Coupling a Functional-Structural Plant Model with a Rhizosphere Model To Gain Multiscale Insights into Plant-Soil-Atmosphere Interactions for Water and Carbon Cycles

To assess the impact of agricultural practices on water and carbon cycles within specific Genome-Environment-Management combinations, understanding the interactions across the Soil-Plant-Atmosphere continuum (SPAC) is crucial.

Indeed, soil water conditions influence carbon concentration and transport, impacting soil carbon physical and biochemical reactions.

The soil water and carbon status affect, in turn, the plant water and carbon dynamics directly via the plant-to-soil water or carbon gradient, and indirectly via plant water status, influencing its inner balance of water (uptake, transpiration and flow) and carbon (assimilation, usage for maintenance and growth, storage, respiration, rhizodeposition, and transport).

Reciprocally, plant water and carbon balances affect the soil carbon cycle in the short term through root water uptake and rhizodeposition. Those rhizodeposits are, for the most part, made of exudates and mucilage. Root exudates are low molecular weight organic compounds that are mainly passively diffused, while mucilage is a fluid made of polymers with high molecular weight created from starch via an active process.

Modelling plant and soil water and carbon processes, along with their interactions, can help to understand better and represent the effects of the underlying feedback loops. In this study, we therefore coupled the Functional Structural Plant Model (FSPM) CPlantBox with the rhizosphere model TraiRhizo, implemented using the porous medium flow and transport solver DuMu^x.

The overall coupled model and multiscale framework includes a module of 3D plant architecture development, and modules to represent flow and transport within the plant, the soil and the perirhizal zone around each root segment. Flows between compartments are solved implicitly via fixed-point iteration, using parallel computation for both the 3D soil and rhizosphere models.

We present a case study in which we simulated the growth of a C3 monocot and observed how changes in soil water content, due to root water uptake, influenced dissolved carbon concentration and (de)activation of the soil microbial communities during a dry spell.

In the future, the model will be applied to assess the impact of small dry spells at various stages of plant development against a baseline scenario. In time, this model could support plant breeding efforts to find root traits that aim for more drought-resistant plants in specific pedoclimatic environments.