Soil Hydraulic Conductivity Controls Soil Moisture Limitation of Transpiration Globally

At a critical soil water content (θ_crit), terrestrial ecosystem fluxes at the soil-vegetation-atmosphere interface transition from energy into water limitation. Understanding and predicting soil, plant and atmospheric mechanisms that control θ_crit are central to interpreting and predicting impacts of drought on ecosystems, including the associated feedbacks to carbon and hydrological cycle. Thanks to the existing monitoring networks, θ_crit can now be estimated globally across soils, biomes and climates. However, the mechanisms and key parameters that explain θ_crit as a result of soil-, plant-, and climate-interaction remain elusive. Here, we show that the soil hydraulic conductivity function determines mean and variability of θ_crit. The underlying concept to calculate θ_crit assumes that soil moisture limitation of transpiration is triggered by a loss in soil hydraulic conductivity around the roots. Taking soil-specific hydraulic properties into account, our soil-plant hydraulic model predicts the observed mean and variance of θ_crit as a function of soil textural classes. In coarse textured soils, θ_crit is small due to the lower absolute soil hydraulic conductivity and its steeper decline with soil drying compared to fine textured soils. The increasing variability of θ_crit in fine-textured soils is explained by (i) the wide range of hydraulic conductivity values for similar soil textures as a result of soil structure formation and (ii) by the higher sensitivity to plant traits and climate for soils with less steep hydraulic conductivity curves (i.e., loamy soils). The corresponding critical soil matric potential (h_crit) is also soil texture specific, and it covers a broad range of values, from values close to field capacity in sandy soils (h_crit ca. -100 hPa) to values close to the wilting point in clay soils (h_crit ca. – 1 MPa). The model implies that climate change has a smaller effect on θ_crit in sandy soils, suggesting that soil texture modulates climate effects on water use and photosynthesis globally. Overall, our results prove the prominent role of soil hydraulic conductivity for water limitation of ecosystem fluxes and for plants’ potential to adjust to water limitations subject to alterations due to climate change.