Foraging capability of extraradical mycelium of arbuscular mycorrhizal fungi to soil phosphorus patches and evidence of carry-over effect on new host plant

The foraging capability of extraradical mycelium (ERM) of arbuscular mycorrhizal (AM) fungi to high soil phosphorus (P) supply and carry-over effect of ERM on new host plants remain to be explored. This study investigated whether the inhibition of ERM at high P supply varies between fungal species and whether ERM have carry-over effects when they encounter new hosts. A two-compartment device was designed to spatially separate hyphae from host roots and the carry-over effect was investigated using two-stage growth experiments. P fertilizers were applied only to hyphal compartments at a gradient of 0, 35, 150, 300, or 1,000 mg Pa (TM) kg(-1). Maize plants were inoculated with Glomus mosseae, Rhizophagus irregularis or remained uninoculated. The biomass of G. mosseae in the hyphal compartment was on average higher than that of R. irregularis and the difference was greater at the second harvest. The biomass of G. mosseae increased slightly at P35 and thereafter decreased significantly with increasing P level. The biomass of R. irregularis increased slightly at P35 and did not differ significantly at higher P levels from that at P0. In general, inoculation significantly increased the biomass and P uptake of donor and receiver plants. The mycorrhizal growth responsiveness (MGR) and mycorrhizal phosphorus responsiveness (MPR) of both donor and receiver plants decreased at higher P supply level. The ratios of MGR and MPR between donor and receiver plants were close to 1 for both fungal species and were independent of P level. Inoculation with R. irregularis significantly enhanced root P uptake efficiency and alkaline phosphatase activity (ALP) in both donor and receiver plants across all P levels. Our results demonstrate that fungal species differ in their responses to soil P availability. R. irregularis was less sensitive to soil P gradients, and compensated for growth promotion and P uptake of the host by increasing ALP and root P uptake efficiency. The carry-over effects of ERM imply that the maintenance of mycorrhizal networks is of particular importance in intensive agricultural ecosystems.