Ecosystem water use efficiency (WUE), defined as the ratio of gross primary production (GPP) over evapotranspiration (ET), is a crucial variable that describes the tradeoff between ecosystem carbon uptake and water loss. However, the effects of soil moisture on WUE remain unclear. Based on hourly observations from 36 forest eddy-covariance sites globally and two percentile-based statistical models, we investigate the effects of soil water supply (i.e., volumetric soil water content, SWC) and vapor pressure deficit (VPD) on carbon-water coupling at the hourly timescale. We further decompose WUE into two components, i.e., the ratio between GPP and plant transpiration (T), WUE_t (GPP/T), and the ratio of plant transpiration to total evapotranspiration, T/ET. Results show that the sensitivities of WUE to SWC significantly increased with the increase of VPD. The sensitivities of WUE to SWC are high at high VPD, due to a rapid decrease of T/ET with SWC at high VPD. At low VPD, WUE is largely independent of SWC. The contribution linked to WUE_t is much weaker and no significant relationships are observed between the sensitivities of WUE_t to SWC and VPD. Our study suggests that the change of ecosystem WUE with SWC is largely due to changes in T/ET, while WUE_t remains rather constant.

生态系统用水效率 (WUE) 定义为初级生产总值 (GPP) 与蒸散量 (ET) 的比率，是描述生态系统碳吸收和水损失之间权衡的关键变量。然而，土壤水分对 WUE 的影响仍不清楚。基于来自全球 36 个森林涡流协方差站点的每小时观测值和两个基于百分位数的统计模型，我们研究了土壤水分供应（即土壤体积含水量，SWC）和蒸汽压亏缺（VPD）对碳水耦合的影响以小时为单位。我们进一步将 WUE 分解为两个分量，即 GPP 与植物蒸腾量 (T) 的比值，WUE _t(GPP/T)，以及植物蒸腾量与总蒸发量之比，T/ET。结果表明，随着VPD的增加，WUE对SWC的敏感性显着增加。WUE 对 SWC 的敏感性在高 VPD 时很高，因为 T/ET 在高 VPD 时 SWC 迅速下降。在低 VPD 下，WUE 在很大程度上独立于 SWC。与 WUE _t相关的贡献要弱得多，并且未观察到 WUE _t对 SWC 和 VPD的敏感性之间存在显着关系。我们的研究表明，随着 SWC 的生态系统 WUE 的变化主要是由于 T/ET 的变化，而 WUE _t保持相当稳定。