Close linkage between available and microbial biomass phosphorus in the rhizosphere of alpine coniferous forests along an altitudinal gradient

The rhizosphere is a hotspot of soil phosphorus (P) transformation, which profoundly influences the P status of plants. Although P is projected to limit the ability of forests to serve as a carbon sink, it remains unclear how rhizosphere P availability responds to changing environments in alpine forests. Here, we investigated changes in rhizosphere available P across a series of altitudinal bands (2850 m, 2950 m, 3060 m and 3200 m) in alpine forests and examined the potential regulators of rhizosphere P availability, including temperature and soil biotic and abiotic properties. The results showed that rhizosphere P availability decreased up to the 3060 m site but then increased at the 3200 m site. A structural equation model showed that temperature and soil properties (pH and organic carbon content) indirectly affected rhizosphere available P through amorphous iron/aluminum oxides and microbial biomass P, which had negative and positive effects on rhizosphere available P, respectively. Thus, sorption by soil minerals and turnover of microbial biomass P may be key processes regulating P availability. In contrast, soil organic acids and acid phosphatase, which may promote the release of P by ligand exchange and mineralization, respectively, did not show a positive relationship with rhizosphere available P. Overall, our findings highlight the potential role of microbial biomass as a labile P pool that provides readily available P by turnover and protects P from sorption by soil minerals, which could help in elucidating the mechanisms by which plants maintain their P nutrient supply in alpine ecosystems under environmental changes.