Soil properties are key determinants for the development of exudate gradients in a rhizosphere simulation model

The rhizosphere is increasingly recognised as a key compartment of the soil. The aim of this work is to explore the role of soil, plant and solute properties on the development of exudate gradients in the rhizosphere. To this end, a numerical model of solute movements around a single root was developed and analysed for its sensitivity to several parameters including soil water content, lifetime of exudates, root exudation rates, among others. To keep the model as simple as possible, water content is assumed to be homogenously distributed around the root. In the sensitivity analysis, the thickness of the simulated rhizosphere (e.g. the maximum distance to which exudates moved) ranged between 0.2 and 61 mm with an average of about 7 mm, which is of the same order of magnitude as published values. Similarly, the time needed to reach steady state between exudation and degradation of exudates ranged from a few hours to more than 100 days with an average of 62 days. In the sensitivity analysis, adsorption properties solute's lifetime and soil water content appears to be key determinants of both the extent of the rhizosphere and the time to steady state and therefore should have a fundamental role in determining the degree of interactions between neighbouring roots and with other soil organisms.