Modelling the interactions between root system architecture, root functions and reactive transport processes in soil

Soil-plant models always oversimplified the representation of soil chemical processes or root system. The objectives of the study were (i) to present a model overcoming such limitations, and (ii) to illustrate its relevance for the modelling of soil-plant interactions. We coupled a root system architecture (RSA) model with a reactive transport model using a macroscopic approach. The two models were coupled sequentially using Fortran-C++ interoperability. We used the resulting model to investigate the case of phosphorus (P) acquisition from hydroxyapatite (HA) in an alkaline soil as induced by P and calcium (Ca) uptake and pH variations in the root zone. Important model parameters were issued of the literature and we tested its sensitivity to selected soil properties. Model sensitivity to grid size and time increment was evaluated as well. The simulations revealed that HA dissolution can contribute very substantially to P nutrition in case of rhizosphere alkalisation thanks to Ca and P uptake. Root-induced acidification was much more efficient at acquiring P, suggesting that ammonium-fed plants should be more P efficient. The variations of dissolved P in the root zone partly agreed with the observations, suggesting that P release was rather controlled by desorption when alkalisation occurs. The presence of more soluble minerals as well as the increase of Ca uptake should enhance P acquisition by crops. We developed a new model and demonstrated the interest of the mechanistic description of geochemical processes with a spatially-explicit distribution of roots in soil while modelling soil-plant interactions. Results of its first application to P acquisition from a mineral source in an alkaline soil were overall consistent with the literature.