Root System Scale Models Significantly Overestimate Root Water Uptake at Drying Soil Conditions

Abstract

Soil hydraulic conductivity (ksoil) drops significantly in dry soils, resulting in steep soil water potential gradients (ψs) near plant roots during water uptake. Coarse soil grid resolutions in root system scale (RSS) models of root water uptake (RWU) generally do not spatially resolve this gradient in drying soils which can lead to a large overestimation of RWU. To quantify this, we consider a benchmark scenario of RWU from drying soil for which a numerical reference solution is available. We analyze this problem using a finite volume scheme and investigate the impact of grid size on the RSS model results. At dry conditions, the cumulative RWU was overestimated by up to 300% for the coarsest soil grid of 4.0 cm and by 30% for the finest soil grid of 0.2 cm, while the computational demand increased from 19 s to 21 h. As an accurate and computationally efficient alternative to the RSS model, we implemented a continuum multi-scale model where we keep a coarse grid resolution for the bulk soil, but in addition, we solve a 1-dimensional radially symmetric soil model at rhizosphere scale around individual root segments. The models at the two scales are coupled in a mass-conservative way. The multi-scale model compares best to the reference solution (−20%) at much lower computational costs of 4 min. Our results demonstrate the need to shift to improved RWU models when simulating dry soil conditions and highlight that results for dry conditions obtained with RSS models of RWU should be interpreted with caution.

Field scale plant water relation of maize (Zea mays) under drought – impact of root hairs and soil texture

“…But it does not consider rhizosphere processes and we are currently testing and developing approaches to upscale coupled rhizosphere and root hydraulic models to 1D simulation models. Despite not considering rhizosphere processes, the model predicted the onset of water stress, here defined as the time when the transpiration rate was reduced, quite accurately and more accurate than comparisons of simulations with models that include rhizosphere processes or not would suggest (Khare et al 2022). These simulation model comparisons as well as lab experiments investigating the role of rhizosphere processes considered non-growing root systems, a single drying cycle, and a uniform initial soil mositure distribution.…”

“…compaction and loosening of the soil around roots and the impact of root exudates on rhizosphere properties (Landl et al 2021). Efficient numerical approaches have been implemented in 3D root architecture models to simulate the impact of rhizosphere processes on water and nutrient uptake (Khare et al 2022;Mai et al 2019;Schröder et al 2009) showing that including rhizosphere processes in simulation models leads to lower predictions of water and nutrient uptake. The model that was used in this paper and that is based on the approach introduced by Couvreur et al ( 2014) is an upscaled 1D version of the detailed 3D root models and therefore considers root hydraulics and reproduces processes like root water uptake compensation, redistribution and hydraulic lift.…”

Field scale plant water relation of maize (Zea mays) under drought – impact of root hairs and soil texture

“…compaction and loosening of the soil around roots and the impact of root exudates on rhizosphere properties (Landl et al 2021) and to account for the effect of root hairs on soil-root contacts and the mitigation of water potential drops in the rhizosphere. Efficient numerical approaches have been implemented in 3D root architecture models to simulate the impact of rhizosphere processes on water and nutrient uptake (Khare et al 2022;Mai et al 2019;Schröder et al 2009) showing that including rhizosphere processes in simulation models leads to lower predictions of water and nutrient uptake. The model that was used in this paper and that is based on the approach introduced by Couvreur et al ( 2014) is an upscaled 1D version of the detailed 3D root models and therefore considers root hydraulics and reproduces processes like root water uptake compensation, redistribution and hydraulic lift.…”

Development and calibration of the FSPM CPlantBox to represent the interactions between water and carbon fluxes in the soil-plant-atmosphere continuum

“…ψ m,soil can thus be used by the xylem module (Dirichlet boundary condition), see Eqn. (29). The coupling between CPlantBox and DuMu x is made available via a python binding within the root-soil interaction module DuMu x -ROSI [29].…”