Plant and Soil 211: 111–119, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 111 Dynamics of phosphorus in the rhizosphere of maize and rape grown on synthetic, phosphated calcite and goethite I. Bertrand, P. Hinsinger ∗ , B. Jaillard and J.C. Arvieu UFR de Science du Sol, Institut National de la Recherche Agronomique, place Pierre Viala, F-34060Montpellier cedex 2, France Received 29 April 1998. Accepted in revised form 16 Sepember 1998 Key words: calcite, goethite, maize, phosphate, H + release, Brassica napus L., Zea Mays L. Abstract In calcareous soils the dynamics of phosphorus is controlled by calcite and iron oxides such as goethite which strongly retain P and consequently maintain low P concentrations in soil solution. Plants can drastically change chemical 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 induce the mobilization of P in the rhizosphere of maize and rape supplied with NO 3 -N or NH 4 -N and grown on synthetic phosphated calcite or goethite as sole source of P. With P-calcite, the mobilization of P was generally related to the acidification of the rhizosphere. With P-goethite, rhizosphere acidification induced some increase of DTPA-extractable Fe and hence dissolution of goethite. Rhizosphere P was concomitantly depleted but the mechanisms involved are less clear. The difference in behavior of the two species is discussed. Introduction The dynamics of phosphorus in calcareous environ- ments is largely controlled by two major mineral components that are iron oxides and calcium carbon- ates (Matar et al., 1992). Calcium carbonates, which are largely dominated by calcite, can immobilize sub- stantial amounts of phosphate ions by both adsorption processes and precipitation of various types of calcium phosphates (Arvieu and Bouvier, 1980; Freeman and Rowell, 1981). Iron oxides including crystalline iron oxyhydroxides such as goethite can also contribute a large proportion of the sorption of P by calcareous soils (Matar et al., 1992; Ryan et al., 1985). Because of these strong interactions with the soil solid phase, phosphate concentrations in the soil solution of cal- careous soils are often very low. To cope with such adverse conditions, plants have evolved various mech- anisms that influence the chemical conditions at the ∗ Fax No: +33 0467632614. E-mail: hinsinger@enjam.inra.fr root-soil interface and enhance the solubilization of soil P in the rhizosphere (e.g. Hinsinger, 1998). For the acquisition of inorganic P, previous works have shown that the mechanisms that are likely to operate are related to release of protons and to the excre- tion of organic anions by roots (Hinsinger and Gilkes, 1996; Hinsinger, 1998). Some authors have moreover shown that some species like oilseed rape respond to P-deficiency by an enhanced release of either protons (Moorby et al., 1988) or organic anions (Hoffland et al., 1989). These phenomena may induce some dis- solution of both iron oxides and calcium carbonate or phosphate. In addition, ligand exchange reactions may occur and organic anions may be directly involved in the release of sorbed P into soil solution (Gerke et al., 1994). Gahoonia et al. (1992) have also suggested that hydroxyl or bicarbonate ions excreted by species such as ryegrass when fed with nitrate may be responsible for some desorption of P from iron oxide surfaces. The aim of this work was to test whether proton or hydroxyl release can induce the mobilization of P in 7793IS.tex; 17/09/1999; 12:36; p.1 DISK: CP: PIPS No.: 191388 (plsokap:bio2fam) v.1.15