Coordinated variation in elemental composition and morphology in leaves, but independence in roots across Chinese grasslands

Abstract The biogeochemical niche (BN) hypothesis, which states that each species has a specific elemental concentration and stoichiometric proportions, has yet to be tested in different organs from plants spanning a broad environmental spectrum in grasslands. As a novel aspect, we tested the BN hypothesis by analysing leaf and root elemental composition (i.e. carbon, nitrogen and phosphorus concentrations and its ratios), as well as leaf and root morphological traits (i.e. leaf thickness, specific leaf area, leaf tissue density, root diameter, specific root length and root tissue density) from 139 species across 82 sampling sites in northern China's grasslands. We also investigated the relationships of the BN, determined by leaf and root elemental composition and its stoichiometry, with the morphological niche (MN), defined by leaf and root morphological traits in these grasslands. We found that a substantial amount of the variance in leaf and root elemental concentrations, their ratios and morphological traits was explained by legacy effects (phylogenetic + species). This demonstrates that both elemental composition and morphology can serve as valuable tools for identifying species niches. Additionally, leaf BN was highly associated with root BN. These findings introduce a new potential paradigm for understanding BN, highlighting a significant connection between species‐specific elemental compositions across different organs. Leaf BN and MN were also strongly correlated, supporting the leaf economics spectrum. However, root BN and root MN were independent of each other, indicating the presence of a multidimensional root economics spectrum. Synthesis. Our findings demonstrate the broad applicability of the BN hypothesis and the relationship between BN and MN in different plant organs. In this regard, our results suggest that elemental composition and morphological traits co‐evolve in leaves but evolve differently in roots among species in grasslands.