Study On Productivity And Sustainability Evaluation Of Chuandi Agro-ecosystem In Loess Hilly Region

Carbon cycle in paddy ecosystem strongly affects the uptaking /emitting of greenhouse gases and global climate change. CO₂ exchange between the ecosystem and the atmosphere is a key part of carbon cycle.Measurements of the net exchange of carbon dioxide between paddy ecosystem and the atmosphere not only benefit to understand well the mechanism of carbon cycle and its modeling and evaluating, but also help to determinate the annual carbon source or sink strength of the paddy ecosystem and to assess its contribution to the budget of CO₂ in the atmosphere, especially in subtropical region.In the research, CO₂ fluxes from paddy ecosystem in subtropical hilly region were measured continuously using eddy covariance technique. The objectives were to assess the accuracy of eddy covariance method, investigate the variation of CO₂ fluxes on daily and seasonal temporal scales, analyze the relationship between CO₂ fluxes and environmental factors, and to quantify the annual net ecosystem exchange (NEE) from the paddy ecosystem. Moreover, application of chamber method in the observation of CO₂ fluxes from paddy fields was investigated.The main conclusion and innovations were showed in the following:(1) Energy imbalance was found during the observation and measurement of CO₂ fluxes using eddy covariance technique in paddy ecosystem in subtropical hilly region. The sum of sensible and latent heat (H+LE) might be underestimated if (Rn-G) was accurate. The energy balance ratio (EBR) of (H+LE) to (Rn-G) was averagely 0.85 from May to Aug, which was lower than that of other periods (0.92) during a year, that is, the extent of energy balance closure was relatively lower from May to Aug. The annual EBR was 0.87 on average with a mean imbalance of 13%. It showed that eddy covariance technique could be reliablely applied in the observation and measurement of CO₂ fluxes.(2) The NEE between paddy ecosystem and the atmosphere was the balance of photosynthesis and respiration processes. A notable diurnal pattern of CO₂ fluxes was observed, with uptaking CO₂ from the atmosphere (negative value) during the daytime and emitting CO₂ to the atmosphere (positive value) in the nighttime. Photosynthetic photon flux density (PPFD) and temperature were the main factors for the daily change of CO₂ fluxes. A rectangular hyperbolic light-response function could be used to describe the relationship of CO₂ flux and PPFD. The absolute values of CO₂ fluxes increased with the increment of PPFD. When PPFD was higher than 1000μmol m^-2 s^-1, light saturation was observed. The carbon dioxide fluxes response differently to light in different growing stages of rice. In the blooming stage, the quantum yield (a) and the maximum rate of photosynthesis assimilation (Pmax) were higher than that in tillering and ripening stages. Moreover, these light response parameters in late rice growing season were general higher than that in early rice growing season.(3) In nighttime, respiration from soil and plants (ecosystem respiration, Reco) with U* (friction velocity) >0.1 m s-1 changed exponentially with the increase of soil temperature at the depth of 5 cm (T5). Reco during the early rice-growing season was more sensitive to temperature than that during the late rice-growing season. Moreover, CO₂ fluxes were affected by drainage. When the paddy was drained, net CO₂ uptake from the atmosphere in daytime was less, and in nighttime, CO₂ emission was greater than when the paddy was flooded. Soil moisture was proved to be the dominant factor for controlling CO₂ emission during the drainage period.(4) Leaf area index (LAI) was a critical influential factor of the seasonal pattern of daily CO₂ flux. A significant positive correlation was found between LAI and the daily gross primary production (GPP). In addition, the cumulative GPP was consistent with the change of rice total biomass. During the growing season of late rice, the absolute value of NEE was about 319 g C m^-2, which was higher than that during early rice growing season (about 232 g C m^-2).(5) The annual trend of daily values of GPP, Reco and the absolute value of NEE behaved higher from Jun. to Sep. and lower during the other monthes. Photosynthecially active radiation (PAR) and mean daily air temperature (Ta) were two main influential factors for controlling the annual trend of GPP and NEE. The response of GPP and NEE to PAR and Ta could be described by binary linear functions, respectively. The annual GPP, Reco and NEE in paddy ecosystem were 5861.3 g CO₂ m^-2, 3385.8 g CO₂ m^-2 and -2475.6 g CO₂ m^-2, respectively. It showed that paddy ecosystem in subtropical region was a sink of atmospheric CO₂ with a net absorbing rate 2.5 kg m^-2 a-1. However, the estimated annual NEE was strongly affected by friction velocity threshold and the reference temperature used in the respiration-temperature function. (6) A new method with an order kinetics equation (ER) was put forward to simulate the change rate of CO₂ concentration versus measurement time and to calculate CO₂ flux observed by closed chamber method. This method could solve the lack of linear regression method (LR) to a great extent, which was used very often now but usually underestimated CO₂ flux in sunny daytime. ER method was proved to be a new feasible means to observe and calculate CO₂ flux including photosynthesis process.(7) CO₂ emission rates measured by closed chamber method appeared exponential relationships with air and soil temperatures at the depth of 0, 5, 10 and 15 cm, respectively, which was consistent with the result from the eddy covariance measurement. There was a very marked power function relationship between CO₂ cumulative emission amount and rice biomass. CO₂-C net uptaking from atmosphere increased with rice growing by power function.