Professor Indiana University Bloomington Bloomington, Indiana, United States
Abstract: Soil water potential (ΨS) controls a large number of biophysical processes, including the function of leaves, roots, microbes, and the drive of water through the soil-plant-atmosphere continuum. It’s a notably important variable that is rarely measured in-situ. Volumetric water content (θ) is widely measured in situ and often used to model carbon and water fluxes where soil water retention is absent. The soil water retention curve connects θ and ΨS. This relationship is highly non-linear and dependent on the complex interactions between soil texture and structure. Pedotransfer functions (PTFs) provide soil water retention parameters with empirical equations founded upon the relationship between simple soil physical properties (usually only % sand, silt, clay) and soil water retention. While widely used, PTFs fail to account for the effect of a specific capillary/root matrix on soil hydraulics. Lab-derived water retention curves could increase the accuracy of ΨS estimates, thus enabling more robust linkages between hydroclimate and plant function. This study aims to explore the agreement between lab-derived and PTF soil water retention parameters. It will also investigate the ecological relevance of ΨS by linking lab-derived measurements to ecosystem flux. Lab-derived curves were assembled using the combination HYPROP evaporation and the WP4C dew point method. AmeriFlux sites were chosen to represent a wide range of soil textures and environments. Gap-filled gross primary productivity (GPP) estimates from each site were used to explore the relationship of lab-derived ΨS to ecosystem flux. Results found that lab-derived water retention curves could reduce conceptual uncertainty about how ecosystem fluxes respond to soil water deficits. Lab-derived curves illustrated the effect of organic matter on soil retention capabilities better than PTFs. Lab-derived ΨS had a stronger correlation with GPP than θ, as indicated by R-squared values of 0.981 versus 0.752 at US-MMS and 0.781 versus 0.531 at US-UMB. These results develop a necessary understanding of site-specific soil water retention curves and stress the importance of soil water potential measurements moving forward.