Contrasting response of root traits of arbuscular mycorrhizal and ectomycorrhizal trees to phosphorus availability in subtropical forests

Soil phosphorus (P) availability regulates plant performance in many forests, but the mechanisms under-pinning these effects are unclear, which limits our ability to adequately understand plant community dynamics in these systems. We conducted a field survey and a soil P manipulation experiment in a subtropical forest to investigate how root functional traits, mycorrhizal colonisation and putative pathogen infection of arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) tree seedlings respond to gradients of soil P. In the field, AM seedling roots were more heavily infected by pathogenic fungi than ECM seedling roots, while the opposite was seen for mycorrhizal colonisation. In the P manipulation experiment, mycorrhizal colonisation was enhanced and pathogenic infection was reduced when soil P availability was low. Root functional traits, including root-to-shoot ratio and root branching intensity showed opposite trends in their response to P availability between AM and ECM plants. Our findings suggest that AM and ECM seedlings have contrasting strategies to deal with P limitation: AM plants produce more fine roots, while ECM plants invest in mycorrhizal colonisation for P acquisition whilst also reducing the likelihood of pathogen infection. These contrasting strategies may regulate interspecific competition and contribute to the stable coexistence of different types of mycorrhizal plants.