The scaling relationship between nitrogen (N) and phosphorus (P) concentrations ([N] and [P], respectively) in leaves manifests plants’ relative investment between the two nutrients. However, the variation in this relationship among taxa as well as its causes was seldom described.

The analysis was based on a dataset including 2483 leaf samples from 46 genera of angiosperm woody plants from 1733 sites across China. We calculated the leaf N–P scaling exponent (β_L) with an allometric equation ([N] = α[P]β), for each genus, respectively. We then performed phylogenetic path analyses to test how the climate and soil niche conditions of these genera contributed to the inter-genus variation in β_L.

The genera living with lower soil P availability presented a more favoured P uptake relative to N, as shown by the higher β_L, suggesting a resistant trend to P limitation. Additionally, genus-wise β_L was positively correlated with soil N–P scaling exponents (β_S), implying that the variation in leaf nutrients is constrained by the variability in their sources from soil. Finally, climatic factors including temperature and moisture did not affect β_L directly, but could have an indirect influence by mediating soil nutrients. Phylogeny did not affect the inter-genus variation in β_L along environmental gradients. These results reveal that the trade-off between N and P uptake is remarkably shaped by genus niches, especially soil nutrient conditions, suggesting that the β_L could be considered as a functional trait reflecting characteristics of nutrient utilization of plant taxa in response to niche differentiation.