Oxalate effects on solution phosphorus in a calcareous soil

Thermodynamics predict that in calcareous soils where the equilibrium activity of solution orthophosphate (P(SO)) is controlled by any calcium-phosphate mineral, calcium oxalate (CaC2O4) precipitation will increase P(SO) by depressing Ca2+ ion activity in soil solution and enhancing dissolution of the P-controlling mineral. Plants and/or associated microflora capable of imparting effective levels of oxalic acid (H2C2O4) to soil solution, may significantly improve their phosphorus nutrition via this mechanism. In previous work, we presented evidence for calcium oxalate (CaC2O4) formation in the rhizosphere of vesicular-arbuscular mycorrhizal (VAM) western wheatgrass (Agropyron smithii Rydb.). Herein, we report on further studies conducted to evaluate the production and effects of indigenous oxalate on P(SO) in a calcareous soil. In greenhouse experiments, we measured P(SO) and soluble oxalate (OX(S)) concentrations in leachate from bare soil columns and soil columns planted to western wheatgrass (Agropyron smithii Rydb.). To evaluate the role of VAM in the production and exudation of H2C2O4, the planted treatments consisted of western wheatgrass which was inoculated with a native mix of Glomus spp. of VAM fungus, and plants that were noninoculated. Changes in soil oxalate content (OX(t)) were monitored following extraction with 0.1 N HCl at the termination (harvest) of each experiment. Solution pH and OX(t) successfully predicted P(SO) concentration in post-harvest saturation extracts of the soils (multiple linear regression model R2 = .925***). However, no consistent treatment differences in total or solution oxalates for either experiment were observed and neither plants nor VAM could be established as the oxalate production sources in these systems. A comparison of column-leachate pH and total P(SO) to thermodynamically modeled stabilities for hydroxyapatite in equilibrium with calcite (CaCO3) and CaC2O4, indicates that total P(SO) was controlled in the soils by the solubility of hydroxyapatite in equilibrium with calcium oxalate monohydrate (whewellite) at rhizosphere CO2 concentrations between 1 and 100 times ambient atmospheric levels.