The Mechanism Of Plant Effect On Greenhousegas Emissions In Yangtze Estuary Wetland

The global warming as well as the impact on the future global change induced by the increases of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) concentrations is one of the most important global environmental problems that the word-wide governments and scientists pay close attentions. Wetland that is the important ecological buffer zone between the terrestrial ecosystem and aquatic system, often become an important source of greenhouse gases, and is closely related to global change. The native species of Scirpus mariqneter (S. mariqueter), Phragmites and the invasive species of Spartina alterniflora (S. alterniflora) widely distributed in the intertidal zone of Yangtze Estuary, which was the third largest estuary in the world. Although several researches on greenhouse gas (GHG) emissions in Liaohe Estuary, Yellow River Estuary, Min River Estuary et al. were carried out, the special research on plants’effects on GHG is rarely reported. The existed research on plants’effects on GHG emissions word-wide also mainly focused the CH4 emissions, and the comprehensive effects of plants on both CH4 and N2O were rarely reported.This research was supported by the Key project of basic research in Shanghai "The emissions and response and feedback of CH4 and N2O to eco-environment evolution" and the Youth fund project of national natural science fund "The mechanism of N2O production and emissions of river network in Yangtze river delta". The best protected and most complete Dongtan wetland was chosen as the research object. The complex and multi-disciplinary methods including environment science, ecology and geography were adopted in the research. The emission and mechanism of the GHG was systematically studied by using the complex methods of field monitoring, sampling, laboratory experiment and indoor simulation. The following four parts of contents were carried out during the four years’research from 2011 to 2014. Firstly, the emission GHG fluxes of different plant communities were monitored; Secondly, the contribution of the three different plant species to GHG emissions were quantified by carrying out the field experiment of clipping and pulling out the plants; Thirdly, the pore-water GHG concentrations in the sediment covered by different plant species were measured systematically and its implications for GHG fluxes were also analysed; Lastly, the production rates and consumption potential of GHG in the sediment were also measured. By comprehensively considering both the production dynamics and indoor simulation experiment, the mechanism of plants’ effects on GHG emissions in Yangtze Estuary was systematically revealed. The main conclusions of the research are as follows:(1) The annual average CO2 fluxes in Phragmites marsh is -103.06 mgCO2·m-2·h-1, which acts as a net sink of atmospheric CO2; The annual average CO2 flux in S. alterniflora marsh is 295.93 mgCO2·m-2·h-1 acting as the net source of atmospheric CO2; The high-frequency observation for S. mariqueter marsh revealed that it acted as a net source of atmospheric CO2 from February to May and the average flux during this period is 83.25 mgCO2·m-2·h-1. In the vigorously growing season, the community acted as a CO2 sink with the average flux reaching-31.76 mgCO2·m-2·h-1. The CH4 emission fluxes across the sediment-air interface under different plant species all reached to the peak value in summer. The annual average CH4 fluxes in Phragmites, S. alterniflora, and S. mariqueter marshes are 2.65mgCH4m·m-2·h-1,0.83mgCH4m·m-2·h-1 and 1.08mgCH4m·m-2·h-1 repectively. The N2O fluxes across the sediment-air interface in Yangtze Estuary exhibited frequent change from the source and sink. The N2O emission fluxes in Phragmites marsh ranged from-15.73μgN2O·m-2·h-1～10.13μgN2O·m-2·h-1 and the annual average flux is-0.26μgN2O·m-2·h-1; The N2O emission fluxes in S. alterniflora marsh ranged from-1.32μgN2O·m-2·h-1～14.75μgN2O·m-2·h-1 and the annual average flux is 3.98μN2O·m2·h-1; N2O fluxes in S. mariqueter marsh showed the biggest emission range from -94.35μN2O·m-2·h-1～64.00μgN2O·m-2h-1 with the annual average emission reaching 1.02μgN2O·m-2·h-1.(2) All of the S. mariqueter, Phragmites and S. alterniflora act as the effective promoters for CH4 emissions by transporting the underground CH4 upward to atmosphere through their developed aerenchyma. Phragmites transports the most CH4 to atmosphere with the flux increment reaching 1.99 mgCH·m-2·h-1 during its growing period. The flux increments induced by S. alterniflora and S. mariqueter are 0.45 mgCH4·m-2·h-1 and 0.43 mgCH4·m-2·h-1 , respectively. The three kinds of wetland plants all exhibited negative influence on sediment-air N2O emissions. The average reduced N2O emissions by Phragmites, S. alterniflora and S. mariqueter are 1.27μgN2O·m-2·h-1,0.80μgN2O·m-2·h-1 and 6.22μgN2O·m-2·h-1, respectively. Although possessing the least biomass, S. mariqueter showed the biggest negative influences on N2O emissions. The concrete influences of Phragmites and S. alterniflora differed in different seasons. Compared with the Phragmites and S. alterniflora, the native species of S. mariqueter acted most stably as the inhibitor for N2O emissions. In the time scale of 100 year, the equivalent emissions (e-emission) of CH4 and N2O caused by S mariqueter are 8.03 mgCO2-C·m-2·h-1 and-0.65 mgCO2-C·m-2·h-1. The e-emission of CH4 and N2O caused by Phragmites are 37.35 mgCO2-C·m-2·h-1 and 0.1 mgCO2-C·m-2·h-1. The e-emission of CH4 and N2O caused by S. alterniflora are 8.39 mgCO2-C·m-2·h-1 and 0.065 mgCO2-C·m-2·h-1, respectively.