| Wetland ecosystem was regarded as "kidney of the earth" and "library of natural species",who had enormous ecological service value,and played an significant role in the global carbon cycle.Natural coastal wetlands generally had strong ability of carbon sequestration,but the habitat changes caused by wetland reclamation are likely to result in great influence on the function of ecosystem source and sink.In the background of global climate changes,it was very necessary to strengthen the research on CO2 flux between coastal reclaimed wetland ecosystem and atmosphere.Therefore,we observed the CO2 flux between coastal reclamed wetland ecosystem of Chongming Dongtan and atmosphere from January 2013 to December 2014, continuously, based on the eddy covariance method. Using The FSAM (The Flux Source Area Model) Model to analyze the CO2 Footprint distribution in chongming dongtan coastal reclaimed wetland,rescarching the dynamic characteristics of CO2 flux in different temporal scales and the influence of related environmental factors to CO2 flux.The main research results were as follows:(1) In the prevailing wind direction of the growing season (the direction of 135°～225°),the Xm (the location where the footprint function get the maximum vaule) under stable atmospheric state was 96.84m,and 90% of the flux information came from the range of upwind 41.04～378.20m and vertical upwind-79.73～ 79.73m;The Xm under unstable atmospheric state was 75.28m,90% of the flux information came from the range of upwind 33.83-257.07m and vertical upwind-82.29-82.29m.While,in the prevailing wind direction of the dormant season(the direction of 315°～45°),the Xm under stable atmospheric state was 82.68m,90% of the flux information came from the range of upwing 36.73-282.49m and vertical upwind-120.31～120.31m;the Xm under unstable atmospheric state was 56.49m,90% of the flux information came from the range of upwind 25.90～179.90m and vertical upwind -76.30～76.30m.(2) Similar patterns of footprint distribution were found in the prevailing and non-prevailing wind directions,showing that the source area was much larger under stable atmospheric state than under stable atmospheric state,regardless of the season,growing or dormant,and in similar stable astmosphere,the source area was larger in the growing season than that in the dormant season.In the dormant season,the range of observed vertical upwind direction was larger in the prevailing wind dierction than in the other direction,which may be explained by the high σv/u* ratio(standard deviation of horizontal wind velocity fluctuation vs friction wind velocity)under such conditions.(3) The flux tower location is relatively reasonable.Around the tower,there was farmland on the north,fish ponds and windmills on the west,and artificial vegetation of the park on the east and south,but the rate of contribution from all of them to the CO2 flux was less than 10%.(4) The monthly average diurnal variation of net ecosystem CO2 exchange (NEE) varied obviously among differnt seasons but all appeared as the typical "U" type curve.The variation of nighttime NEE (NEEn) is relatively stable,but the variation of daytime NEE (NEEd) was strongly influenced by solar radiation.The value of NEEd often showed a decrease trend with the increasing of radiation intensity,but also could appear the phenomenon of photosynthesis "nap" at noon.(5) The daily cumulative value of NEE were often positive in winter while negative in other seasons,by the influence of gross primary productivity (GPP) and ecosystem respiration (Reco),but the wetland ecosystem would be also a carbon source in some days of summer when the plants grow vigorously,and realeased CO2 into the atmosphere.During the whole observation period,the daily cumulative value of NEE ranged from-30.406 to 8.397 g·m-2·d-1,and the proportion of negative value reached 62.47%.(6) During the whole obserbation,the ecosystem released CO2 into the atmosphere in every month of winter,but absorbed carbon from the atmosphere in every month of other seasons.January had the greatest CO2 emission capacit,while July had the greatest CO2 sequestration capacity. Throughout the whole year,the study area showed to be a strong sink of CO2.The cumulative value of NEE could reach up to-1115.654 g·m-2 in 2013,then the CO2 sequestration capacity had a certain degree of diminished in 2014,but the cumulative value of NEE could also reach to -1003.719 g·m-2.The absolute value of the ratio of Reco and GPP (Z) were greater than 1 in every month of winter,but less than 1 in every month of other seasons.During year 2013 to 2014,the value of Z was 0.745,showed that 74.5% of the gross primary productivity was consumed by ecosystem respiration.(7) The photosynthetic active radiation (PAR) was the main influence factor to NEEd,which could explain 22.0%～70.5% changes of daytime NEE,and there was rectangular hyperbolic correlation between PAR and NEEd.The ranges of apparent quantum yield (a),maximum photosynthetic rate (Pmax) and dark respiration rate (Re) between March to November were 0.008～0.0473,5.886～27.369 and 0.066～5.069 μmol·m-2·s-1,respectively.The average value of a and Re in 2013 was less than that in 2014,and the average value of Pmax was closed to that in 2014.(8) Temperature is one of the most important environmental factors to NEE.It showed a significant exponential function correlation between NEEn and temperature,while significant negative linear function correlation between NEEd and temperature.Compared with the soil temperature in the 10 and 30 cm layer(Ts10, Ts3o),the air temperature (Ta) could explain the variation of NEEn and NEEd better. As far as Q10 vaule,it changed in the range of 2.096～2.718 in this study area.It was more sensitive that NEEn response to soil temperature than to air temperature,and it showed the order of Q10(Ta)<Qio(Tsio)<Q10(Ts3o).(9) Soil water content and soil salinity was important environmental factors to NEE.When the soil water content in 10cm layer (SWC10) was less than 45%,there was significant positive correlation between SWC10 and NEEn,so was that between SWCio and NEEd;When SWC10 was more than 45%, there was no significant correlation between SWCio and NEEn(in 2013) or there was significant negative correlation (in 2014),but there was significant positive correlation between SWC10 and NEEd.While,soil water content in 30cm layer(SWC3o),soil salinity in 10cm and 30cm layer(Ss10 and Ss3o),were significantly positive correlated to NEEn(except for Ssio in 2013),and significant negative correlated to NEEd. |
PDF Full Text Request