Wheat, canola and grain legume access to soil phosphorus fractions differs in soils with contrasting phosphorus dynamics

Despite the high phosphorus (P) mobilizing capacity of many legumes, recent studies have found that, at least in calcareous soils, wheat is also able to access insoluble P fractions through yet unknown mechanism(s). We hypothesized that insoluble P fractions may be more available to non-legume plants in alkaline soils due to increased dissolution of the dominant calcium(Ca)-P pool into depleted labile P pools, whereas non-legumes may have limited access to insoluble P fractions in iron(Fe)- and aluminium(Al)-P dominated acid soils. Four crop species (faba bean, chickpea, wheat and canola) were grown on two acid and one alkaline soil under glasshouse conditions to examine rhizosphere processes and soil P fractions accessed. While all species generally depleted the H₂O-soluble inorganic P (water Pi) pool in all soils, there was no net depletion of the labile NaHCO₃-extractable inorganic P fraction (NaHCO₃ Pi) by any species in any soil. The NaOH-extractable P fraction (NaOH Pi) in the alkaline soil was the only non-labile Pi fraction depleted by all crops (particularly canola), possibly due to increases in rhizosphere pH. Chickpea mobilized the insoluble HCl Pi and residual P fractions; however, rhizosphere pH and carboxylate exudation could not fully explain all of the observed Pi depletion in each soil. All organic P fractions appeared highly recalcitrant, with the exception of some depletion of the NaHCO₃ Po fraction by faba bean in the acid soils. Chickpea and faba bean did not show a higher capacity than wheat or canola to mobilize insoluble P pools across all soil types, and the availability of various P fractions to legume and non-legume crops differed in soils with contrasting P dynamics.