Taxonomically conservative root phenolic compounds underpin biogeochemical niche separation between grasses and forbs in temperate steppes of northern China

BACKGROUND AND AIMS: The temperate steppes are dominated by perennial grass and forb species and biogeochemical niche (BN) separation exists universally in leaves between grasses and forbs at different scales. However, the underlying physiological mechanisms still remain unresolved. METHODS: We measured macronutrients, micronutrients and total phenolic compounds in both leaves and roots of nineteen grass and thirty-nine forb species across the Inner Mongolia grassland in northern China, and investigated relationships between root phenolic compounds and BN separation in both leaves and roots at functional group level. KEY RESULTS: Forbs displayed significantly higher concentrations of both leaf and root macronutrient (N, P, S, K, Ca and Mg) than grasses regardless of species and functional groups. Although concentrations of leaf micronutrients (e.g. Fe and Mn) were higher in forbs than in grasses, concentrations of root Fe and Mn were extremely low in forbs than in grasses. Metabonomics and physiological analyses showed that roots of forb plants synthesized and secreted higher phenolic compounds than grass plants at both species and functional group levels. Concentrations of root macronutrients (C, N, P and Ca) were positively correlated with concentrations of total root phenolic compounds, but concentrations of root micronutrients (Fe and Mn) were negatively associated with concentrations of total root phenolic compounds between grasses and forbs. In addition, multiple leaf mineral concentrations (including P, Mg, Ca, Fe, Mn, Zn and Cu) were positively correlated with concentrations of total root phenolic compounds between grasses and forbs. CONCLUSIONS: Our findings underscore that forb species have evolved both higher nutrients and higher phenolic compounds compared to grass species in steppes of northern China. Root phenols-facilitated nutrient acquisition and transportation are involved in leaf and root BN separation between grasses and forbs. These findings may provide valuable insights in predicting species coexistence and biogeochemical cycling in temperate steppes.