Plant species diversity increases as soil phosphorus availability declines during long-term ecosystem development1,2. The increase in plant species diversity is associated with a decline in above-ground functional diversity, because leaf traits converge on a high phosphorus-use efficiency strategy on old and infertile soils3,4. In contrast, the response of below-ground traits that directly influence nutrient acquisition remains poorly understood3,5; yet it might be key to understanding how soil fertility drives patterns of plant species diversity1. Here we show a marked increase in the richness and diversity of plant nutrient-acquisition strategies with declining soil phosphorus availability during long-term ecosystem development in a global biodiversity hotspot. Almost all nutrient-acquisition strategies currently known were found in plants from the most infertile soils, despite these being some of the most phosphorus-impoverished soils on Earth. Mycorrhizal plants declined in relative abundance by >30%, although the decline was compensated by an increase in non-mycorrhizal, carboxylate-exuding species that ‘mine’ phosphorus from the soil using different strategies. Plant species richness within individual nutrient-acquisition strategies also increased dramatically, with the species richness of many strategies more than doubling between the youngest and oldest soils. These results reveal increasing functional diversity of below-ground traits related to nutrient acquisition during ecosystem development, suggesting that no single combination of traits, including those related to nutrient-acquisition strategies, is superior to all others at extremely low soil fertility. Furthermore, the increasing diversity of nutrient-acquisition strategies with declining soil fertility, despite functional convergence of above-ground traits4,6, suggests that fundamentally different plant community assembly processes operate above- and below-ground.
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G.Z. was supported by a scholarship from the Paul Hasluck Bequest administered by the Kwongan Foundation. We acknowledge the assistance of many people in conducting the flora surveys. Funding was provided by research awards from UWA and a DECRA (DE120100352) from the Australian Research Council (ARC) to E.L. H.L. was supported by the Australian Research Council (ARC DP0985685). We thank D. Agudo and A. Bielnicka for laboratory support. We acknowledge the Department of Parks and Wildlife (Western Australia) and the Shires of Dandaragan, and Coorow for permission to conduct research on land under their administration. We acknowledge the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments.