Changes in ectomycorrhizal fungal community composition and diversity along a two-million year coastal dune chronosequence in a biodiversity hotspot

Abstract

Ectomycorrhizal (ECM) fungi enhance plant nitrogen (N) and phosphorus (P) acquisition. During long-term soil and ecosystem development, plant growth shifts from being N-limited to P-limited. However, how community composition and diversity of ECM fungi change during ecosystem development remains unknown. We studied ECM communities and diversity along a two million year Australian dune chronosequence, which shows a clear shift from N- to P-limitation of plant growth with increasing soil age. We hypothesised that ECM community composition strongly varies along the chronosequence, reflecting soil properties, and that diversity increases during pedogenesis. We grew two ECM plant species in soils collected from five distinct chronosequence stages, and quantified ECM operational taxonomy units (OTU) and diversity on roots using next-generation sequencing. To distinguish changes in ECM community composition due to soil properties vs ECM inoculum, we used three soil treatments: i) unaltered soil from each stage (?as is?), ii) triple-pasteurised, mixed soil from all five stages, with addition of live soil inoculum from each particular stage (?specific inoculum?), and iii) triple-pasteurised soil from particular stages, with addition of mixed live inoculum from all five stages (?average inoculum?). We obtained 175 unique ECM OTUs. ECM differed considerably among chronosequence stages in the ?as is? treatment, and these differences were associated with shifts in soil pH and nutrients. Furthermore, OTU richness declined with soil age in the ?as is? treatment. In contrast, there were no differences in ECM community composition or richness in the other two soil treatments. We suggest that strong changes in ECM community composition and diversity along this chronosequence do not simply reflect differences in soil abiotic properties or ECM inoculum, but reflect complex long-term interactions between these factors. Finally, we surmise that the decline in OTU richness is due to few ECM species being effective at acquiring P from severely P-impoverished soils.