To explore the relationship between water and carbon utilization and key factors to keep high water use efficiency (WUE), a 2-yr experiment was conduct by covering 0 and 0.6 kg m~(-2) straw to the surface of soil with plant densities of 1.0 × 10~5, 7.5 × 10~4, and 5.5 × 10~4 plants ha"1 in North China Plain during summer maize growing seasons of the 2012 and 2013. Results showed that straw mulching not only increased grain yield (GY), WUE, and carbon efficient ratio (CER) but also inhibited CO2 emission significantly. WUE positively correlated with CER, GY and negative correlated with evapotranspiration (ET) and CO2 emission. CER had the larger direct effect on WUE compared with ET and CO2 emission. The results indicate that straw mulching management in summer maize growing seasons could make sense for inhibiting CO2 emission.
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