Deciphering climate, soil, and phylogenetic controls on leaf nitrogen and phosphorus stoichiometry of terrestrial plants

Leaf nitrogen (N) and phosphorus (P) are essential nutrients constraining plant growth and ecosystem productivity. Stoichiometric relationships between N and P concentration have been explored at both regional and global levels in recent decades, especially for leaves, resulting in the increasing number of available data. For example, threshold values of leaf N:P ratios and a 2/3 power law of leaf N-P scaling relationship have been proposed, and are widely used to detect nutrient limitation in terrestrial ecosystems. However, our recent work has revealed that significant variation in the scaling exponent of leaf N against P concentration appeared not only across plant functional and taxonomic groups, but also along climatic and soil fertility gradients. This suggests the N vs. P scaling exponent to be a relatively plastic “trait” that is shaped by local climate environments, soil nutrient conditions, and phylogeny in conjunction, with important feedbacks to ecosystem function and crucial insights into traditional mechanistic ecosystem models, which have mostly modelled the controls on leaf N and P stoichiometric characteristics through the relationships of foliar nutrient concentrations with soil nutrient availability across plant functional types (PFT), ignoring the important differences within PFTs and species along climatic gradients.

To further identify relationships of leaf N against P concentrations with multiple controls across the globe, we compiled a comprehensive leaf N and P dataset. Totally, this dataset contains 12,453 individual records spanning 2,158 sites worldwide, including 203 families, 1,291 genera and 3,361 species. The climatic, soil variables and phylogenetic information provided for each individual record included: (1) geographical location information; (2) topography indices; (3) sampling period during the field campaigns and experiments; (4) taxonomic information; (5) life form (i.e. angiosperm vs. gymnosperm, monocotyledonous vs. dicotyledonous, woody (including coniferous, deciduous broad-leaved and evergreen broad-leaved woody) vs. herbaceous plants; (6) climate indices, including mean annual temperature, mean annual precipitation, aridity index, maximum cumulative water deficit; (7) pairwise soil N and P concentrations for part of the whole records; (8) corresponding soil class information from the Harmonized World Soil Database; (9) corresponding atmospheric CO₂ concentration and bulk atmospheric N deposition during the sampling period; and (10) specific leaf area (TRY Plant Trait Database).

Moreover, we built empirical models of varying complexity, which provides critical insight into the relative importance of plant phylogeny, soil properties and climatic variables in shaping global-scale leaf N and P stoichiometry of terrestrial plants. Then, we assessed the power of the modelled optimal maximum Rubisco carboxylation rate standardized to 25 ℃ (V_cmax25), based on leaf N concentrations, as an independent predictor to investigate the power of the state-of-the-art optimality model driven by climate variables alone for photosynthetic traits. We further explored the use of the N~P scaling relationship for each specific species, life form and PFT in parameterizing current ecosystem models. Collectively, our work may provide an important benchmark for connecting plant elemental stoichiometry with next-generation earth system models, which can enhance our predictions of ecosystem functioning and vegetation dynamics, especially in the context of rapidly environmental changes with increasing CO₂ and N deposition.