Diurnal and seasonal variation of carbon dioxide exchange over a film-mulched cotton field under drip irrigation in northern Xinjiang

To study the diurnal and seasonal variation of CO_2 fluxes and to explore the effects of biotic and abiotic factors on the seasonal dynamics of CO_2 exchange for cotton fields,CO_2 fluxes were measured using Eddy covariance (EC) methods at a film-mulched cotton field under drip irrigation in Wulanwusu(85°49'E,44°17'N,469 m a.s.l.), which is located in northern Xinjiang in the northwest part of China and has a continental climate.The net ecosystem carbon exchange(NEE ) was partitioned into gross ecosystem production(GEP) and ecosystem respiration(R_(eco)). Diurnal changes of NEE in each growth stage and evolution of daily GEP,R_(eco) and NEE in cotton growing season were analyzed.Except for the emergence and seedling stage,the daytime CO_2 fluxes varied with maximum assimilation occurred closing to the noon while the nighttime fluxes changed little.Overall,the evolution of daily GEP,R_(eco) and NEE was closely related to the leaf area index(LAI).Absolute values of GEP,Reco and NEE increased to the maximum of 11.8,8.0 and 6.2 g C·m~(-2)·d~(-1) respectively in the flower-boll stage when the LAI reached peak.The mean daily NEE was 2.6,1.6,-1.2,-2.8 and 0.5 g C·m~(-2)·d~(-1) for the emergence,seedling,squaring,flower-boll and boll-opening stage,respectively.The integrated NEE value for the whole growing season was about—122.2 g C/m~2 and for the whole year of 2010 was 65 g C·m~(-2)·a~(-1),showed that the cotton fields behaved as a carbon sink in growing season and weak carbon source for the year.Partial correlation coefficients between daily CO_2 fluxes and meteorological factors showed that air temperature was the most important one influencing the dynamics of carbon fluxes during the growing season.The rise in air temperature increased assimilation of carbon dioxide while vapor pressure deficit (VPD ) exerted significant limitation on the carbon fluxes.