| The research of eco-hydrological process of farmland ecosystems based on CO2/H2O cycles has become a hot issue in the field of agronomy, ecology and hydrology. Quantitative research on water and carbon fluxes is essential for irrigation water optimal allocation and CO2 sequestration in irrigation district. In this research, CO2/H2O fluxes from paddy ecosystem and wheat-maize rotation ecosystem were measured continuously though long-term observation using eddy covariance technique. The objectives were to further study the variations of H2O/CO2 and the mechanism of environmental response in these two typical and important farmland ecosystems of China.To ensure the data quality, this thesis carried out a series of flux corrections and calculations, and then conducted data quality control and analysis. The results proved that WPL corrections, coordinate rotation and mean fluctuation computations could significantly affect the CO2/H2O fluxes, while the effect of storage term on the total fluxes could be neglected. Steady state test showed the quality of the data from paddy ecosystem was reliable. Unclosed energy balance was found during the observation and measurement of CO2/H2O fluxes using open-path eddy covariance system in paddy ecosystem of Poyang-Lake plain and wheat-maize rotation ecosystem of North China plain. The annual energy balance ratio (EBR) of paddy system was 0.884 with an imbalance of 11.6% and the annual EBR of wheat-maize rotation ecosystem was 0.850 with an imbalance of 15%. The footprint analysis showed that the observation data mainly came from the target area, during growing periods of rice, there were about 90% of data came from the target zone in the daytime, while 70?80% in the nighttime. The appropriate gap-filling method was chosen according to data type and gap length, and various meteorological factors were synthetically considered in the process of data interpolation, and piecewise interpolation was carried out according to underlying surface and meteorological conditions, which could effectively improve the precision of data gap-filling.Based on water-carbon fluxes observation data from paddy field in 2011?2013 and wheat-maize rotation field in 2003?2005, this research, in terms of water and carbon transfer, revealed the ecological hydrology change progress from different time scales, and analyzed the similarities and differences between two farmland ecosystems. The results showed that, there were obvious variable characteristics in hours, days and seasonal scales. Diurnal variation of ET displayed a "inverted U-shaped" curve with the maximum peaks occurred generally at 12:00-12:30, while diurnal variation NEE displayed a "U-shaped" curve with the maximum uptake peaks of CO2 occurred usually at 13:00. During the crop growing seasons, farmland ecosystems absorbed CO2 (NEE was negative) during the daytime, whereas CO2 emissions (NEE was positive) during the nighttime. Farmland ecosystem characterized by obvious carbon sinks because the carbon uptakes much higher than carbon emissions. The diurnal ET processes of farmland ecosystems within a year showed a bimodal trend. ET of paddy fields was mainly driven by meteorological factors, whereas ET of wheat-maize rotation fields was mainly affected by regulation of crop growth. The night ET accounted for under 10% of the daily ET during the growth period, while 10?20% during the non-growing season, and crop water consumption occurred mainly in the daytime. The diurnal of NEE processes of paddy fields with the cropping pattern of "rice-rice-astragalus sinicus" displayed a trimodal trend, whereas that of wheat-maize rotation fields showed a bimadal trend. Two kinds of farmland ecosystems'NEE were mainly driven by the crop leaf area index (LAI), and the largest carbon uptakes occurred in the period with the largest LAI. The largest carbon uptakes and emissions in the wheat-maize fields were greater than that in the paddy fields. After considering the carbon in the grain after harvest, the ecosystem of early rice fields and wheat-maize fileds turned from a strong carbon sink into a weak carbon sink or source, and late rice fileds turned into a medium carbon sink. There were nearly equal total amount of NEE during the crop growing season, and the NEE's interannual difference between two farmland ecosystems mainly reflected in the crop non-growing season. Studies showed that, transplanting the late rice as early as possible could improve the quantity of carbon assimilation. Compared with the cropping pattern of "rice-rice-winter fallow", the "rice-rice-astragalus sinicus" could show a certain degree of carbon sink in winter.This thesis analyzed the correlation between CO2/H2O fluxes and meteorological factors such as net radiation (Rn), air temperature (T), humidity (VPD) and wind speed (v), and the correlation between CO2/H2O fluxes and LAI, in different time scales, and discussed the mechanism of environmental response on CO2/H2O fluxes. Results proved that, the main control factors of ET/NEE were different in time scales. Net radiation was the control factor for ET in hour's and day's scale, and the correlation between ET and meteorological factors was affected by LAI and soil water content (SWC). The mean v was the main control factor for ET of paddy fileds in nighttime. The main control factor for ET of paddy filed was T in seasonal scale, whereas that of wheat-maize fields was affected by the changes of SWC due to rainfall and irrigation. Photosynthesis-light response model in the form of non-rectangular hyperbola could be used in describing the relationship between NEE and photosynthetically active radiation (PAR) in hour's scale. PAR=1000 ?mol/m2/s might be a threshold that light use above the crop canopy, when PAR<1000? mol/m2/s, PAR was the most important factor for limiting net photosynthesis rate. The main control factor turned into T in the seasonal scale. The critical temperatures for carry out photosynthesis of paddy field ecosystem and wheat-maize field ecosystem were 10? and 5?, respectively. The changes of carbon sink and source were performed before and after critical temperature. LAI was the main control factor during crop growing season in seasonal scale, whereas T in crop non-growing season, and NEE showed asymmetry circular response on T. |
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