Species-dependent phosphorus acquisition strategy modulates soil phosphorus cycle in the subalpine forest of eastern Tibetan Plateau

The availability of phosphorus (P) in soils will ultimately determine forest productivity because of increasing P limitation in terrestrial ecosystems. However, how root exudates affect the availability of soil P in subalpine forests remains unclear. Here, bulk soils (BS) and rhizosphere soils (RS) under Abies fabri and Rhododendron decorum were respectively collected in the early-, mid- and late-growing seasons in a subalpine forest of eastern Tibetan Plateau, and low molecular weight organic acids (LMWOAs), microbial biomass P and P fractions were analyzed to decipher the effects of the plants on soil P availability. The P fractions in both BS and RS showed a distinct difference between A. fabri and R. decorum because of their different P acquisition strategies. The ericoid mycorrhiza-associated R. decorum sequestered soil available P through organic P mineralization, while the ectomycorrhizal mycorrhiza-associated A. fabri directly or indirectly acquired both the organic and inorganic P pools. Seasonal variations in soil available P further revealed that the difference in the P acquisition by the two species was closely associated with their growing stages. The increase in the concentrations of available P in RS of A. fabri was significantly related to the LMWOAs that dominated by citric acid, likely through the desorption or ligand exchange rather than acidification effect because of limited range of soil pH in the mid-growing season, while organic P mineralization contributed to available P for R. decorum in the early-growing season. The results of this study indicate that LMWOAs can significantly promote P availability in RS of A. fabri, mycorrhizal types and plant growing stages drive plant P acquisition, which results in the coexistence patterns of different species in the same habitat.