Abstract
Arbuscular mycorrhizal fungi (AMF) facilitate ecosystem functioning through provision of plant hosts with phosphorus (P), especially where soil P is limiting. Changes in soil nutrient regimes are expected to impact AMF, but the direction of the impact may depend on context. We predicted that nitrogen (N)-only enrichment promotes plant invasions and exacerbates their P limitation, increasing the utility of AMF and promoting AMF diversity. We expected that enrichment with N, P and other nutrients similarly promotes plant invasions, but decreases the benefit and diversity of AMF because P is readily available for both native and exotic plants. We tested these hypotheses in eucalypt woodlands of south-western Australia, that occur on soils naturally low in P. We evaluated AMF communities within three modified ground-layer states representing different types of nutrient enrichment and associated plant invasions. We compared these modified states to near-natural reference woodlands. AMF richness varied across ground-layer states. The moderately invaded/N-enriched state showed the highest AMF richness, while the highly invaded/NP-enriched state showed the lowest AMF richness. The reference state and the weakly invaded/enriched state were intermediate. AMF richness and colonisation were higher in roots of exotic than native plant species. AMF community composition differed among ground-layer states, with the highly invaded/NP-enriched state being most distinct. Distinctions among states were often driven by family-level patterns. Reference and moderately invaded/N-enriched states each supported distinct groups of zero-radius operational taxonomic units (zOTUs) in Acaulosporaceae, Gigasporaceae and Glomeraceae, whereas Gigasporaceae and Glomeraceae were nearly absent from the highly invaded/NP-enriched state. Further, Diversisporaceae and Glomeraceae were most diverse in the moderately invaded/N-enriched state.
Synthesis. Both the nature of soil nutrient enrichment and plant provenance matter for AMF. N-only enrichment of low-P soils increased AMF richness, likely due to introduction of AMF-dependent exotic plant species and exacerbation of their P-limitation. In contrast, multi-nutrient enrichment, decreased AMF richness potentially due to a decrease in host dependence on AMF, regardless of host provenance. The changes in AMF community composition with nutrient enrichment and plant invasion warrants further research into predicting the functional implications of these changes.
