The Tibetan Plateau (TP) has the largest area of alpine grasslands in the world. Among the various grassland types, alpine steppe covers 72 million hectares in the central-western TP, where the climate is even drier than in other alpine ecosystems. Rapid climate warming (0.34 degrees C per decade) since the 1970s has caused significant atmospheric dryness-that is, an increase in the vapor pressure deficit-despite increases in both precipitation and soil moisture on the TP. However, it remains controversial whether the availability of atmospheric or soil moisture has a stronger role in the growth of vegetation. We used eddy covariance to measure the CO2 fluxes in a grazed alpine steppe over three consecutive years. Our results showed that the alpine steppe acted as a net CO2 sink of 47.9-72.7 g C m(-2) yr(-1) despite significant grazing. The CO2 fluxes showed a clear seasonal pattern, largely regulated by climate factors, although human activities were also documented. The soil moisture content dominated the seasonal variation in the net ecosystem productivity: the importance of soil moisture to the net ecosystem productivity was 49.0%, whereas the importance of the vapor pressure deficit was 36.5%. We found that the surface soil moisture, rather than the water content of soil layers deeper than 10 cm, affected the net CO2 uptake more strongly, although its role may have been affected by intense rainfall. This study emphasizes that the availability of surface soil water rather than atmospheric dryness regulates CO2 uptake in alpine steppe ecosystems, suggesting that a warming-wetting climate will favor the net uptake of CO2 by alpine steppes.