Coupling of net ecosystem CO2 exchange and evapotranspiration of irrigated maize field in arid areas

Abstract

Understanding water and carbon fluxes over agroecosystem are fundamental for water resources management in arid areas. However, their dynamics and coupling at different temporal scales have not been well understood. Therefore, in this study, net ecosystem CO2 exchange (NEE) and evapotranspiration (ET) over irrigated maize field were monitored using eddy covariance system during two crop growing seasons in 2017 and 2018. The dynamics of NEE, ET and the resulted water use efficiency (WUE, defined as NEE/ET), and their responses to the meteorological and hydrological factors were investigated. At hourly scale, when ET increased at noon, NEE reached the plateau ahead of ET, which resulted in decreased WUE. The maize WUE changed during the growing season. Average daytime WUE at the heading stage was the largest: 9.52 and 8.57 g (CO2) kg−1 (H2O) in 2017 and 2018, respectively. Photosynthetically active radiation (PAR) was the primary driver controlling diurnal dynamics of NEE and ET in irrigated maize field, while the opposite effects of vapor pressure deficit (VPD) on NEE and ET were identified. When the VPD increased from 0 to 3.3 kPa, the slope of the linear relationship between ET and PAR increased; whereas, the slope of the linear relationship between NEE and PAR decreased. At daily scale, the relationship between NEE and ET was generally linear with a slope of 8.59 g CO2 kg−1 H2O, which could be attributed to their similar sensitivities in response to canopy conductance, PAR and soil water content. In addition, the effect of irrigation on WUE was different depending on the crop development stage and crop physiological activities. This study highlights that high VPD would decrease WUE of maize growing in irrigated agroecosystems in arid areas.