Uncoupling the effects of phosphorus and precipitation on arbuscular mycorrhizas in the Serengeti

The Serengeti grassland is characterized by antiparallel gradients of soil phosphorus (P) and precipitation. We hypothesized that grasses associate with arbuscular mycorrhizal (AM) fungi to ameliorate water stress and improve nutrient acquisition; and, that geographic patterns in AM fungal abundance relate to nutrient and water limitation of host plants. We conducted a factorial experiment to uncouple the interacting effects of soil type, P, and water availability on AM fungal abundance. Maize was grown in pots of soil collected from three locations across the natural gradients. Full factorial treatments of +/- P fertilization and high/low water were administered to all three soils. Abundance of AM hyphae in soil was reduced by fertilization in high-P soil and increased with fertilization in low-P soil. Phosphorus uptake efficiency of mycorrhizas was greatest in low-P soil. Water-limited plants grown in arid region soil allocated relatively more biomass to AM fungi. The formation of AM fungi in each soil was most strongly linked to the most limiting belowground resource. Interactions among soil properties, water availability and variation in the community composition of AM fungi are likely to influence the abundance and function of AM symbioses the Serengeti.