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 nutrientacquisition 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 ecosystemdevelopment, 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 aboveground traits4,6, suggests that fundamentally different plant community assembly processes operate above- and below-ground.