Interannual Variability Of Carbon And Water Fluxes In A Rain-fed Maize Ecosystem And Its Climatic And Biotic Controls

Elevated CO₂and water deficit are two hot issues in global climate change.Cultivated land occupies approximately 40%of the terrestrial surface,and 30%of that is used for agriculture.Interannual variability（IAV）in net ecosystem carbon production（NEP）plays an important role in the processes of the carbon cycle,but the long-term trends in NEP and the climatic and biotic control of IAV in NEP still remain unclear in agroecosystems.Evapotranspiration is a key factor in crop productivity in rain-fed agriculture.Global climate change alters the interaction effects of atmospheric demand and soil moisture on evapotranspiration,with unknown changes to crop productivity in rain-fed agriculture.Thus,knowledge of the inter-annual variability in ET and its controlling factors could help develop appropriate agriculture water management schedule and model crop production.We investigated interannual variability in energy,water,carbon and yield and they climatic and biotic controls using an eddy-covariance dataset for 2005–2020 for rain-fed spring maize in northeastern China,and decomposed it into transpiration（T）and evaporation（E）with the Shuttleworth-Wallace model in order to provide a theoretical basis for simulating the carbon and water cycle and its effect on water use efficiecy under the background of climate change.The main findings are as follows:1.The average EBR of the data from 2005 to 2020 is 0.75,indicating that the energy of the research site is basically closed,and the observed data are reliable and can be used for subsequent analysis.The annual of net radiation was 2333±145W m^-2,surface heat flux was29±20 W m^-2,sensible heat was 790±147 W m^-2,latent heat was 991±121 W m^-2,Bowen ratio was 0.81±0.2 W m^-2.Rn had no significant change trend and G showed no significant trend change,H increased significantly and LE decreased significantly indicating evapotranspiration significant decrease.The Bowen ratio had significant increase trend and LE was significantly negatively correlated with H indicating that more energy was allocated to sensible heat,thus reducing the latent heat.DR had significant increase trend over time,at the same time UR also showed significant increased trend but surface albedo had no significant change trend,indicating that the underlying surface had no obviously change.Although DLR showed no significant change trend,ULR significantly increased,and temperature was significantly positively correlated with ULR,indicating that ground emitting longwave radiation heated the air and caused a significant increase in temperature.2.the annual of ET was 418±51mm,T was 247±50 mm,E was 182±19,and the ratio of E and ET was 0.44±0.07.The evapotranspiration and transpiration of maize farmland had significant decreasing trends.There was no significant changing trend for evaporation.Precipitation exhibited no significant trend;however,soil water content and groundwater table depth decreased with time,accompanied by significantly increased air temperature and vapor pressure deficit.Meanwhile,an increased use of groundwater resulted in a lowering of the groundwater table depth.Decreasing trend of ET was controlled by T,rather than E.T was mainly controlled by canopy conductance and soil water content.CO₂ concentrations and vapor pressure deficit exhibited indirect effects on T by reducing canopy conductance,while air temperature,groundwater table depth and precipitation had negative effect on T via reducing soil water content.Our results indicated that the decreasing ET may be more severe with crop physiological adaptability for a decreased available soil water,highlighted the urgency of appropriate agricultural water management practices for improving soil water supply to maintain yield of rain-fed farmland.3.Average annual NEP was 270±31 g C m^-2yr^-1,with no significant changes over time.The effects on interannual variability in NEP of gross ecosystem productivity（GEP）that was mainly controlled by soil water content（SWC）and leaf area index（LAI）,were more than those of respiration（RE）that was controlled by temperature and LAI.Further,maximum daily NEP(NEP_max)that was dominated by summer vapor pressure deficit explained the largest fraction of annual anomalies in NEP,followed by carbon dioxide uptake period（CUP）that was defined by the beginning date（BDOY）and the end date（EDOY）of CUP.The variability in BDOY was mainly determined by spring precipitation and the effective accumulated temperature,and the variability in EDOY was determined by autumn precipitation,SWC and LAI.NEP may decrease with declining precipitation in the future due to decreasing GEP,NEP_max,or CUP,and irrigation and residues cover may be useful in efforts to maintain current NEP levels.Our results indicate that interannual variability in NEP in agroecosystems may be more sensitive to changes in water conditions（such as precipitation,SWC and VPD）induced by climate changes,while temperature may be an important indirect factor when VPD is dominated.4.There was a significant positive correlation between yield and photosynthetic capacity.Environmental and biotic variables acting via photosynthetic capacity explained 67%of the variance in yield.The standardized direct effect of photosynthetic capacity on yeild was 0.48.Water use efficiency（WUE）,canopy stomatal conduction(g_sc)and Soil volumetric water content were the main controlling factor for photosynthetic capacity with the standardized direct effects were 0.65,0.42 and 0.32.So the standardized indirect effects of the three variables on ET change were 0.32、0.20 and 0.15。photosynthetic capacity increased with WUE increased and decreased with SWC decreased,so photosynthetic capacity did not change significantly from year to year because of the opposing effects offset.The standardized indirect effects of CO₂ and VPD via WUE,g_scand SWC on ET change were 0.04and-0.22.The effect of CO₂ on WUE was positive,but the effect on g_sc was negative,thus it was offset the effect of CO₂ on yield.It is indicated that In soil water deficit condition,CO₂fertilization is beneficial to maintain maize yield.5.At the economic yield level,WUE had a significant increase trend which indicated that the WUE of crops increased,and the main influencing factors wre photosynthetically active radiation and CO₂.While at the ecosystem level,WUE increased significantly,indicating that WUE of maize agroecosystem increased,and the main influencing factors were photosynthetically active radiation,temperature,precipitation and CO₂.