Seasonal and inter-annual variations in carbon fluxes and evapotranspiration over cotton field under drip irrigation with plastic mulch in an arid region of Northwest China

Xinjiang is the largest semi-arid and arid region in China, and drip irrigation under plastic mulch is widely used in this water-limited area. Quantifying carbon and water fluxes as well as investigating their environmental drivers over cotton fields is critical for understanding regional carbon and water budgets in Xinjiang, the largest cotton production basin of China. In this study, an eddy covariance (EC) technique was used to measure the carbon and water fluxes of cotton field under drip irrigation with plastic mulch in the growing seasons of 2009, 2010, 2012 and 2013 at Wulanwusu Agrometeorological Experiment Station, a representative oasis cropland in northern Xinjiang. The diurnal patterns of gross primary production (GPP), net ecosystem exchange (NEE) and evapotranspiration (ET) showed obviously sinusoidal variations from June to September, while the diurnal ecosystem respiration (R-es) was stable between daytime and nighttime. The daytime hourly GPP and ET displayed asymptotic relationships with net solar radiation (R-net), while showed concave patterns with raising vapor pressure deficit (VPD) and air temperature (T-a). The increases in hourly GPP and ET towards the maximum occurred over half ranges of VPD and T-a. The seasonal variations of GPP, NEE and ET were close to the cotton phenology, which almost reached the peak value in July. The cumulative GPP averaged 816.2 +/- 55.0 g C/m(2) in the growing season (from April to October), and more than half of GPP was partitioned into NEE (mean value of -478.6 +/- 41.4 g C/m(2)). The mean seasonal ET was 501.3 +/- 13.9 mm, and the mean water use efficiency (WUE) was 1.0 +/- 0.1 (mg C/g H2O)/d. The agro-ecosystem behaved as a carbon sink from squaring to harvest period, while it acted as a carbon source before the squaring time as well as after the harvest time.