Coordinated variations in leaf and root biogeochemical niches.

The biogeochemical niche (BN) hypothesis posits that each species has a specific elemental composition. However, the BN of roots and its interaction with leaf BN have largely been neglected until now across diverse environmental conditions. We investigated the relationships between the elemental compositions of leaves and roots, phylogeny, and environmental variables, as well as the connection between leaf and root BN. We analyzed the concentrations of carbon, nitrogen, phosphorus, potassium, calcium, and magnesium in the leaves and roots of 12 394 individuals from 1238 species. Consistent with the BN hypothesis, despite significant differences in elemental concentrations and their ratios between leaves and roots, we observed strong legacy (phylogenetic + species) signals in the species-specific elemental compositions. This finding confirms that the elemental compositions of leaves and roots can contribute to identifying species niches. Our study revealed a higher phylogenetic conservatism for BN in leaves than in roots and provided evidence of a tight association between the species-specific BN of leaves and roots. Our results underscore the broad applicability of the BN hypothesis across diverse species and biomes and demonstrate the critical role of evolutionary legacy in driving coordinated dynamics in both above- and belowground ecological niches.