Dynamics of phosphorus fractions in the rhizosphere of fababean (Vicia faba L.) and maize (Zea mays L.) grown in calcareous and acid soils

The dynamics of soil phosphorus (P) fractions were investigated, in the rhizosphere of fababean (Vicia faba L.) and maize (Zea mays L.) grown in calcareous and acid soils. Plants were grown in a mini-rhizotron with a thin (3 mm) soil layer, which was in contact with the root-mat, and considered as rhizosphere soil. Hedley sequential fractionation was used to evaluate the relationship between soil pH and P dynamics in the rhizosphere of fababean and maize. Soil pH influenced the dynamics of P fractions in both calcareous and acid soils. Fababean and maize roots decreased rhizosphere pH by 0.4 and 0.2 pH units in calcareous soil, and increased rhizosphere pH by 1.2 and 0.8 pH units in acid soil, respectively, compared with the no-plant control. The acid-soluble inorganic P fraction in the rhizosphere of calcareous soil was significantly depleted by fababean, which was probably due to strong rhizosphere acidification. In contrast, maize had little effect on this fraction. Both fababean and maize significantly depleted the alkali-soluble organic P fractions in calcareous soil, but not in acid soil. Fababean and maize utilised different P fractions in soil, which was partly due to their differing abilities to modify the rhizosphere. This study has decoupled successfully the effects of chemically induced pH change from plant growth effects (such as mineralisation and P uptake) on P dynamics. The effect of soil pH on plant exudation response in P-limited soils has been demonstrated in the present study.