Pinus armandii ectomycorrhizas mobilize aluminium‐bound phosphorus via carboxylates along a subtropical altitudinal gradient

In the low phosphorus (P) bioavailability, acid‐weathered soils of subtropical southwest China, Pinus armandii employs diverse P‐acquisition strategies (PAS). However, the relative contributions of ectomycorrhizal (ECM) roots versus non‐mycorrhizal roots, and how these strategies shift along altitudinal gradients to mobilize specific cation‐bound P pools, remain poorly understood.

We sampled bulk and rhizosheath soil and root tissues of P. armandii across an altitudinal gradient (2000–3000 m above sea level). To identify the soil P pools targeted by different PAS, we integrated measurements of P–Al, P–Fe and P–Ca stoichiometry into the Hedley fractionation. PAS were quantified using aluminon–agarose assays and leaf manganese concentrations ([Mn]) for carboxylates, alongside fluorometric and staining assays for ECM and non‐mycorrhizal root phosphatase activity.

Carboxylate exudation was substantially greater from ECM root tips than from non‐mycorrhizal root segments. Sites with higher ECM colonization exhibited stronger gel discoloration and elevated leaf [Mn], indicating that ECM roots were the primary source of carboxylates. Across elevations, NaOH‐extractable inorganic P (NaOH‐Pi) was consistently depleted in rhizosheath soil relative to bulk soil and was associated exclusively with Al rather than Fe or Ca. Furthermore, the slope of NaOH‐Pi versus NaOH‐Al was 36.6% lower in rhizosheath soil than in bulk soil, and rhizosheath NaOH‐Pi declined with increasing leaf [Mn], demonstrating preferential mobilization of Al‐bound inorganic P through ECM‐mediated carboxylate release. In contrast, evidence for acquisition of Al‐bound organic P was restricted to the lowest elevation, where phosphatase activity was greater in ECM roots than in non‐mycorrhizal roots and rhizosheath phosphatase activity exceeded that of bulk soil.

Synthesis . Pinus armandii acquires P primarily through ECM‐mediated carboxylate release that mobilizes Al‐bound inorganic P. By integrating P‐cation stoichiometry into Hedley fractionation, our approach links plant P‐acquisition strategies to specific mineral‐associated P pools in natural soils. These findings demonstrate how ECM symbioses construct rhizosheath nutrient niches through mobilization of Al‐bound P, providing new mechanistic insight into plant adaptation to bioavailable P‐poor ecosystems.