Abstract / Description
The distinct co-occurrence of soil water content (SWC) and vapor pressure deficit (VPD) influences ecosystem water use efficiency (WUE) by modifying the synergistic relationship between gross primary productivity (GPP) and evapotranspiration (ET), yet how they impact each other remains unclear in agricultural ecosystems. Based on long-term eddy covariance flux data (2005–2014) observed at a rainfed maize site in Northeast China, we examined how SWC and VPD affect WUE by decomposing it into gross primary productivity to transpiration ratio (GPP/T) and transpiration to evapotranspiration ratio (T/ET). Results showed that WUE was more sensitive to VPD than SWC. Increasing VPD directly suppressed WUE under all soil moisture conditions, whereas SWC had a context-dependent effect: higher SWC reduced WUE under low VPD but enhanced WUE under high VPD. The underlying mechanism was that changes in GPP/T (plant physiological regulation) dominated the WUE responses to both SWC and VPD (contributing 70.25–83.30% and 67.89–87.96%, respectively), while T/ET (evapotranspiration partitioning) played a minor role (