Response of arbuscular mycorrhizal fungi to soil phosphorus patches depends on context

Foraging strategies in arbuscular mycorrhizal fungi (AMF) for heterogeneously distributed resources in the soil remain to be explored. We used nylon-mesh bags of 30 µm to simulate patches of different phosphorus (P) supply levels (Expt 1) and P forms (organic v. inorganic, Expts 1 and 2). In Expt 1, host maize (Zea mays) was unfertilised; in each pot, five P-enriched bags were supplied with either Na-phytate or KH2PO4 at P rates of 0 (P0), 50 (P50), 100 (P100), 150 (P150) and 200 (P200) mg P kg–1. In Expt 2, maize plants were supplied with 20 (P20) or 50 (P50) mg P kg–1, and five P-enriched bags were supplied with different P forms (Na-phytate, lecithin, RNA, KH2PO4) and a nil-P control. Three fungal species (Funneliformis mosseae, Rhizophagus irregularis, and Glomus etunicatum) were compared in Expt 1, and the first two species in Expt 2. In Expt 1, the hyphal-length density (HLD) of G. etunicatum was not significantly different among different P levels when supplied with KH2PO4, whereas the HLD of R. irregularis tended to increase at higher P supply (above P50) in the Na-phytate treatment. The HLD of F. mosseae increased at P150 when supplied with KH2PO4, and increased at P100 and P150 in the Na-phytate treatment relative to P0. APase activity levels were more related to P supply level, in particular with F. mosseae inoculation and uninoculated control, showing that P200 significantly reduced APase relative to P0. In Expt 2, greater hyphal growth of both fungal species tended to occur with KH2PO4. At P20, the HLD of R. irregularis in treatments with KH2PO4 and lecithin, and of F. mosseae with KH2PO4, were higher than in P0. At P50, the HLD of F. mosseae was higher than of R. irregularis; but P form had no significant influence on HLD of F. mosseae, whereas the HLD of R. irregularis in the P-amended treatment (except with Na-phytate) was higher than in P0. APase activity did not differ significantly between the two fungal species. Highest APase activity generally occurred with lecithin, with no significant difference among the other P forms. Our results indicate that the response of AMF to P-enriched patches is complex, and both the form and amount of P supplied should be considered. Variations between AMF in the proliferation of hyphae to heterogeneous nutrient patches might be a mechanism by which these species can maintain diversity in intensive agricultural ecosystems.