Interaction between root growth allocation and mycorrhizal fungi in soil with patchy P distribution

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Many plants preferentially grow roots into P-enriched soil patches, but little is known about how the presence of arbuscular mycorrhizal fungi (AMF) affects this response. Lotus japonicus (L.) was grown in a low-P soil with (a) no additional P, (b) homogeneous P (28 mg pot−1), (c) low heterogeneous P (9.3 mg pot−1), and (d) high heterogeneous P (28 mg pot−1). Each P treatment was combined with one of three mycorrhiza treatments: no mycorrhizae, Glomus intraradices, indigenous AMF. Real-time PCR was used to assess the abundance of G. intraradices and the indigeneous AMF G. mosseae and G. claroideum. Mycorrhization and P fertilization strongly increased plant growth. Homogeneous P supply enhanced growth in both mycorrhizal treatments, while heterogeneous P fertilization increased biomass production only in treatments with indigenous AMF inoculation. Preferential root allocation into P-enriched soil was significant only in absence of AMF. The abundance of AMF species was similar in P-enriched and unfertilized soil patches. Mycorrhization may completely override preferential root growth responses of plants to P- patchiness in soil. The advantage of this effect for the plants is to give roots more freedom to forage for other resources in demand for growth and to adapt to variable soil conditions.