Long-term growth stage-specific dynamics and environmental drivers of water use efficiency in a wheat–maize rotation agroecosystem: Evidence from a 22-year eddy covariance record in the North China Plain

Ecosystem water use efficiency (EWUE), defined as the ratio of gross primary productivity (GPP) to evapotranspiration (ET), reflects water–carbon coupling in agroecosystems. However, previous whole-season scale assessments have rarely resolved growth stage-specific differences in long-term EWUE trends. Based on a 22-year eddy covariance record spanning 2003–2024 from a winter wheat–summer maize rotation system in the North China Plain, this study quantified growth stage-specific trends in ET, GPP, and EWUE and evaluated the environmental factors associated with stage-specific EWUE variability. Results showed the significant interannual trends in whole-season EWUE for either crop, but clear stage-specific changes: maize EWUE declined significantly during the silking-to-maturity stage (−0.06 g C kg –1 H 2 O yr –1 ), whereas wheat EWUE increased significantly during the sowing-to-green-up stage (0.10 g C kg –1 H 2 O yr –1 ). Boosted regression tree analysis showed that sunshine hours and vapor pressure deficit were the leading explanatory variables for maize silking-to-maturity EWUE variability, whereas photosynthetically active radiation, diurnal temperature range, and air pressure were the dominant explanatory variables for wheat sowing-to-green-up EWUE. Counterfactual detrending analysis showed that the observed increases in sunshine hours and vapor pressure deficit reduced maize silking-to-maturity stage EWUE, as removing their trends increased ΔEWUE over time by 0.005 and 0.002 g C kg –1 H 2 O yr –1 , respectively. For wheat, diurnal temperature range and sunshine hours promoted sowing-to-green-up EWUE, whereas air pressure partly offset this increase, with ΔEWUE trends of −0.025, −0.005, and 0.016 g C kg –1 H 2 O yr –1 after removing their trends, respectively. These findings highlight the need to assess EWUE dynamics at the growth stage scale, as long-term environmental changes can alter crop water–carbon coupling differently across key phenological stages under climate change.