Measurements of net ecosystem carbon dioxide (CO2) exchang (NEE) were made, using eddy covariance, to investigate the biophysical regulation of a temperate desert steppe characterized drought in Inner Mongolia, China during 2008. The half-hourly maximum and minimum NEE were −3.07 and 0.85 mol CO2 m−2 s−1 (negative values denoting net carbon uptake). The maximum daily NEE was −6.0 g CO2 m−2 day−1. On an annual basis, integrated NEE was −7.2 g C m−2 y−1, indicating a weak carbon sink. The light response curves of NEE showed a rather low apparent quantum yield (a) and saturation value of NEE (NEEsat). Moreover, a and NEEsat varied with canopy development, soil water content (SWC), air temperature (Ta), and vapor pressure deficit (VPD). Piecewise regression results suggested that the optimal SWC, Ta, and VPD for half-hourly daytime NEE were 12.6%, 24.3 ◦C, and 1.7 kPa, respectively. The
apparent temperature sensitivity of ecosystem respiration was 1.6 for the entire growing season, and it was significantly controlled by soil moisture. During the growing season, leaf area index explained about 26% of the variation in daily NEE. Overall, NEE was strongly suppressed by water stress and this was the dominant biophysical regulator in the desert steppe.