Differential mobilization of P in the maize rhizosphere by citric acid and potassium citrate

The release of organic acid anions from plant roots into soil has been hypothesized to be a mechanism for enhancing phosphorus availability in the rhizosphere. Although these compounds are excreted from the cytoplasm as organic acid anions (e.g. citrate, malate), when the H+-ATPase is also upregulated there is evidence to suggest that they enter the soil as organic acids (e.g. citric acid, malic acid). The aim of this study was to evaluate the role of citric acid (H-citrate) and potassium citrate (K-citrate) in the mobilization and plant uptake of P from two acid soils contrasting in their P availability. Our results indicated that the mobilization of P from a KH233PO4 labelled patch of soil was soil type dependent, was controlled by its intrinsic P status, and that more P was made available by K-citrate than H-citrate. Similarly, the uptake of 33P from the rhizosphere by Zea mays L. was greatest in the presence of K-citrate in comparison to H-citrate. However, a significant increase in shoot 33P content was only observed in the more acidic soil with high P sorption potential (Haplic podzol) while no significant increase was observed in the less acidic soil with low P sorption potential (Eutric cambisol). We conclude that the chemical form of organic acid anion excretion may have a significant impact on its P mobilization capability. The contrasting results with the two acid soils indicate that organic acids may not provide a universal mechanism for enhancing P uptake from soil.