<b>Coordinated variation in elemental composition and morphology in leaves, but independence in roots across Chinese grasslands</b>

1. 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.2. As a novel aspect, we tested the BN hypothesis by analyzing 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.3. 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.4. <i>Synthesis.</i> 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.1. 生物地球学生态位（biogeochemical niche, BN）假说提出，每个物种均具有特定的元素浓度与化学计量比例，但目前该假说尚未在涵盖广泛环境梯度的草地植物不同器官中得到验证。 2. 作为一项创新性研究内容，本研究针对中国北方草地82个采样点的139个植物物种，分析其叶片与根系的元素组成（即碳、氮、磷浓度及其相互比值）以及叶片和根系的形态性状（即叶片厚度、比叶面积、叶片组织密度、根系直径、比根长以及根系组织密度），以此验证BN假说。此外，本研究还探讨了由叶片与根系元素组成及其化学计量特征定义的BN，与由上述草地植物叶片和根系形态性状定义的形态生态位（morphological niche, MN）之间的关联。 3. 本研究发现，叶片与根系的元素浓度、元素比值以及形态性状的大量变异可由系统发育与物种的遗留效应解释。这表明元素组成与形态性状均可作为识别物种生态位的有效工具。此外，叶片BN与根系BN呈现高度关联。本研究结果为理解BN提供了全新的潜在范式，凸显了不同器官中物种特异性元素组成之间的紧密联系。叶片BN与MN同样呈现显著相关，这支持了叶经济谱（leaf economics spectrum）理论。但根系BN与根系MN则相互独立，这表明存在多维的根经济谱（root economics spectrum）。 4. <i>Synthesis.</i> 本研究结果证实了BN假说在不同植物器官中的广泛适用性，以及BN与MN之间的关联。据此，本研究结果表明，草地植物物种的叶片中元素组成与形态性状协同演化，但根系中二者的演化模式则存在差异。