Study On Ecological Response And C,N Reserves Of Kandelia Candel?Linn.? Druce And Aegiceras Corniculatum?L? Blanco Wetlands Under Doubling CO₂ Mole Fraction

Elevated atmospheric CO₂ not only contributes to global climate change,but also has a significant impact on plant primary productivity.Mangrove is a woody plant community in the intertidal zone of tropical coast and the most important forest buffer zone and ecological barrier between land and sea.Mangrove has important effects on C?carbon?reserves,C fixation potential,N?nitrogen?cycle and soil decomposition.However,the study of C and N reserves in mangrove wetlands under global climate change is still insufficient especially under the situation of doubling CO₂ Mole Fraction.This thesis assumes that atmospheric CO₂ doubling has an important effect on wetland C and N reserves by affecting the flow and distribution of mangrove C.Taking the mangrove wetland?110°35'14.47"E,19°54'51.48"N?in Dongzhai Harbor,Hainan Island as the research object,the open-top box method is used to simulate the atmospheric environment situation in which CO₂ mole fraction doubled from 350?mol/mol to 700?mol/mol.By simulating the coastal tidal dry-wet alternating environment,this thesis studies the changes of C and N contents in Kandeliacandel?Linn.?Druce and Aegiceras corniculatum?L?Blanco,analyzes the relationship between them and mangrove biomass,and discusses the structure and diversity of microbial community and its response to CO₂ mole fraction doubling.Based on the data of laboratory simulation and field survey,the equations of C and N reserves in mangrove wetland system are established to predict the change trend of C and N reserves in mangrove wetland system.It provides a theoretical basis for the study of mangrove wetland eco-environmental response under atmospheric CO₂ mole fraction doubling.?1?Effects on plant traits and biomassThe doubling of CO₂ mole fraction promoted the growth and thickening of annual Kandelia candel?Linn.?Druce and Aegiceras corniculatum?L?Blanco.Meanwhile,the sponge tissue of Kandelia candel?Linn.?Druce and Aegiceras corniculatum?L?Blanco increased by7.24?m and 2.91?m respectively.Compared with 350?mol/molCO₂,the biomass of roots,stems and leaves of Kandelia candel?Linn.?Druce and Aegiceras corniculatum?L?Blanco.increased significantly?P<0.05?.The biomass of Kandeliacandel?Linn.?Druce leaves and Aegicerascorniculatum?L?Blanco stems increased most by 20.53%and 9.35%respectively.The total biomass of Kandeliacandel?Linn.?Druce and Aegicerascorniculatum?L?Blanco increased by 17.22%and 6.4%,respectively,which has significant difference.The biomass of underground part of the two mangrove plants was significantly lower than that of above ground part.The results showed that the effect of doubling CO₂ mole fraction in atmosphere on the above-ground biomass of mangrove plants was more obvious than that of the underground part of mangrove plants.?2?Effects on C and N contents and microorganisms in mangrove wetland systemThe concentration of TC?total carbon?and TN?total nitrogen?did not change with the increase of CO₂ mole fraction.The concentration of dissolved C in water is not changed in the short term by this physical process of dissolution.Therefore,it can be considered that the change of TC and TC in water is the difference caused by the influence of plant system.The doubling of CO₂ mole fraction had a significant effect on the content of C and N in water and soil of Kandeliacandel?Linn.?Druce and Aegicerascorniculatum?L?Blanco simulated wetland system.TOC?total organic carbon?concentrations in water and soil increased significantly,IC?inorganic carbon?concentrations decreased,TC concentrations showed an increasing trend,and the whole system served as a"reservoir"for C accumulation.The mass concentration of NO₃^-in water increased,while the mass concentration of NH₄⁺decreased significantly in the later stage,which has significant difference?P<0.05?.Nitrification plays a dominant role in the system at the late stage of culture,and the concentration of TN in the water body increases as a whole.In the soil of the system,the mass fraction of TN showed a decreasing trend.With the increase of atmospheric CO₂ mole fraction,the TN in the soil was rapidly consumed,and the mineralization intensity of TN in the soil was enhanced.Phospholipid fatty acids in wetland soils were analyzed.The results showed that the microbial population in wetland soils changed under CO₂ mole fraction,C17:0,which was characterized by nitrification ability,increased significantly,and nitrification activity was enhanced.C16:1?5c and cycC17:0,which characterize denitrification,increased significantly.The denitrification of soil microorganism population was also increased,and the activity of soil microorganism in wetland was increased.The microbial biomass of wetland soils increased by38.4%after CO₂ mole fraction doubling.Among them,the microbial biomass of gram-positive bacteria increased by 44%;The microbial biomass of gram-negative bacteria and fungi increased by 33.4%and 30.9%,respectively;The microbial biomass of wetland soil increased significantly,while the microbial C source in soil mainly came from the sediment and exudate of root system.The microbial C and N sources in soil changed with the increase of sediment and exudate of root system,which accelerated the cycle of soil C and N.In the simulated wetland system of Kandelia candel?Linn.?Druce and Aegiceras corniculatum?L?Blanco,the biomass of plants was significantly increased under 700?mol/mol CO₂.As a result,the TOC of wetland soil is greatly improved,more TOC is accumulated,and the quality of TC is improved.The TN conversion rate and N mineralization rate in the system were increased rapidly,and the N content in the soil decreased obviously with the N entering the water body,and the quality of TN decreased.?3?Effects on C and N reserves of mangrove plants The plant height?H?,basal diameter?D?,biomass?C?and seedling quality index?QI?of Kandelia candel?Linn.?Druce and Aegiceras corniculatum?L?Blanco were positively correlated with C content?TC,TOC,IC?and N content?TN,NH₄⁺-N,NO₃^--N?in soil and water.The seedling quality index?QI?was significantly correlated with TC in water?0.915,0.943?and soil?0.905,0.878?at CO₂ concentration of 350?mol/mol and 700?mol/mol,respectively,indicating that TC in water and TN concentration in soil could promote the growth of Kandelia candel?Linn.?Druce and Aegiceras corniculatum?L?BlancoUnder 700?mol/molCO₂ CO₂,the total C reserves of Kandelia candel?Linn.?Druce and Aegiceras corniculatum?L?Blanco were 17.2%and 5.6%higher than those under 350?mol/mol,respectively,and the C reserves increased significantly.N reserves decreased by 0.6%and 8.6%respectively,mainly due to dilution effect caused by biomass increase.Through field investigation and analysis,Kandelia candel?Linn.?Druce is the main mangrove community in Dongzhai Harbor,Hainan Province,followed by Aegiceras corniculatum?L?Blanco.Substituting the measured stem height and basal diameter of Kandelia candel?Linn.?Druce and Aegiceras corniculatum?L?Blanco into the C reserves equation constructed according to the laboratory data,under the situation of 350?mol/molCO₂,the estimated values of C storage equation were 32.78±0.2t/hm² for Kandelia candel?Linn.?Druce and 25.12±0.64t/hm² for Aegiceras corniculatum?L?Blanco,while the measured values are33.34±1.92t/hm² and 23.86±1.73t/hm²,respectively.The measured values is in good agreement with that of estimated values.According to the established C storage equation,the C storage of Kandelia candel?Linn.?Druce and Aegiceras corniculatum?L?Blanco were 96.61±2.13t/hm²and 63.22±1.69t/hm² respectively under 700?mol/molCO₂.