Characteristic Of Cycle Of Carbon And Water And Modeling Of Carbon Flux In Winter Wheat Agro-ecosystem

Carbon cycle and water cycle of terrestrial ecosystem have become two hot issues and kernel questions in global change science.There are three processes in carbon cycle of terrestrial ecosystem include plant photosynthsis,autotrophic respiration and heterotrophic respiration,among them,plant photosynthsis can connect by stomatal behavior with transpiration which can control leaf water dissipation,and then the interactional coupling cycle of carbon and water is formed in soil-plant-atmosphere system,and every dynamic change in coupling cycle will have a significant impact on the climate system. Agro-ecosystem is affected most seriously by human activities,also is the main accepter for climate change,the comprehensive study on mechanisms process,variation tendency and response to environment of carbon and water cycle are helpful to understand the potential probability of easing global CO₂ concentration increase in agro-ecosystem and its contribution to global change,and also can lay the foundation for building the coupling model of carbon-water cycle at ecosystem-scale.In this study,leaf photosynthesis-transpiration rate,soil respiration rate and related meteorological factors in typical winter wheat agro-ecosystem were measured by using LI-6400R portable photosynthesis-transpiration system,LI-6400R-09 soil CO₂ flux chamber and eddy covariance system et al.at the Yucheng Comprehensive Experimental Station of the Chinese Academy of Sciences during the main growing season in 2007 and 2008.We analyzed and discussed the characteristics of cycle of carbon and water,and after having obtained important physiological parameters based on field experiment,a process-based, multilayer two-leaf model which calculates energy and material cycle separately for sunlit and shaded leaves was developed,then characteristic of CO₂ fluxes above the canopy was simulated.The main results are as follows:1.Characteristics of carbon and water cycle in the relationship of leaf photosynthesis -stomatal conductance-transpiration.It is quite different for leaf net photosynthesis rate(Pn) measured in different height layer after separated into sunlit and shaded leaves in different growing stage,and so transpiration rate(Tr) and water use efficiency(WUE),and these resulted from the joint effects of photosynthetically active radiation(PAR),stomatal conductance,CO₂ concentration,temperature and other factors.Stomatal limited analysis is an effective method to explain the daily variation regularity;With PAR increasing,Pn presented a rectangular hyperbolic curve increase,and Tr was linear rise,then leaf level WIdE showed a rectangular hyperbolic curve increases with " light saturation" under the influence of Pn and Tr;With the CO₂ concentration enrichment,Pn displayed a rectangular hyperbolic increase,but Tr decreased wavily result from stomatal conductance change,then caused an increase in leaf level WUE,and the response of Tr to stomatal movement which caused by CO₂ concentration change was faster than the response of Pn.2.Characteristics of carbon cycle in the process of soil respiration and the contribution of root respiration,each of the different components of soil respiration had a clear diurnal variation and a seasonal variation,temperature played an important role and root biomass had a synergistic effect to the seasonal variation,5-10cm soil temperature can explaine about 49%～65%of the seasonal variation while root biomass about 44%.Integrated the root exclusion method and the root biomass extrapolation method,this study considers the contribution of root respiration to total soil respiration of winter wheat has a range from 32%～45%.3.Characteristics of energy balance and carbon,water cycle at Agro-ecosystem-scale. Leaf is the basic unit for plant vital activities,which can drive material(carbon and water) circular flow through turbulent exchange and there has a very close correlation between energy transmission and material cycle in SPAC.In this study,Ago-ecosystem-scale energy and material fluxes showed significant diurnal and daily variations when they were measured directly using the eddy covariance(EC) technique,while,compared to conventional observation methods,EC presented an underestimation trend.The ecosystem water use efficiency(WUEe) can describe the relationship of carbon and water cycle at Agro-ecosystem-scale,and the diurnal variation of WUEe was different in April and May. Also,we found that the daily average value of WUEe always higher than that of leaf level WUE,one reason maybe lie in that they can not be compared directly for using two kinds of different measurement methods,the other reason may be associated with the energy fluxes underestimate by EC technology.4.Simulation of winter wheat canopy CO₂ fluxes by a multiplayer two-leaf model. Considering many biophysics processes such as the atmosphere,hydrology,plant leaf N content et al.a multiple layer,two-leaf canopy model for predicting of mass transportation and energy exchange between plant canopies and the atmosphere has developed after dividing the canopy height into three layers(top,middle and bottom),and the sunlit and shaded leaf area index was computed respectively at different layers through introducing a foliage clumping index,it is important to distinguish the spatial light distribution and so leaf photsynthesis.The simulation results showed that the model performed well after compareing with eddy covariance CO₂ fluxes data,the modeled fluxes can explain about 78.5%of the measured in this study,the modeled diurnal variation of CO₂ fluxes have a higher precision at fine daytime than cloudy and nighttime.The clumped architecture of winter wheat canopy makes the stratification between sunlit and shaded leaves essential because the fractin of shaded leaves is much larger in clumped canopy than in random canopy,and shaded leaves play an importance role of about 35.7%to total gross primary productivity(GPP) in this study, while the leaves in top layer is about 80%,the modeled total net primary productivity(NPP) during the study period is 626.3gC·m^-2.