget in coastal wetland after invasion of.S. alterniflora.We settled sampling sites in each plant community, and sampled greenhouse gases monthly by using static chamber-GC technique for one year in Yancheng National Nature Reserve. Meanwhile, three treatments have been conducted in each community which was transparent chamber, opaque chamber and clipping treatment, respectively. We also sample the aboveground biomass at the end of growing season inorder to studying the effects of plant on greenhouse gases emission. The environmental factors, including air temperature, soil temperature, surface water depth, soil moisture and PAR also have been measured when the gases were taking in order to studying the effects of environmental factors on emission by regression analysis. Furthermore, we took soil samples from 0-15cm layer underground in May, August, December in 2011 and March in 2012 to analysis the difference in TN,TC and SOC content between S. alterniflora and native species. We also discussed the possible mechanisms that changed the soil carbon and nitrogen content by integrating soil and CO2 emission. Our main results are:(1) Based on the CO2 emission, we found that there is no significant difference in NEE between S. alterniflora and P. australis communities, but both of them were significantly lower that that from mudflat and S. salsa community; ecosystem respiration rate and GPP in S. alterniflora community was significantly higher than that in other communities. NEE was negatively significantly correlated to air temperature, soil moisture, and surface water (P<0.05), while the ecosystem respiration was positively significantly correlated to temperature, soil moisture and PAR (P<0.05). There were no significant difference in soil respiration across communities, and the soil respiration was not significantly correlated to any environmental factor because of the inhibition effect from inundation.(2) The increase of TN, TC and SOC following the invasion of S. alterniflora was determined by three possible mechanisms. First of all, S. alterniflora prefers to allocating more biomass underground which is a more continuous and direct carbon source for soil. Second, the surface water inhibited soil respiration process in wetland, which contributed to the insignificant difference across communities and the insensitive to variation of environmental factors, and kept the stability of soil carbon pool. Last, S. alterniflora has a significantly higher GPP than native species which could allocate more photosynthetic product into soil potentially, and thereby, promote accumulating of TN, TC and SOC.(3) Coastal wetland was mainly expressed as a CH4 emitting source. The biomass provided the substrate for methangentic bacteria in soil, and therefore promoted CH4 generation. The vascular plants influenced CH4 emission significantly by acting as a high efficient transportation path. However, that S. salsa suppressed CH4 generation and transportation processes by exudating O2 into rhizosphere.(4) Temperature and surface water are the two main influential factors in determining CH4 emission. It is because, on the one hand, the methangentic bacteria responded to temperature faster; and, on the other hand, the surface water created a better anaerobic environment in soil, and therefore promoted the CH4 generation and emitting process.(5) The coastal wetland was mainly expressed as N2O absorbing sink. The results indicated that the plants suppressed the N2O generation in soil by competing NH4+ and NO3- with nitrification and denitrification process in soil.(6) There was significantly negatively relationship between N2O emission from transparent/opaque chambers and surface water, which probably because the anaerobic environmental created by inundation promoted the denitrification process in soil, and thereby converted N2O to N2. The absence of significant relationship between N2O emission under clipping treatment and surface water was due to the competition effect for nitrogen from plant was more influential than inhibition effect from surface water.(7) In our study, the ecosystem respiration, CH4 and N2O emission in costal wetland were 3358.5 g m-2 yr-1,23.2 g m-2 yr-1 and -76.7 mg m-2 yr-1, respectively。 The ecosystem respiration and CH4 emission rate in this study was comparable to other studies. But, it is worth noting that the CH4 emission had a clear seasonal pattern, and the low sampling frequency may cause underestimation. The N2O emission in our study was lower than others, which probably because plant inhibited N2O generation process by competing nitrogen in soil, and the inundation promoted N2O deoxidation by denitrificantion process. (8) We found that S. alterniflora is the main contributor, up to 55.1%,to the total CO2e in coastal wetland by converting greenhouse gases (CO2, CH4 and N2O) into CO2e, and the CO2 is still the main contributor, up to 80.5%-85.8%, to total CO2e among CO2, CH4 and N2O.