CO₂ Flux And Light Use Efficiency Of Farmland Based On Eddy Covariance

Farmland ecosystem is an important component of the terrestrial ecosystem. Carbon cycle of farmland impacts on global climate change significantly. Carbon dioxide exchange characteristics and the factors influencing carbon cycle between farmland and the atmosphere is a key to understand global carbon cycle. It can not only help to understand the mechanism of carbon cycle, but also make a progress on evaluating and modeling the annual carbon source or sink strength of the farmland ecosystem.In this research, carbon dioxide fluxes in Huaihe River valley were measured continuously in a rice-wheat rotation farmland ecosystem using eddy covariance technique in Shouxian, Anhui. The energy balance closure was analyzed to evaluate its reliability. The objectives are:to investigate daily and seasonal variation of carbon dioxide fluxes, to analyze the relationship between carbon dioxide fluxes and environmental factors, to quantify the annual net ecosystem exchange (NEE) and to calculate the light use efficiency of rice using field data.The main conclusion are showed that:(1) The energy balance of farmland ecosystem in Shouxian does not closed. The averaged ratio of energy balance is 0.88, the level of energy imbalance is 0.12. The ratio of energy balance of daytime is bigger than night, spring and summer are bigger than autumn and winter. The discussion shows that there are many reasons could cause energy imbalance, but not all of them could affect the credibility of the carbon dioxide flux data. In general, it shows that eddy covariance technique could be reliably applied to the observation and measurement of carbon dioxide fluxes.(2) Daily variation of carbon dioxide flux in wheat and rice growing season has distinct patterns. The farmland ecosystem absorbs carbon dioxide during the daytime and releases carbon dioxide into the atmosphere at night. During the growing season of wheat and rice, carbon dioxide flux variation in farmland ecosystem is influenced by crop phenology period. Generally, the absorption peak is higher in jointing and heading stage than tillering and maturity stage. The whole fertility period of wheat and rice shows obvious characteristics of carbon sink, the amount of carbon storage is 485 g C·m-2 and 442 g C·m-2, respectively. Air temperature and photosynthetically active radiation are two main environmental factors of carbon dioxide flux during the fertility period in daytime. The relationship between photosynthetically active radiation and carbon dioxide flux in daytime can be described in a hyperbolic equation. Different growing stages of wheat and rice have different utilization efficiency of photon quanta or maximum photosynthetic rate. Over all, the utilization efficiency of light energy of rice is higher than wheat. At night, the carbon dioxide flux in wheat and rice growing season is positively exponential correlated with temperature. By analyzing several temperature indexes, the soil surface temperature is the most suitable for fitting respiration flux. Photosynthetically active radiation is the main factor influencing seasonal changes of NEE during the growing seasons of wheat and rice. In the daytime, NEE in wheat growing season is sensitive to temperature, but during rice growing season it’s sensitive to photosynthetically active radiation.(3) Latent heat flux of the rice-wheat rotation farmland ecosystem is basically same as the annual variation of net radiation, which shows two high peaks from April to May and from July to September. Sensible heat flux and soil heat flux is relatively small.69% of the net radiation absorbed by the farmland ecosystem is used for latent heat exchange. The patterns of annual changes of NEE (absolute), GPP and Reco in farmland ecosystem are similar to the net radiation. The annual net carbon storage is 1.02 kg C·m-2, in which the carbon sequestration of wheat and rice growing seasons reached 91% of the whole year. PAR and Ta are the main influencing factors to the changing of GPP and NEE. U* critical value will influence the total amount of NEE.(4) There is a good correlation between fraction of absorbed photosynthetically active radiation index and leaf area index in paddy ecosystem. An obvious diurnal and seasonal variation pattern of light use efficiency is found in paddy ecosystem. In daily changes, light use efficiency is higher at the sunrise and sunset than at noon. The light use efficiency is significantly correlated with temperature and absorbed photosynthetically active radiation. Light use efficiency decreases first and then increases as temperature increasing. Negative logarithmic correlation is found between light use efficiency and absorbed photosynthetically active radiation. In seasonal changes, light use efficiency increases first and then decreases with the increasing of temperature.