Study On Dynamics Of CO₂ Flux In Swan Islet Wetland And Effects Of Environmental Factors

The Swan Islet Wetland ecosystem (E112°31′36″-112°36′90″, N29°46′71″-29°51′45″) are typical periodical flooded marshes. As a part of Yangtze river mid anddownstream wetland ecosystem, it is meaningful to make better understanding of thepotential implications of the global climatic warming on this marsh. In this article, wetried to elucidate the dynamics of CO₂ flux in this marsh from three level, ecosystem,plants and soil.1 Carbon budget in Swan Islet Wetland ecosystem(SIW)In order to study the dynamics of primary production in the wetland ecosystem, wemeasured the ecosystem/atmosphere carbon dioxide exchange in SIW from May 2005to April 2006. The results showed that the environment factors did together contribute tothe production of the ecosystem. Significant correlations were found between the NEEand the variation of soil temperature, air temperature and the light density respectively.In the growing season, SIW serve as a net carbon sink, but the ecosystem should be takeas a carbon source accounting for the CO₂ release in winter.2 Photosynthesis characteristics of representative speciesNet photosynthesis (Pn) of Carex cinerascens was measured in SIW over diurnaland annual periods to evaluate the effects of environmental factors on photosynthesis. Amarked seasonal pattern of Pn occurred with the greatest average photosynthesis duringautumn, spring and summer (9.79 9.15, and 8.47μmol m^-2 s^-1, respectively) compared towinter (4.53μmol m^-2 s^-1). Average apparent photosynthetic efficiency (AQY) of C.cinerascens was 1.43% and varied seasonally with the highest values in winter (1.67%) followed by spring (1.43%), autumn (1.33%), and summer (1.29%). Seasonalmeasurements of diurnal photosynthesis showed that daily Pn was variable among theseasons at SIW. From the parallel response of net photosynthesis to the variation ofseasonal light and temperature, it suggested that the light and the temperature be the keyfactors affecting the growth of wetland plants. According to the analysis of artificial networks, Pn showed highest sensitivity to vapor pressure deficit(VPD). The compacts ofvariation in water level on Growth of C. cinerascens was also investigated in our study.The roots of C. cinerascens were exposed to different extent of partial flooding (-5, -15,-25, 35cm from the soil surface) and gas exchange, chlorophyll content, N uptake, Puptake as well as morphological characteristics of C. cinerascens were measured. Pnand transpiration rate (E) of C. cinerascens was greatly affected by the variation ofwater level whereas no detectable compacts was observed on stomatal conductance (gs)and water usage efficiency (WUE). Similar changes found in chlorophyll content andnitrogen content paralleled well to the photosynthesis. Depth of ground water tablewithin specific range has a slight influence on the photosynthetic performance of C.cinerascens, but it have a strong influence in the allocation of source between the leafand the root. As a typical flood pulsed marsh, different tolerance of wetland plants tolong-term of soil flooding are correlated with the distribution pattern and plants survival.In field, the effect of soil flooding on the photosynthesis characteristics and the effectsof soil flooding on photosystemâ…¡(PSII) activity were analyzed at the level ofindividual leaves of Phragmites australis, C. cinerascens and Hemarthria altissima.Pronounced deficiency of Pn followed by a significant decrease of gs was only found inC. cinerascens during flooding. Stomatal limitation was still the main factor brought bylong-term of soil flooding affected photosynthesis in C. cinerascens. No significantdifference in PSII photochemical efficiency was observed among the three speciesbetween the two hydroconditions, which suggested no damage to the photochemicalreactions in plant. Among the three species, similar values of Pn, gs, E and WUEbetween two hydroconditons, lower respiration rate (Rd) and no midday depression indicated that H. altissima adapt to the environmental stress and wetter sites better thanP. australis and C. cinerascens.3 Soil respirationSoil respiration, a substantial component of the wetland carbon budget, has beenstudied extensively at the ecosystem, regional, continental, and global scales, but littleprogress has been made toward understanding soil respiration over temperate wetlandlandscapes. Soil respiration is often influenced by soil temperature (Ts), soil moisture(Ms), type of vegetation and water table, and these factors vary widely among the patchtypes within a landscape. We measured soil respiration from May 2005 to April 2006.Multiple peaks of seasonal soil respiration were functions of soil temperature andmoisture dynamics. Positive relationships were found between soil respiration and soiltemperature and air temperature respectively. The Q10 differed from 1.55 to 2.12, thehigher Q10 value with soil temperature means stronger relationship between soilrespiration and soil temperature. Soil moisture also showed a strong compaction on soilrespiration r²=0.5825 P＜0.0001). Compared with the climate factors, land using is thekey factor affecting the carbon cycle in soil. In our study, soil bulk density, soil nutrientlevel, microbial activity and soil respiration were measured to evaluated the compactsbrought by the David's deer. The results showed that soil compaction reduced soilrespiration. The reduction of soil respiration was attributed to the decreased soilporosity and microbial activity.