A mechanistic model to describe the effect of complexing root exudates on transport and uptake of soil nutrients

Abstract

Root exudates may affect nutrient concentration in soil solution. This is often called chemical mobilisation because once a nutrient gets into soil solution, it becomes mobile. The action of root exudates may be by solubilisation of sparingly soluble salts, desorption of strongly adsorbed ions or by complexation. The latter is considered in this paper. Complexation decreases ionic concentration in solution, subsequently decreasing ion uptake. On the other hand the increased total concentration will allow for higher transport to the root. Furthermore, the root may or may not be able to absorb the whole complex. To evaluate the effect of these processes, a simulation model was developed and run with an exudation rate as found for citrate in plants. The results show that through complexation total concentration in solution increases, but that of the free ion concentration decreases, leading to a drastic decrease in uptake. If the whole complex is absorbed by the root or the ion is released at the root surface, uptake increases. Uptake is then more dependent on exudation rate than on the original ionic concentration in solution. Complexation of ions by root exudates will be helpful in reducing uptake of Al and some heavy metals, but will increase uptake of Fe complexed by siderophores. Complexation of P will increase uptake only if the complex is broken down at the root surface.