Soil moisture in root zone is highly heterogeneous in space, while its effect on water transport safety in soil-plant-atmosphere continuum (SPAC) remains poorly understood. In this study, we conducted vertical spilt-root experiments in R. pseudoacacia using loamy clay and sandy loam soils in greenhouse, and measured the dynamics of midday transpiration rate, predawn and midday leaf water potential with the lower root zone remaining drought and the upper root zone undergoing the drought-rewatered-drought process. The plant supply-demand hydraulic model was calibrated with the measured data, indicating that the model could efficiently simulate SPAC water transport in R. pseudoacacia under the condition of vertical soil moisture heterogeneity. On this basis, we set various combinations of soil moisture in the upper and lower root zones under different soil types and atmospheric evaporative demands in the model, and simulated the variations of indicators describing water transport safety in SPAC, including actual transpiration rate (E), the critical leaf transpiration rate at hydraulic failure (E_crit), hydraulic safety margin (HSM), and percentage loss of soil-plant hydraulic conductance (PLK). The numerical simulations suggested that the water transport safety in SPAC varied substantially with vertical soil moisture heterogeneity, and the responses were impacted by soil types and atmospheric evaporative demand. With decreasing soil moisture in the upper root zone (SMC_up), E_crit, E and HSM remained steady at first and then decreased rapidly when SMC_up below a threshold, while PLK exhibited an opposite trend. With decreasing soil moisture in the lower root-zone (SMC_down), the curves of E_crit, E and HSM presented a descending trend, while the curve of PLK went up. Water transport safety in SPAC declined with decreasing SWC_up and SWC_down and became more sensitive to SWC_up with a lower SWC_down. SWC_down had greater impact on water transport safety in SPAC under coarser-textured soil with higher atmospheric evaporative demand. The results were supplemental to the traditional analysis of soil water availability to plants under homogeneous condition, and would be helpful for analyzing functional difference of water storage in different soil depths as well as for optimizing water resource management.

根区中的土壤水分在空间上高度异质，而对于土壤-植物-大气连续体（SPAC）中水运输安全的影响仍然知之甚少。在这项研究中，我们进行了在垂直溢出根实验刺槐使用壤质粘土和温室沙壤土，并测量正午蒸腾速率，与下根区域剩余干旱和上部黎明前和正午叶水势的动态根区经历了干旱-再水-干旱过程。植物供需液压模型与所测量的数据进行校准，表明该模型可在有效地模拟SPAC水运刺槐在垂直土壤水分非均质条件下。在此基础上，我们在模型中设置了不同土壤类型和大气蒸发需求下上下根区的土壤水分的各种组合，并模拟了描述SPAC中水运输安全性的指标（包括实际蒸腾速率（E））的变化。在出现液压故障的临界叶蒸腾速率（Ë_暴），水力安全系数（HSM）和土壤植物水力传导率（PLK）的损失百分比。数值模拟表明，ＳＰＡＣ的水运安全性随垂直土壤水分异质性的不同而有很大差异，其响应受到土壤类型和大气蒸发需求的影响。随着上根区土壤水分的减少（SMC_上升），E _crit，E和HSM最初保持稳定，然后当SMC_上升到阈值以下时迅速下降，而PLK呈现相反的趋势。随着下部根区土壤水分的减少（SMC_下降），E _crit，E的曲线HSM呈下降趋势，而PLK曲线上升。在SPAC水上运输安全谢绝SWC减少_了和SWC_下来并成为SWC更加敏感_了以较低的SWC_下来。SWC_下降对质地较粗，土壤蒸发量较高的土壤中SPAC的水运输安全影响更大。该结果补充了均质条件下植物对土壤水分有效性的传统分析方法，对分析不同土壤深度储水量的功能差异以及优化水资源管理将有帮助。