A Study On Dynamics Of Light Use Efficiency Of A Desert Steppe Ecosystem In Yanchi,Ningxia

Light use efficiency（LUE）refers to the efficiency of vegetation photosynthesis transforming absorbed energy into organic dry matter,being a key indicator for evaluation of the ecosystem productivity.However,the understandings of the characteristics of light use efficiency in desert steppe ecosystem at different time scales and its controlling mechanism are still limited.Moreover,the studies examined how LUE responds to different sky conditions as climate model has predicted a year-by-year decrease in radiation quantity,which may significantly impact the photosynthetic characteristics of the desert steppe ecosystem.Here,we made a continuous in situ measurments of carbon flux in a desert steppe ecosystem using the eddy-covariance techniques.The specific objectives are to（1）analyze the dynamics of light use efficiency（LUE,a ratio of gross ecosystem productivity（GEP）to absorbed photosynthetically active radiation（APAR））at different time scales in relation to environmental and biological factors,as compared with the apparent quantum yield（α）derived from rectaular hyperbolic curve,（2）examine the effects of diffuse radiation on the photosynthetic characteristics of desert steppe ecosystem under different sky conditions.Main results and conclusions were as follows.（1）Theαand LUE of the desert steppe ecosystem showed similar seasonal variation in both years2014-2015.The seasonal changes ofαin 2014 and 2015 are consistent,and they gradually increased in early growing season and reached a maximum at the mid-growing season,and then gradually decreased.The seasonal changes of LUE in 2014 is similar to the seasonal change trend ofα;The seasonal changes of LUE in 2015 with the start of the change of the growing season is gradually increased and a small trough appears in July and then continues to increase.It peaks in September and then declines with the end of the growing season.The average values ofαin 2014 and 2015 were 0.0075μmol CO₂μmol^-1 PAR and 0.0098μmol CO₂μmol^-1 PAR,respectively;And the average values of LUE were 0.0019 g C MJ^-1and 0.0017 g C MJ^-1,respectively.LUE had obvious daily variations.LUE decreased and reached the lowest at around 13:00-14:00 every day,and then increased gradually.This change trend of LUE is mainly because the photosynthetically active radiation reaches its maximum at about 13:00 every day,and the change rate of APAR is greater than the change rate of GEP.（2）The seasonal variation ofαis mainly affected by:T_a,T_s,VPD,andαincreases with the increase of temperature（T_a and T_s）and VPD.αgradually decreased with the increase of REW in 2014,but there was no obvious correlation between the changes in REW andαin 2015.With the increase of temperature（T_a and T_s）,LUE increased first and then decreased.The LUE gradually decreases with the increase of VPD,and increases with the increase of REW.The light use efficiency（αand LUE）rapidly increases with the increase of NDVI.When gs is within a certain range,the light use efficiency increases with the increase of gs.The rate of change of APAR on a daily scale is significantly greater than the rate of change of GEP,resulting in a change in LUE that first decreases and then increases.Photosynthetically active radiation will not only affect the ecosystem productivity,but also affect the variation of LUE in ecosystems.（3）Path analysis results show that the main influencing factors of changes inαand LUE are different.The total effect of the change ofαis mainly related to T_s,followed by the effect between gs and VPD.There is no direct effect between the changes of T_a,NDVI andα,but it will affect the change ofαby affecting gs.The total effect of the change of LUE is mainly related to the VPD,followed by the effect between T_a and T_s.The biological factors gs and NDVI have less effect on the change of LUE,and there is no direct effect between the changes of REW and LUE.（4）The LUE was higher in thin cloudy conditions in both years.As the clearness index（CI）increased,the LUE decreased when the sky conditions change from cloudy sky conditions to clear sky conditions.Theαwas greater under clear sky conditions than that under cloudy sky conditions.As CI increases,αalso shows a gradual increase.The value of the GEP under thin cloudy conditions was greater than that under thick cloudy and clear sky conditions,and the thin cloudy conditions are the main proportions of different sky conditions.The GEP in 2014 and 2015 showed a trend of increasing first and then decreasing with the increase of CI.The GEP reached a maximum value of 0.69 g C m^-2 day^-1when the CI was about0.5 in 2014,and the GEP reached a maximum value of 0.55 g C m^-2 day^-1 when the CI was 0.46 in 2015.（5）The diffuse radiation did encourage a potential increase in the ecosystem production for desert steppe ecosystems.GEP peaked when the CI was around 0.5.The P_max of the cloudy skies was 23.7%higher than that of the clear skies.When the sky was cloudy,the PAR_dif became a positive driver of GEP.In addition,unstressed conditions with lower VPD（1≤1 k Pa）,lower T_a（T_a≤20℃）,and non-stressed water conditions（REW≥0.4）were more conducive to enhancing ecosystem photosynthesis.The light response process under the diffuse radiation condition had higher light use efficiency and higher maximum photosynthetic rate of the desert steppe ecosystem.It was suggested that the diffuse radiation benefit LUE and GEP at our site.In summary,light use efficiency had obvious seasonal trend,and the influence of environmental factors on light use efficiency varied with time scales.The diffuse radiation did encourage a potential increase in light use efficiency and the ecosystem production for the desert steppe ecosystem at our site.In the context of the decreasing trend of diffuse radiation in northwest China and the trend of increasing cloud thickness in sky conditions,changes in sky conditions and diffuse radiation should be considered when establishing a model of light use efficiency in desert grassland ecosystems.