Soil phosphorus availability modifies the relationship between AM fungal diversity and mycorrhizal benefits to maize in an agricultural soil

Arbuscular mycorrhizal (AM) fungi are very abundant in many agricultural soils and are important in increasing and maintaining the sustainability of agricultural systems. However, high soil fertility, especially high available soil phosphorus (P) status, often limits AM fungal development in intensively managed agricultural systems and results in the C-costs to the host plant outweighing any benefits from AM colonization. Some promising approaches can minimize negative effects on soil biota while providing the desired agricultural benefits (e.g. reduced soil tillage and organic amendments). However, the influence of these agricultural practices on the benefits of root mutualistic associations remains poorly understood. Here, we collected indigenous AM fungi from a long-term field experiment with different application rates of cow manure and conducted a pot experiment with different soil P levels. Using high throughput sequencing we demonstrate that increasing cow manure application changed the community composition of AM fungi and reduced community diversity in the field. Phosphorus addition in a bioassay experiment produced a mycorrhizal growth response (MGR) and the mycorrhizal P response (MPR) shifted from positive to negative, regardless of the origin of the inoculum. An increase in intraradical and extraradical AM fungal structures enhanced maize shoot biomass and P uptake under low (no P added) and moderate (20 mg P added kg−1 soil) P levels but the opposite occurred at a high P level (250 mg P added kg−1 soil). The MGR was higher in the treatment with higher community diversity at the moderate P level but there was no significant correlation at the other two P levels examined. We conclude that available soil P may regulate the abundance and diversity of AM fungi and their symbiotic function in intensive cropping systems. Our results indicate that managing the soil phosphorus level to favor a rich and diverse AM fungal community can enhance crop production and result in a more sustainable agricultural system.