Incremental acidification reveals phosphorus release dynamics in alkaline vertic soils

Phosphorus (P) available to plants in alkaline, vertic soils is thought to be buffered by the dissolution of various calcium phosphate minerals (CaP), driven by pH and the concentration of Ca and/or P in solution. To investigate this hypothesis we incrementally acidified 6 alkaline vertic soils of CaP 300-6000 mg kg-1 in the presence or absence of an anion exchange P sink. Following the early removal of solution and sorbed P sources, P recovery remained low until soil pH passed key thresholds. These thresholds varied little between soils and with the sink (pH6.0-6.3), and soil pH buffer capacity affected the amount of acid required to approach the thresholds. Dissolution of CaP species occurred 0.7-1.0 pH units higher where the solution P concentration was kept below 1μM using the anion exchange membrane sink compared to acidification without the sink. The data support the hypothesis that rhizosphere acidification may increase the availability of CaP minerals to plants; however, the dependence of P release dynamics on pH buffering capacity may put P release beyond the reach of some plant species. Consequently, research is necessary to quantify both plant acidification potential in these soils and the effect of concomitant removal of Ca on release of CaP species into plant available forms.