Research On Energy Balance And Carbon-water Flux Of The Summer Maize Farmland Ecosystem In The Lower Reaches Of The Yellow River Plain

Under the background of global warming and shortage of freshwater resources,carrying out long-term observations of carbon and water fluxes between terrestrial ecosystems and the atmosphere will not only help people to deeply understand the relationship and control mechanism between global change and the carbon and water cycle of terrestrial ecosystems.It also provides important data for building the climate change impact assessment models on agricultural production.The lower reaches of the Yellow River plain is the most important food production area in my country with a long history of planting.The main land use method in this area is agricultural land.Studying the dynamic characteristics and environmental response mechanisms of carbon and water fluxes in farmland ecosystems in the lower reaches of the Yellow River plain can accurately assess the energy exchange and material transfer between the land-air system in this region,and provide a theoretical basis for sustainable agricultural development in the core area of food production.Provide scientific data support for global climate change and international carbon-water cycle research.In this study,the summer maize farmland ecosystem at the Henan Fengqiu Farmland Ecosystem National Field Scientific Observation and Research Station in the lower reaches of the Yellow River plain as the research object.Based on the vorticity correlation system and the total factor meteorological observation system,the long-term positioning observation experiment was carried out.The energy balance characteristics,distribution characteristics and closure levels of summer maize farmland ecosystems at different time scales were mainly studied;Elucidation of the dynamic change law of CO₂ fluxes in summer maize farmland in day,season and different growth period,and analyzed the carbon source and carbon sink characteristics of the summer maize farmland at different growth periods.The variation characteristics of water vapor flux,evapotranspiration and water use efficiency under different time scales were discussed.Finally,the correlation between each energy flux and environmental meteorological factors was analyzed respectively.The conclusion is:（1）The diurnal variations of net radiation flux（Rn）,sensible heat flux（H）,latent heat flux（LE） and soil heat flux（G）on the surface of summer maize farmland all showed an inverted“U”shap with low in the morning and evening and high at noon.The peak values of net radiation flux,sensible heat flux and latent heat flux generally appeared at 12:00～14:00,while the peak values of soil heat flux lagged behind 1～2 h.In the early period,the radiation flux was mainly converted into sensible heat flux.In the vigorous growth period and late period,it was mainly converted into latent heat flux,and the relationship between sensible heat flux and latent heat flux was“one goes away,the other goes up”.In addition,30%energy was not closed in farmland,and turbulent flux at night is obviously underestimated.The reasons for this phenomenon are as follows:system bias of observation instrument;random error of observation instrument sampling;flux losses at high and low frequencies;neglect of other energy absorption terms and loss of advection.（2）The CO₂ flux during the whole growth period of summer maize had obvious diurnal and seasonal variation characteristics,and the change trend was“U”shap.Carbon emission at night（early and late stage）and carbon absorption during daytime（growth and development period）.The overall trend of the gross primary productivity（GPP）and ecosystem respiration（Reco）showed an inverted“U”shap,lower in early and late growth periods,higher in mid-growth periods.In the early period,farmland was mainly performed soil respiration,and R_eco was significantly larger than GPP;Later crop development,photosynthes was greater than respiration,and GPP is greater than R_eco.The farmland ecosystem of summer maize is a system with high carbon sink effect.During the whole growing season of summer maize,the farmland ecosystem absorbed about 11971.9μmol of CO₂.Correlation analysis showed that CO₂ flux was mainly related to net radiation and photosynthetically active radiation.（3）The diurnal variation of water vapor flux showed an inverted“U”shap,with a high value at noon,a low value at morning and evening,and the peak value at 12:00～14:00.The summer maize farmland had always shown the role of water vapor source and transported water vapor to the atmosphere through evapotranspiration.Water vapor flux showed a good response to photosynthetically active radiation and temperature,which were quadratic functions,and showed a significant linear relationship with net radiation,which increased with the increase of net radiation.（4）The variation law of daily evapotranspiration in summer maize farmland showed an inverted “U”shap,and the peak appears at about 12:00～14:00.The total evapotranspiration during the growing season of summer maize was 325 mm,which was 138.6 mm more than the total precipitation（186.4 mm）.When the precipitation occurred,the evapotranspiration increased significantly and then decreased.There was a lag period of about 1 d in time.The correlation between evapotranspiration and photosynthetically active radiation,net radiation and vapor pressure difference was the greatest.In addition,the variation trend of water use efficiency（WUE）was relatively stable during the whole growth period,ranging from 0.08～8.53μmol·mmol^-1.Correlation analysis found that water use efficiency was significantly negatively correlated with saturated water vapor pressure difference,net radiation,atmospheric temperature,average wind speed,photosynthetically active radiation,and soil heat flux,and was significantly positively correlated with air relative humidity and precipitation.