Abstract
Arbuscular mycorrhizal fungi (AMF) are known to enhance phosphorus (P) acquisition by plants, but it has been suggested that AM strategy becomes ineffective in strongly weathered soils, where P availability is extremely low. Using high-throughput DNA sequencing, we determined changes in the diversity and composition of AMF communities along a 2 M year old sand dune chronosequence in the south-western Australia global biodiversity hotspot. AMF have thus far been largely ignored in biodiversity hotspots, and it is unknown how these communities vary during long-term ecosystem development. We show that AMF spore density decreased from 90,000 spores m-³ in the young dunes to 55,000 spores m-³ in the old dunes. Mycorrhizal colonisation was dominated by Rhizophagus species where it peaked to 24% in 120,000 year old dunes and then dropped again to 9% in 2 M year old dunes. This reduction in colonisation was perhaps due to antagonistic effects of non-mycorrhizal, cluster-rooted Proteaceae plants that dominate on the oldest soils, or to the very low P levels in the soil (<10 ppm total P). Multivariate analyses of the operational taxonomic units (OTUs) show that richness and diversity of the AMF communities are the greatest in the 120,000 year old dunes. A sharp 50% loss of OTUs richness was found in the oldest dunes. The AMF community structure in the mid-aged dunes also differ from the younger and older dunes. Our results are consistent with the view that the AM strategy becomes less effective when P availability declines to very low levels.