(3) Compared with the weak natural N2O production and weak plants’influence on N2O emissions, the N2O fluxes across the sediment-air interface response sharply to imported nitrate and the flux increased positively with the NO3-N concentrations. However, the N2O flux didn’t response to the NH4+-N addition, which suggested that denitrification rather than the nitrification process is the most probable production source of N2O in Yangtze Estuary wetland.(4) The pore-water CO2 concentrations ranged from 4.58mg·L～1-99.82 mg·L-1, 5.49 mg·L-1～133.25 mg·L-1 and 0 mg·L-1～148.81 mg·L-1 in Phragmites, S. alterniflora and S. mariqueter marshes, respectively. The pore-water CH4 concentrations also showed apparent vertical changes and it increased with the sediment depth in the surface 20cm layers. Totally, the pore-water CH4 concentrations ranged from 0.013 mg·L-1～0.95 mg·L-1,0.13 mg·L-1～0.55 mg·L-1 and 0.42 mg·L-1～3.61 mg·L-1 in Phragmites, S. alterniflora and S. mariqueter marshes, respectively. Restricted by the low concentration of sediment inorganic nitrogen, N2O in the sediment pore water is rarely detected, while, the N2O concentration showed a quick accumulation after the nitrate addition.(5) By Pearson correlation analysis, only the pore-water CH4 concentration at the depth of 10cm～20cm where the S. mariqueter roots distributed showed the significant correlation with CH4 fluxes. Although the CH4 concentration increased steeply under this depth, only the CH4 existed in the rhizosphere could be effectively transported to the atmosphere. By the field comparison experiment of clipping and pulling out the plant, the pore-water CH4 concentration is higher in non-vegetated marsh than that in the normal vegetated marsh suggesting that the plant exerts a more important influence on CH4 oxidation compared with the promotion effects by secreting root exudates. However, in consideration of the low CH4 flux in the non-vegetated marsh, the S. mariqueter exerted more important influence on CHU transportation compared with its influence on CH4 production and consumption.(6) The maximum CO2 production rates in S. alterniflora, Phragmites and S. mariqueter marshes all appeared in summer with the production rates reaching 1291.41 ngCO2·g-1·h-1,758.72 ngCO2·g-1·h-1 and 910.34 ngCO2·g-1·h-1 respectively; The maximum and minimum CH4 production rates in the sediments also appeared in summer and winter, respectively. The annual average natural CH4 production rates in S. alterniflora, Phragmites and S. mariqueter marshes are 0.88 ngCH4·g-1·h-1,5.63 ngCH4·g-1·h-1 and 1.68 ngCH4·g-1·h-1. Different with CH4 production, the potential CH4 oxidation showed no apparent seasonal variation with the annual average potential rates in S. alterniflora, Phragmites and S. mariqueter marshes reaching 232.09 ngCH4·g-1·h-1,118.63 ngCH4·g-1·h-1 and 95.49 ngCH4·g-1·h-1, respectively.(7) Restricted by the low concentration of NO3--N, the natural N2O production in all the plant marshes are very weak showing no significant variations in Yangtze Estuary. The annual average natural N2O production rates in S. alterniflora, Phragmites and S. mariqueter marshes are 0.13 ngN2Og·-1·h-1,0.037 ngN2O·g-1·h-1 and 0.077 ngngN2O·g-1·h-1, respectively. Compared with the weak N2O production, the sediments still retained strong N2O consumption potential and the N2O annual average consumption potential in S. alterniflora, Phragmites and S. mariqueter marshes are 33.50 ngngN2O·g-1·h-1、25.34 ngngN2O·g-1·h-1 and 25.92 ngngN2O·g-1·h-1, respectively. Besides, the N2O consumption potential showed the significant response to temperature.(8) The sediment denitrification showed apparent seasonal variation with the highest value appeared in summer, but the N2O:N2 ratio didn’t show the same variation. In the vertical sediment profile, denitrificaiton rates in surface sediment layers are apparently higher than that in deeper sediment. By correlation analysis, temperature, SWC and nitrate concentration are the key factors influencing the denitrificaiton process rather than the N2O:N2 ratio.(9) By comprehensively considering the low concentration of extractable inorganic nitrogen, the weak N2O production, the strong N2O consumption potential and the inhibition effects of S. mariqueter on N2O emissions, the indoor simulation of plant transportation for N2O suggested the S. mariqueter exerted contrary influences on CH4 and N2O emissions. The sedge can transport the underground CH4 to atmosphere and the atmospheric N2O to the rhizosphere. Besides, the N2O transported into sediment could be converted to N2 in the second half step of the denitrification process by N2O reductase and finally act as the N2O sink.