The interaction of extracellular enzymes with the solid phase of the soil affects their mobility and their catalytic properties. In particular, adsorption on clay minerals is known to shift the optimum pH of the catalytic activity towards alkaline values. Two conflicting interpretations of this phenomenon have been proposed: a surface pH effect and a pH-dependent modification of the conformation of the adsorbed enzyme. To assess the two mechanisms, we studied the adsorption on montmorillonite of two extracellular acid phosphatases of the ectomycorrhizal fungus Hebeloma cylindrosporum, and its consequences on catalytic activities. The results obtained are better interpreted by a pH-dependent modification of enzyme conformation due mainly to electrostatic interactions with the clay surface. At low pH, the two positively charged enzymes unfold on the negatively charged montmorillonite surface. At high pH, both the enzymes and the clay are negatively charged, and adsorption decreases. Adsorption and modification of conformation are largely irreversible, which should be taken into account when considering the fate of enzymes in soil. Finally, the comparison with the effect of clays on the catalytic activities of intracellular enzymes raises the hypothesis of a selection pressure of the soil solid phase leading to more stable extracellular enzymes.ActivitC enzymatique extracellulaire dans Ie sol: effet du pH et de la force ionique sur l'interaction avec la montmorillonite de deux phosphates acides sCcrdteds par le champignon ectomycorhizien Hebeloma cylindrosporum RCsumC L'interaction des enzymes extracellulaires avec la phase solide du sol modifie leur mobilitt et leur proprittts catalytiques. En particulier, I'adsorption sur les minCraux argileux provoque une augmentation du pH optimal de I'activitt catalytique. Deux interprktations opposkes de ce phtnombne ont Ctt propostes: un effet de pH de surface et une modification de conformation de I'enzyme adsorbte qui dtpend du pH. De faGon ? I tvaluer ces deux mtcanismes, I'adsorption sur la montmorillonite de deux phosphatases acides extracellulaires du champignon ectomycorhizien Hebeloma cylindrosporum a Ct C CtudiCe, ainsi que les constquences de I'adsorption sur leurs activitts catalytiques. Les rtsultats obtenus s'interprbtent mieux par une modification de conformation des enzymes dtpendant du pH et principalement due aux interactions klectrostatiques avec la surface d'argile. A bas pH, les deux enzymes positivement chargtes se dtnaturent sur la surface de montmorillonite chargke ntgativement. A pH tlevt, les enzymes et I'argile sont ntgativement chargtes, et ]'adsorption diminue. L'adsorption et la modification de conformation sont largement irrkversibles, ceci devrait Ctre prise en compte dans I'ttude de I'activitk enzymatique du sol. Enfin, la comparaison avec I'effet des argiles sur I'activitt catalytique d'enzymes intracellulaires soulBve I'hypothbse d'une pression de stlection de la phase solide du sol conduisant A des enzymes extracellulaires plus stables.

The effect of pH and the addition of carboxylates (acetate, oxalate, tartrate, salicylate and citrate) on the solubility of soil phosphate has been investigated to assess the possible effects of root exudates on phosphate availability. The soil was a neutral calcic Luvisol with a large pH buffer capacity. Various concentrations of strong acid (0-20 mmol kg-I) and anion (0-2 mmol kg-') were applied to soil in suspension (0.5 g soil ~m -~) .The effect of 2 mmol kg-' oxalate on the adsorption isotherm of phosphate was also studied. The rate of isotopic exchange was largely unchanged by any of the treatments. Neither pH nor acetate had an effect on phosphate solubility.The addition of di-and trivalent anions increased phosphate solubility somewhat. The effect increased with increasing concentration of anion, and was generally independent of pH and the proportion of anion adsorbed. Oxalate was more efficient in limiting the adsorption of freshly added phosphate than in desorbing native phosphate. The results illustrate that rhizosphere acidification does not necessarily increase phosphate solubility. However, carboxylates, the conjugate bases of organic acids, may play an important role in improving the availability of soil phosphate.Effet du pH et de quelques anions organiques sur la solubilitk du phosphate du sol: conskquences pour la biodisponibilitk du phosphore RCsumC L'effet du pH et de l'addition des carboxylates (acCtate, oxalate, tartrate, salicylate et citrate) sur la solubilitk du phosphate a Ctt CtudiC pour comprendre les constquences possibles de l'exsudation racinaire sur la disponibilitt du phosphate. L,e sol utilist Ctait un Luvisol calcique avec un pH neutre et un pouvoir tampon important. Un acide fort (0-20 mmol kg-I) et les anions organiques (0-2 mmol kg-') ont Ct C apportCs au sol en suspension. L'effet de l'oxalate (2 mmol kg-') sur l'isotherme d'adsorption du phosphate a Ctt mesurC. La vitesse d'kchange isotopique est peu sensible aux traitements Ctudits. Ni le pH ni l'acttate n'ont modifiC la solubilitC du phosphate. L'apport d'anions di-et trivalents est suivi d'une petite augmentation de la solubilitk de phosphate. L'effet augmentait avec les concentrations croissantes de I'anion, et de plus il ttait largement indkpendant du pH et de la proportion de l'anion adsorbke. L'oxalate limitait plus efficacement I'adsorption du phosphate apportt rkcemment qu'il ne dCsorbait le phosphate natif. Ces rksultats illustrent que l'acidification de la rhizosphbre n'augmente pas forckment la solubilitt! du phosphate. Nkanmoins les carboxylates, bases conjugukes des acides organiques, peuvent jouer un r6le important dans I'amClioration de la disponibilitk du phosphate du sol.

No abstract

-The release of oxalate and H + by six ectomycorrhizal isolates (Hebeloma cylindrosporum 1 and 9, Paxillus involutus, Suillus collinitus 2 and 22, Rhizopogon roseolus), able to grow in vitro with NO 3 -as the sole source of N, was measured in response to orthophosphate (Pi) concentration (20, 100 and 500 µM Pi) and CaCO 3 (+500 µM Pi) in the solution. Without CaCO 3 , all isolates excepted H. cylindrosporum ones released oxalate. For each isolate, oxalate production was not related to P-deficiency but was strongly enhanced by CaCO 3 . Whatever the medium composition, H. cylindrosporum alkalinised the solution. Among oxalate-producing fungi, S. collinitus 22 and R. roseolus exhibited an important proton efflux that could be related to transport phenomena for oxalate excretion. CaCO 3 enhanced both oxalate and proton efflux, increasing the chemical action exerted by the fungi on the mineral.