Pore scale modeling of the influence of roots on soil aggregation in the rhizosphere

We present a mechanistic, spatially and temporally explicit microscale model to investigate the interactions between a growing root, its exudates and the soil structure. Our model allows us to simultaneously simulate and study the dynamic rearrangement of soil particles, the input and turnover of organic matter, the root growth and decay, as well as the deposition, redistribution and decomposition of mucilage into the rhizosphere. The interactions between these components are realized within a cellular automaton framework. Mechanistic rules lead to the formation and break-up of soil structures. The most stable configuration is determined by the amount and attractivity of surface contacts between the particles. Alteration of surface types due to addition and decomposition of organic matter and the root growth induced movements of particles result in varying aggregation dynamics over time and space.

We illustrate the capability of our model by simulating the growth and shrinkage period of a fine root in a two-dimensional, horizontal cross section through the soil. We evaluate various scenarios to identify the impact of the root and further influencing factors that shape soil aggregation in the rhizosphere. More precisely, we address how the soil structure formation is influenced by soil texture and the amount of mucilage. We quantify the variations in local porosity due to the change in available pore space as influenced by the root growth. We further identify attractive properties of the soil surface induced by root exudation as key factors for the creation of stable soil structures.