Plant-Mediated Hydraulic Redistribution: A Valve Controlling Watershed Solute Transport?

Authors

Alex Redlins¹* (redlinsa@oregonstate.edu), Pamela Sullivan¹, Kamini Singha²,  Holly Barnard³, Emily Graham⁴, Rahila Yilangai³

Institutions

¹Oregon State University, Corvallis, OR; ²Colorado School of Mines, Golden, CO; ³University of Colorado–Boulder, CO; ⁴Pacific Northwest National Laboratory, Richland, WA

Abstract

Plants passively move water around in the subsurface from wet to dry soil via their roots in a process known as hydraulic redistribution. Soil moisture is an important regulator of microbial activity, organic matter decomposition, biogeochemical cycling, and soil properties. This passive redistribution of soil water may have important implications for a soil’s carbon carrying capacity, nutrient exchange, and soil structure, particularly in water-limited environments such as the Mediterranean climate of the Western U.S. The project seeks to elucidate the relationship between hydraulic redistribution, soil nutrient dynamics, and soil properties through a multi-year study, which will combine data from a controlled greenhouse experiment with in situ data collected from two adjacent hillslopes at the HJ Andrews Experimental Forest; one which is known to experience hydraulic redistribution, and one which experiences little to no hydraulic redistribution. Recent work has revealed two types of ecohydrologic function associated with high and low hydraulic redistribution among the hillslopes of the Andrews Forest. Where trees have access to groundwater, moisture in the upper soil profile increased by nearly 2% daily. In contrast to this, hillslopes where tree access to groundwater is inhibited experience a daily moisture increase in the upper soil of < 0.5%. Both types of ecohydrologic function have been documented on two adjacent hillslopes in the Andrews Forest. The study will build on existing techniques for the identification and quantification of hydraulically redistributed water by combining transpiration and physiological measurements of trees with new geophysical methods for mapping the magnitude, location, geometry, and flow paths of near surface water. In conjunction with these techniques, researchers will be quantifying soil carbon pools via data collection of soil respiration and soil-carbon chemistry analyses. Furthermore, the classification of soil physical and hydrologic properties will be done through field and laboratory methods. Combined, this data will help us understand how surface vegetation influences subsurface water fluxes with the goal of identifying how these biologically mediated water fluxes may, in turn, alter soil-carbon dynamics and soil physical properties.