Phosphorus facilitation and covariation of root traits in steppe species

Summary Different phosphorus (P)‐acquisition strategies may be relevant for species coexistence and plant performance in terrestrial communities on P‐deficient soils. However, how interspecific P facilitation functions in natural systems is largely unknown. We investigated the root physiological activities for P mobilization across 19 coexisting plant species in steppe vegetation, and then grew plants with various abilities to mobilize sorbed P in a microcosm in a glasshouse. We show that P facilitation mediated by rhizosphere processes of P‐mobilizing species promoted growth and increased P content of neighbors in a species‐specific manner. When roots interacted with a facilitating neighbor, Cleistogenes squarrosa and Bromus inermis tended to show greater plasticity of root proliferation or rhizosheath acid phosphatase activity compared with other non‐P‐mobilizing species. Greater variation in these root traits was strongly correlated with increased performance in the presence of a facilitator. The results also show, for the first time, that P facilitation was an important mechanism underlying a positive complementarity effect. Our study highlights that interspecific P‐acquisition facilitation requires that facilitated neighbors exhibit a better match of root traits with a facilitating species. It provides a better understanding of species coexistence in P‐limited communities.