Protozoa stimulate N uptake and growth of arbuscular mycorrhizal plants

Roots of most terrestrial plants interact with arbuscular mycorrhizal (AM) fungi and soil protozoa with each individually increasing nitrogen (N) supply to plants and plant growth. We investigated if AM fungi and protozoa interactively affect plant N acquisition and growth. We used stable isotopes (15N, 13C) to assess plant N uptake, C fixation and C partitioning. Shifts in the microbial community structure were evaluated by analysing phospholipid fatty acids. Protozoa and AM fungi enhanced plant N uptake. Protozoa mobilized N from bacterial biomass and AM fungi increased the translocation of the mobilized N to the host plant; if both were present, plant biomass, C fixation and C allocation to roots increased twofold. Protozoa alone and in the presence of AM fungi altered microbial activity and community structure in the rhizosphere. The results indicate that plant N uptake via mycorrhizal symbiosis strongly benefits from the presence of protozoa.