Studies On The Effects Of Plastic Film Mulching On Water And Carbon Fluxes In Rainfed Spring Maize Croplands

Water,energy and carbon exchange between the land surface and atmosphere is one of the major concerns in the field of ecology.Analyzing the variation and controlling mechanism of water,energy and carbon fluxes is meaning for understanding of material cycle and energy cycle in terrestrial ecosystem.This study is based on eddy covariance technique,soil respiration measurement system,soil evaporation measurement instrument and environmental factors measurement on the experiment in Shouyang County on the Loess Plateau.We observed net ecosystem exchange?NEE?,soil respiration?R_s?and its components in the un-mulching rainfed spring maize field during 2012–2014;observed NEE in the plastic film mulching rainfed spring maize field in 2013 and 2014;and observed evapotranspiration?ET?and its components in the both fields,and energy fluxes in the un-mulching field in 2015 and 2016.with aim to reveal the pattern of material and energy exchange in the rainfed spring maize croplands,and the differences in carbon and water fluxes between the two fields.The main conclusions are summarized as following:1.Plastic film mulching improved the gross primary productivity,decreased the net ecosystem exchange,and increased the carbon sequestration in the rainfed spring maize field1.1.Carbon balance and the important controlling factors of carbon fluxes were determined.During the growing season,the seasonal variations in daily NEE,ecosystem respiration?R_e?,and gross primary productivity?GPP?were mainly controlled by green leaf area index?GLAI?,following by total photosynthetically active radiation?PAR_t?and air temperature?T_a?;during the non-growing season,daily R_e was mainly controlled by air temperature?T_a?and increased after precipitation in the un-mulching field.Only⁶0%of the annual GPP was lost as R_e,resulting in an average annual net CO₂uptake of 509 g C m^-2.Taking into account carbon exported?C_e?and carbon imported?C_i?,the un-mulching field acted as a carbon sink of 25,61,and 24 g Cm^-2 in 2012,2013,and 2014,respectively.1.2.The effects of plastic film mulching on dry yield and carbon sequestration were revealed.Compared with the un-mulching field,total GPP and R_e increased by 6.42%and 3.27%,respectively,and the sequestered carbon from the air increased by 121 g Cm^-2 in the plastic film mulching field in2015 and 2016.Taking into account C_e and carbon imported C_i,the plastic film mulching field acted as a carbon sink of 76 and 41 g Cm^-2 in 2013 and 2014,respectively,which were higher than the corresponding data in the the un-mulching field.Compared with the un-mulching field,total dry yield increased by 1721kg ha^-1 in the plastic film mulching field.We therefore believe that plastic film mulching is good for carbon sequestration and yield.2.Plastic film mulching regulated the water cycle,optimized the evapotranspiration structure,and improved the water effectiveness and water use efficiency in the rainfed spring maize field2.1 The pattern of energy fluxes and the effect of plant growth on surface parameters were determined.Downward longwave radiation offset 83%of upward longwave radiation in both years;19%and 18%of downward shortwave radiation was reflected back to the atmosphere by the land surface in 2015 and 2016,respectively in the un-mulching field.During the growing season,latent heat flux was the largest consumer of net radiation?R_n?;during the non-growing season,sensible heat flux was the dominant consumer of R_n.Soil heat flux?G?and imbalance energy(I_mb)did not exhibit obvious seasonal variation pattern,and G/R_n and I_mb/R_n values were relatively low during different periods.Midday evaporative fraction,daily crop coefficient,midday Priestley–Taylor coefficient,and midday surface conductance increased linearly with increasing GLAI,and the midday Bowen ratio decreased linearly with increasing GLAI during the growing season,indicated that GLAI controlled the development of the surface parameters.2.2.The important controlling factors of ET and its components and the effects of plastic film mulching on ET and its components were revealed.In the both fields,the main controlling factors of ET and plant transpiration?T?were GLAI and R_n,the main controlling factors of soil evaporation were R_n and soil water content,indicated plastic film mulching did not change the important controlling factors of ET and its components.But,compared with the un-mulching field,total ET and E decreased by 37mm and 65 mm,respectively,and total T increased by 28 mm in the plastic film mulching field,indicated that plastic film mulching decreased E significantly,increased T slightly,and decreased water losses.Plastic film mulching not only increased yield,but also performed better in the carbon and water cycles,we therefore recommend the model of plastic film mulching to plant spring maize in the rainfed area in the eastern part of the Loess Plateau.3.Increases in clouds and/or aerosols changed the characteristic of energy input,decreased the gross primary productivity,and increased the resources use efficiency in the rainfed spring maize fieldThe pattern of light use efficiency?LUE?and water use efficiency?WUE?and the mechanism of the effect of clouds and/or aerosols on LUE and WUE were revealed.The peak values of monthly average diurnal light use efficiency?LUE?and water use efficiency?WUE?were found at sunset and at⁷:00 AM,respectively,and the seasonal patterns of LUE and WUE were generally consistent with that of GLAI.During the period of rapid growth,LUE and WUE decreased linearly with increasing clearness index?K_t?.The relationship between ET and available energy was the linear equation,as well as that between GPP and diffuse PAR?PAR_f?.However,GPP only increased sharply with increasing direct PAR?PAR_r?when PAR_r<5 mol PARr m^-2 d^-1.When clouds and/or aerosols increased in the air,the ratio of PAR_f to PAR_t increased,producing a more uniform irradiance of the canopy and resulting in increases in LUE and WUE.