Effect of root-zone vertical soil moisture heterogeneity on water transport safety in soil-plant-atmosphere continuum in Robinia pseudoacacia

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 (SMCup), E-crit, E and HSM remained steady at first and then decreased rapidly when SMCup below a threshold, while PLK exhibited an opposite trend. With decreasing soil moisture in the lower root-zone (SMCdown), 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 SWCup and SWCdown and became more sensitive to SWCup with a lower SWCdown. SWCdown 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.

Automated summary

This study investigated the effects of vertical soil moisture heterogeneity on water transport safety in the soil-plant-atmosphere continuum (SPAC), using split-root experiments with different soil types. The results showed that water transport safety in SPAC varied significantly with soil moisture distribution, soil types, and atmospheric evaporative demand. Understanding these factors is crucial for optimizing water resource management and analyzing the functional differences of water storage in various soil depths.