Mycorrhizal fungi and phosphatase involvement in rhizosphere phosphorus transformations improves plant nutrition during subtropical forest succession

Phosphorus (P) availability is a limiting factor for plant growth in tropical and subtropical regions, but many tropical and subtropical forests maintain high levels of productivity and biodiversity under P-limited conditions, which is why P limitation and plant community biomass increase simultaneously during succession in subtropical forests. Biologically-mediated P transformations in the rhizosphere are expected to increase P acquisition by plants during forest succession in tropical and subtropical regions. In this study, we collected leaf and rhizosphere samples from the dominant tree species in three successional forests (early, middle, and late) at the Dinghushan Biosphere Reserve, Southern China, and measured the leaf P concentrations and biological properties of the rhizosphere: microbial biomass P (MBP), P fractions, acid phosphatase activity, and the biomasses of arbuscular mycorrhizal fungi (AMF) and ectomycorrhizal fungi (EMF). We found that the leaf N:P ratio increased but the rhizosphere microbial biomass N:P ratio decreased from the early- to late-successional stages, indicating more plant P limitation and less microbial P limitation during forest succession. The decreased proportion of occluded P in the rhizosphere was negatively related to the increasing AMF biomass, and the increased proportions of labile organic P (Po) and moderately labile Po were positively associated with increasing AMF biomass but negatively related to the acid phosphatase activity that increased during forest succession. We propose a mechanism for P availability in subtropical successional forests based on AMF processes that increase the transformation of occluded P to labile and moderately labile Po forms, followed by acid phosphatase activity that converts these hydrolysable Po forms into inorganic P. Transformation of occluded P and Po fractions in the rhizosphere to plant-available P will satisfy the increasing P demand of plants during forest succession, thus maintaining the high productivity and biodiversity of P-limited tropical and subtropical forests.