Dynamics of phosphorus in the rhizosphere of maize and rape grown on synthetic, phosphated calcite and goethite

In calcareous soils the dynamics of phosphorus is controlled by calcite and iron oxides such as goethite which strongly retain P and consequently main tain low P concentrations in soil solution. Plants can drastically change c hemical conditions in the rhizosphere, in particular by releasing H+ or OH- or by excreting organic anions. By modifying the dissolution/precipitation and desorption/adsorption equilibria, roots can influence the mobility of soil P. The aim of this work was to test whether H+ or OH- release can indu ce the mobilization of P in the rhizosphere of maize and rape supplied with NO3-N or NH4-N and grown on synthetic phosphated calcite or goethite as so le source of P. With P-calcite, the mobilization of P was generally related to the acidification of the rhizosphere. With P-goethite, rhizosphere acid ification induced some increase of DTPA-extractable Fe and hence dissolutio n of goethite. Rhizosphere P was concomitantly depleted but the mechanisms involved are less clear. The difference in behavior of the two species is d iscussed.