Geochemical and microbial controls of the effect of citrate on phosphorus availability in a ferralsol

Abstract The secretion of organic anions, particularly citrate, by plant roots may be an effective process to mobilize phosphate (PO 4 ) from solid constituents. However, microbial growth and corresponding phosphorus (P) immobilization can act adversely. This experimental study was aimed at studying the relative influence of geochemistry and microbiology on the influence of citrate on P availability in a tropical soil. We carried out water extractions with different citrate concentrations (0–500 μM). A first set of experiment was performed at low temperature (T = 2 °C) in order to have only geochemical effects. We performed two other set of experiments at room temperature (T = 25 °C), without and with addition of glucose and nitrate, in order to investigate the influence of an increase of microbial activity. We measured an overall increase of P availability with citrate concentration in all the treatments. Results obtained at low temperature were consistent with the lack of significant P immobilization by microbial activity. The concomitant increase of dissolved Si, Al and Fe suggested that citrate enhanced the dissolution of PO 4 -adsorbing minerals. This geochemical process should release PO 4 and increase P availability. The overall increase of P availability with temperature further suggested that the dissolution of Al- and Fe-PO 4 minerals was also responsible of the effect of citrate. Lastly, results obtained at 25 °C without addition of glucose and nitrate exhibited a weak effect of microbial activity, as a decrease of P availability was only observed at the lowest citrate concentration (2 μM). The addition of glucose and nitrate enhanced microbial activity as a net immobilization of PO 4 was observed up to 20 μM of citrate. Beyond this threshold, microbial growth appeared limited by other factors as time. Our study revealed that the effect of citrate on P availability was mainly controlled by complex geochemical interactions in a range of temperature conditions. The adverse influence of microbial growth increased with temperature and nutrient supply but revealed limited. The nature and intensity of controlling geochemical processes depend on soil type and fertilization.