Relationship between root water uptake and soil respiration: A modeling perspective

Soil moisture affects and is affected by root water uptake and at the same time drives soil CO₂ dynamics. Selecting root water uptake formulations in models is important since this affects the estimation of actual transpiration and soil CO₂ efflux. This study aims to compare different models combining the Richards equation for soil water flow to equations describing heat transfer and air-phase CO₂ production and flow. A root water uptake model (RWC), accounting only for root water compensation by rescaling water uptake rates across the vertical profile, was compared to a model (XWP) estimating water uptake as a function of the difference between soil and root xylem water potential; the latter model can account for both compensation (XWP_RWC) and hydraulic redistribution (XWP_HR). Models were compared in a scenario with a shallow water table, where the formulation of root water uptake plays an important role in modeling daily patterns and magnitudes of transpiration rates and CO₂ efflux. Model simulations for this scenario indicated up to 20% difference in the estimated water that transpired over 50 days and up to 14% difference in carbon emitted from the soil. The models showed reduction of transpiration rates associated with water stress affecting soil CO₂ efflux, with magnitudes of soil CO₂ efflux being larger for the XWP_HR model in wet conditions and for the RWC model as the soil dried down. The study shows the importance of choosing root water uptake models not only for estimating transpiration but also for other processes controlled by soil water content.