Increase of soil phosphorus bioavailability with ectomycorrhizal tree dominance in subtropical secondary forests

Many forest tree species form symbiotic associations with either arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) fungi to increase access to nutrients. ECM and AM differ in strategies for acquiring nitrogen (N) and phosphorus (P), however, little is known about the degree to which mycorrhiza mediate the effects of tree species on soil P bioavailability outside of the temperate zone. Here, we established a natural gradient with increasing ECM tree dominance in subtropical secondary forests, and investigated the linkages between the ECM tree dominance and soil bioavailable P content. We quantified the contents of four soil bioavailable P pools (CaCl2-P, citric-P, enzyme-P and HCl-P) using the recently developed biologically-based P extraction method, as well as acid phosphatase activity, litter biomass and quality, microbial biomass carbon (C), and soil abiotic variables. We found that CaCl2-P, citric-P and HCl-P increased with ECM tree dominance, while enzyme-P did not. Specially, ECM-dominated soils displayed markedly higher (1.2–2.9 times) citric-P and HCl-P than AM soils, indicating that ECM forests can effectively obtain inorganic P by releasing organic acids and through proton excretion. The acid phosphatase involved in the acquisition of organic P had similar activity between AM and ECM forests. Structural equation models indicated that increasing ECM tree dominance increases litter C/N ratio and soil organic matter, but decreases pH and microbial biomass C/P ratio, all of which are critical in mediating P bioavailability. Collectively, our findings confirm that tree mycorrhizal associations affect soil P bioavailability, which have implications for understanding species coexistence and guiding forest managements in subtropics.