||This project will advance quantitative and predictive understanding of belowground processes, particularly the well-known, but poorly understood, phenomenon of “hydraulic redistribution” (HR). During HR, soil water moves upward, downward, or horizontally from moist to dry soil through plant roots, which serve as conduits connecting soil volumes. Field measurements and regional scale modeling indicate that HR enhances overall moisture availability to plants, leading to increased stomatal conductance (and thus enhanced carbon assimilation). In addition to this direct effect of HR on plant productivity, it has been hypothesized for decades that “hydraulic lift” (upward HR of deep soil water to dry, nutrient-rich surface soil) may also stimulate enhanced soil microbial activity, thus indirectly affecting plant productivity via soil nutrient availability. In combination, these direct and indirect effects of HR on ecosystem processes have large biogeochemical implications, however the current generation of terrestrial ecosystem models and earth system models do not include a representation of HR. Using a linked suite of empirical experiments, small-scale mechanistic modeling, and terrestrial ecosystem and earth system modeling, we will explore HR’s impact on terrestrial carbon, nitrogen, water, and energy cycles through both the direct and indirect pathways. Greenhouse experiments will assess the effect of HR on plants and soil microbes, as a function of soil moisture, texture, and plant transpiration patterns. Root-scale mechanistic modeling will capture both hydraulic and biogeochemical aspects of HR’s influence belowground, aiming to reveal dominant controllers of HR and soil microbial response. Large scale modeling will draw from greenhouse and field data (from 4 Ameriflux sites), and incorporate information from mechanistic modeling in order to improve the representation of HR in earth system models and to quantify the effects of HR on terrestrial ecosystems in past and future regional climates. The work addresses two major questions: 1) How do HR-related belowground processes influence the past and future regional climate in North America? 2) How does the feedback from the atmosphere modify the strength of HR’s impact on the terrestrial carbon, nitrogen, water and energy cycles?