Effect of pH and some organic anions on the solubility of soil phosphate: implications for P bioavailability

Summary 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−1) and anion (0–2 mmol kg−1) were applied to soil in suspension (0.5 g soil cm−3). The effect of 2 mmol kg−1 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. Resume L'effet du pH et de l'addition des carboxylates (acetate, oxalate, tartrate, salicylate et citrate) sur la solubilite du phosphate a eteetudie pour comprendre les consequences possibles de l'exsudation racinaire sur la disponibilite du phosphate. Le sol utiliseetait un Luvisol calcique avec un pH neutre et un pouvoir tampon important. Un acide fort (0–20 mmol kg−1) et les anions organiques (0–2 mmol kg−1) ont ete apportes au sol en suspension. L'effet de l'oxalate (2 mmol kg−1) sur l'isotherme d'adsorption du phosphate a ete mesure. La vitesse d'echange isotopique est peu sensible aux traitements etudies. Ni le pH ni l'acetate n'ont modifie la solubilite du phosphate. L'apport d'anions di- et trivalents est suivi d'une petite augmentation de la solubilite de phosphate. L'effet augmentait avec les concentrations croissantes de l'anion, et de plus il etait largement independant du pH et de la proportion de l'anion adsorbee. L'oxalate limitait plus efficacement l'adsorption du phosphate apporte recemment qu'il ne desorbait le phosphate natif. Ces resultats illustrent que l'acidification de la rhizosphbre n'augmente pas forcement la solubilite du phosphate. Neanmoins les carboxylates, bases conjuguees des acides organiques, peuvent jouer un role important dans l'amelioration de la disponibilite du phosphate du sol.