The hidden Serengeti—Mycorrhizal fungi respond to environmental gradients

•We observed an extremely high abundance and diversity of AM fungi.•Support for the Functional Equilibrium Model was nuanced and expands our understanding of AM fungal ecology.•There was little response to grazing. This suggests that either AM fungi are stable in regards to large ungulate grazing, or that responses are ephemeral.•Responses to the phosphorous (P) gradient suggest that P limitation influences AM fungal structures across the gradient. Although new production of AM fungal hyphae is greatest in low-P sites, the standing crop of hyphae is greatest in the dry, high-P sites.•There was a very tight inverse relationship between soil P and annual rainfall. Our models suggest that AM fungal biomass is directly influenced by P and indirectly influenced by rainfall.•The community of AM fungal spores was responsive to numerous environmental factors. Responses by individual families were varied, suggesting that niche partitioning occurs in response to environmental gradients.•Within the family of Gigasporaceae, we observed evidence for temporal partitioning between auxiliary cells and spores. Previously, this has only been documented in laboratory cultures. Our study provides the first field evidence that auxiliary cells may function as P storage units and are precursors to spores.

Mycorrhizas influence plant productivity, community composition, nutrient dynamics and soil structure, yet the factors determining the abundance and diversity of these symbioses in natural ecosystems are poorly understood. We studied arbuscular mycorrhizal (AM) fungi (Glomeromycota) in long-term grazed and un-grazed plots at eight sites forming natural gradients of soil properties and rainfall in the Serengeti National Park, Tanzania. Four metrics of AM fungal abundance and diversity were examined: (1) abundance and species composition of spores, (2) standing crop of AM hyphae in the soil, (3) production of new AM hyphae in buried mesh bags and (4) a bioassay of AM fungal inoculum potential. Our first goal was to test the predictions of the Functional Equilibrium Model that AM fungi should be most abundant in grazed plots (because of increased nutrient demands caused by overcompensation) and at sites with the lowest soil phosphorus (P) and rainfall. Our second goal was to use multi-group structural equation models (SEMs) to investigate the interrelated influences of grazing, soil properties, rainfall and root biomass on the abundance and species composition of AM fungal communities. Two multi-group SEMs were generated: one to examine abundance patterns of AM fungi and another to examine patterns in the spores or auxiliary cells of different families of AM fungi. We observed an extremely high abundance and diversity of AM fungal spores. Only one prediction of the Functional Equilibrium Model was supported; production of new hyphae was highest in the lowest P soils. The abundance and species composition of AM fungi varied across sites but the only variable that responded to grazing was the abundance of Glomeraceae spores, with higher abundance in fenced (un-grazed) plots. Multi-group SEMs revealed that rainfall, total soil P, soil organic matter and soil texture were all important predictors, but the relative importance of each factor varied depending upon the AM metric or taxon observed. Our findings suggest niche differentiation among AM fungal taxa and demonstrate that climate and soil properties are important predictors of the abundance and community composition of these important, hidden components in the Serengeti ecosystem.