Responses Of Carbon Characteristics Of Calamagrosits Angustifolia Wetland To Elevated CO₂and Nitrogen Deposition On The Sanjiang Plain

The increasingly increases of atmospheric CO₂and nitrogen deposition can influenceplant photosynthetic capacity and then affect ecosystem structure and functions. Thelong-term continuous simulation of atmospheric CO₂and nitrogen （N） depositionexperiments is significant to predict the future global climate change on wetland ecosystemproductivity and carbon sequestration. This study is based on the results through thecontrolled situ-experiment on the Sanjiang Plain wetland in northeast China. This studyfocus on the photosynthetic characteristics, photosynthetic product and mineral elements ofthe dominant species Calamagrostis angustifolia, and ecosystem carbon, water fluxes andsoil respiration of C.angustifolia wetland under different CO₂and N level, which provide atheoretical basis for the wetland adaptation process to global change. There were three CO₂concentration accomplished using Open Top Chambers （370ppm AC,550ppm EC1and700ppmEC2）, and three N deposition level （0g N/m².a, NN;4g N/m².a, LN;8g N/m².a, HN） suppliedas NH4NO3. Each treatment had three replicates. The main conclusions are as follows:（l） Photosynthetic acclimation phenomena occured when C.angustifolia exposed toelevated CO₂（550ppm and700ppm）. Nitrogen addition weakened photosyntheticacclimation phenomena and photosynthetic acclimation phenomena were not found underhigh nitrogen addition. The stomatal conductance （Gs） and transpiration rate （Tr） weredecreased and the water use efficiency （WUE） was enhanced under elevated CO₂. ElevatedCO₂significantly influnced the maximum net photosynthetic rate （Amax）, lightcompensation point （LCP）, light saturation point （LSP）, maximum carboxylation rate（Vc,max） and CO₂compensation point （г*）. The response of photosynthetic parameters toelevated CO₂varied according to the level of N. Elevated CO₂decreased Amaxand Vc,max,increased LCP, LSP and г*under NN and LN treatment. However, elevated CO₂could notsignificantly affect Amax, dark respiration rate （Rd）, apparent quantum efficiency （AQE）,LCP, LSP, Vc,maxand maximum electron transport rate （Jmax） under HN. At the same time,N-addition enhanced Amaxand AQE, but decreased Rd,LCP and LSP, which showed that Naddition could increased photosynthetic capacity.Elevated CO₂significantly altered the photosynthetic pigment content of leaves inC.angustifolia. The response of photosynthetic pigment content to elevated CO₂variedaccording to the level of N. The chlorophyll a, chlorophyll b, carotenoids and totalchlorophyll content of C.angustifolia were decreased under NN and LN treatment.However, the photosynthetic pigments contents of C.angustifolia grown under elevatedCO₂were not significantly lower than those grown under ambient CO₂under HN treatment.N addition enhanced chlorophyll a, chlorophyll b, carotenoids and total chlorophyll contentof C.angustifolia. The results indicate that the photosynthetic capacity of C.angustifolia declined underlong-term elevated CO₂, but enough N addition could alleviate photosynthetic acclimation.The photosynthetic acclimation occured accompanying with significant decrease of Vc,max.Our results imply that photosynthetic acclimation under elevated CO₂is largely due toRuBP carboxylation limitation.（2） Elevated CO₂significantly increased the starch content of root, the soluble sugarcontent of leaves, stem, root and total organ in C.angusitifoila, but had no significanteffects on the starch content of leaves and stem. The response of soluble protein content toelevated CO₂varied according to the level of nitrogen. Elevated CO₂decreased solubleprotein content of leaves in C.angusitifoila under NN treatment, but increased solubleprotein content under LN and HN treatment. N addition enhanced the starch content andsoluble sugar content of root, stem and total organ, and the soluble protein content ofleaves, but had no significant effect on the starch content and soluble sugar content ofleaves in C.angusitifoila.The response of the mineral element to elevated CO₂varied according to the elementsspecies and the level of N. Elevated CO₂had no significant effects on the organic carbon（C） of leaves in C.angusitifoila. The response of N content of leaves to elevated CO₂variedaccording to the level of nitrogen. Elevated CO₂significantly decreased N content ofleaves in C.angusitifoila under NN and LN level but not under HN treatment. Phosphorus（P） and potassium （K） content of leaves was significantly increased under elevated CO₂.Elevated CO₂significantly influnced C/N, C/P, N/P and N/K of leaves in C.angusitifoila. Naddition significantly increased the content of N, decreased the content of P and K, andincreased N/P and N/K of leaves in C.angusitifoila. Our results show that elevated CO₂andnitrogen addition could change the nutrition pattern of plant and imply that elevated CO₂could facilitate wetland plant to mitigate P and K limitation.（3） The seasonal dynamics of ecosystem carbon fluxes （NEE, ER and GEP） inC.angusitifoila wetland were high in mid-summer and low in the early and late growingseasons in2010and2011. In addition, there were remarkable interannual variabilities inthe ecosystem C fluxes. This was coincided with air temperature in the early growingseason, the soil temperature and the vegetation cover. Elevated CO₂significantly decreasedNEE and evapotranspiration （ET）, but enhanced GEP, ER and water use efficiency （WUE）.Elevated CO₂reduced NEE because the stimulation caused by the elevated CO₂had agreater impact on ER than on GEP. The addition of N stimulated ecosystem C fluxes inboth years and ameliorated the negative impact of elevated CO₂on NEE. Future elevatedCO₂may favor carbon sequestration in C.angusitifoila wetland when coupled withincreasing nitrogen deposition.（4） Soil respiration displayed strong seasonal patterns, with higher values observed inmidsummer and lower values in spring and autumn in all treatments, which mainly due tohigher atmospheric temperature induced higher plant growth and soil microbial activity inthe midsummer. There were significantly exponential relationships between soil respirationand soil temperature. Elevated CO₂significantly enhanced soil respiration by repeatedmeasures ANOVA. The effect of the N addition on soil respiration was significantly increased in the first year, whereas was decreased in the second year when analyzing yearby year. It was indicated that the effect of the N addition on soil respiration had beeninhibited over time. There was a significantly interactive effect between elevated CO₂andN addition on soil respiration, and the combined effect of them significantly increased soilrespiration by20%-29%compared to that in the control chambers.The results suggest thatelevated CO₂will accelerate soil respiration and carbon cycling in C.angusitifoila wetlandregardless of nitrogen deposition level.