Root-induced modifications of the exchange of phosphate ion between soil solution and soil solid phase

The uptake of phosphorus (P) by roots results in a depletion of phosphate ions (PO4) in the rhizosphere. The corresponding decrease in PO4 concentration in the soil solution (C-P) gives rise to a replenishment of P from the solid phase which is time- and C-P-dependent. This PO4 exchange which reflects the buffer power of the soil for PO4 also varies with the composition and the physico-chemical conditions of the soil. As root activity can modify these physico-chemical conditions in the rhizosphere, the question arises whether these modifications affect the ability of PO4 bound to the soil solid phase to exchange with PO4 in soil solution. The aim of the present work was to measure and compare the parameters which describe the amount of PO4 bound to soil solid phase that is capable to replenish solution P for both rhizosphere and bulk soils. The soil sample was a P-enriched, calcareous topsoil collected from a long-term fertiliser trial. Rhizosphere soil samples were obtained by growing dense mats of roots at the surface of 3 mm thick soil layer for one week. Three plant species were compared: oilseed rape (Brassica napus L., cv Goeland) pea (Pisum sativum L., cv. Solara) and maize ( Zea mays L., cv. Volga). The time- and C-P-dependence of the PO4 exchange from soil to solution were described using an isotopic dilution method. The measured C-P values were 0.165 mg P L-1 for bulk soil and 0.111, 0.101 and 0.081 mg P L-1 for rhizosphere soils of maize, pea and rape, respectively. The kinetics of the PO4 exchange between liquid and solid phases of soil were significantly different between rhizosphere and bulk soils. However, when changes in C-P were accounted for, the parameters describing the PO4 exchange with time and C-P between soil solution and soil solid phase were found to be very close for bulk and rhizosphere soils. For this calcareous and P-enriched soil, plant species differed in their ability to deplete PO4 in solution. The resulting changes in the ability of the soil solid phase to replenish solution PO4 were almost fully explained by the depletion of soil solution P.