Plant-Mediated Hydraulic Redistribution: A Valve Controlling Watershed Solute Transport?
Authors
Marc Dumont1* (marc.dumont@mines.edu), Xander Redlins2, Rahila Yilangai3, Kamini Singha1, Holly Barnard3, Emily Graham4, Pamela Sullivan1
Institutions
1Colorado School of Mines, Golden, CO; 2Oregon State University, Corvallis, OR; 3University of Colorado–Boulder, Boulder, CO; 4Pacific Northwest National Laboratory, Richland, WA
Abstract
Plants passively move water in the subsurface from wet to dry soil via their roots in a process known as hydraulic redistribution (HR). This passive redistribution of soil water may have important implications for a soil’s carbon carrying capacity, nutrient exchange, and soil structure, particularly in seasonally water-limited environments such as the Mediterranean climate of the western United States. Soil moisture is an important regulator of microbial activity, organic matter decomposition, biogeochemical cycling, and soil properties. The objective of the project is to elucidate the relationship between HR, soil carbon and nutrient dynamics, and soil properties through a multi-year study within situ data collected from two adjacent hillslopes at Watershed 10 of H.J. Andrews Experimental Forest in Oregon, USA. Here, recent work has revealed two types of ecohydrological functions associated with high and low HR among the hillslopes. Where trees have access to groundwater, moisture in the upper soil profile vary by nearly 2% between night and day. In contrast, hillslopes where tree access to groundwater is inhibited experience a daily moisture increase in the upper soil of <0.5%. To meet objectives, the team relies on the combination of multidisciplinary monitoring to quantify hydraulically redistributed water and its relationship to soil structure and biogeochemistry. In summer 2023, the team began monitoring sap flow, soil moisture, CO2 soil concentration, matrix potential, and soil and tree self-potential. The team also assessed soil structure and chemistry through X-ray computed tomography and high-resolution carbon and nutrient pool characterizations. These measurements are being acquired in four sites, two on each slope with a first site close to the river and a second uphill, where the vegetation have more limited access to groundwater. Last summer’s data provide first insights of interactions between trees, soil water, and geochemical dynamics. Additionally, the classification of soil physical and hydrologic properties via field and laboratory analysis has revealed distinct differences between rooting patterns and soil structural properties between high- and low-HR hillslopes. The team finds that high-HR sites tend to have more homogeneous rooting patterns, where low-HR sites have distinct trends in coarse- and fine-root patterns with depth. This result suggests that where water is more limited, roots may have more targeted rooting strategies. Furthermore, in low-HR sites, it appears that more root mass is dedicated to fine, non-woody absorption roots. High-HR sites also tend to have a higher soil clay fraction and smaller overall soil aggregate distribution than low-HR sites. Self-potential monitoring highlights water flows from the soil to the tree. Initial analyses show that upward water flows are greater at sites near the stream and a strong contrast between high- and low-HR slopes. Combined, these 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.