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

Background/Question/Methods

The Serengeti is a magnificent grazing ecosystem that maintains the migration of large ungulates. This migration is driven by an antiparallel gradient of fertility and precipitation resulting in tall, but less nutritious, grass in the north and short, more nutritious grass in the south. Grasses associate with arbuscular mycorrhizal (AM) fungi, which ameliorate water stress and improve nutrient acquisition. Plant investment in AM fungi is influenced by nutrient and water availability. Severe water limitation restricts the amount of photosynthate available for symbiotic exchange. Field studies showed that mycorrhizas also vary across the antiparallel precipitation/fertility gradient in the Serengeti; abundance of AM fungi was positively correlated with soil phosphorus and negatively correlated with precipitation. We conducted a factorial experiment to uncouple the interacting effects of soil type, phosphorus, and water availability on AM fungal abundance.  Maize was grown in pots of soil collected from three locations across the natural precipitation/phosphorus gradient: high phosphorus and low precipitation (SOT), low phosphorus and high precipitation (KUH), and intermediate phosphorus and precipitation (SER). Full factorial treatments of +/- phosphorus fertilization and drought/luxury watering were administered for all three soils. Plant and mycorrhizal metrics were measured across treatments.

Results/Conclusions

Our results support the expectation that mycorrhizal biomass will increase with phosphorus fertilization when ambient phosphorus levels are low and decrease when ambient phosphorus levels are high.  External hyphal abundance was reduced by fertilization in SOT soil (high phosphorus) and increased with fertilization in KUH soil (low phosphorus).  Plant allocation to AM fungi was also strongly influenced by water.  Plants in drought-inducing treatments allocated relatively more biomass to AM fungi, especially in SOT soils, suggesting partial mediation of AM symbioses by water limitation in the Greater Serengeti Ecosystem.