Global patterns and controlling factors of non-leaf litter nitrogen and phosphorus stoichiometric characteristics

As a key carrier of carbon (C) and nutrient cycling in terrestrial ecosystems, the initial stoichiometric characteristics of litter not only directly regulate the subsequent decomposition processes but also exert important impacts on soil fertility, microbial activity, and ecosystem productivity. Non-leaf litter is an important component of litter, and can account for more than 40% of the total litter production. Compared with leaf litter, non-leaf litter exhibits marked differences in morphological structure, tissue density, and nutrient concentration, and therefore would play distinct roles in decomposition rate, nutrient release, and their coupling with soil nutrient cycling. However, available studies in the literature have predominantly focused on leaf litter, with comprehensive assessments on the stoichiometric characteristics of non-leaf litter remaining limited. Here, we constructed a database comprising 2993 observations extracted from 398 publications to systematically examine the concentrations of nitrogen (N) and phosphorus (P), as well as the N:P ratio across different types of non-leaf litter (i.e., bark, branch, flower, fruit, root, stem, and coarse woody debris) at the global scale. We also assessed the influences of plant functional types (leaf type, mycorrhizal association, taxonomic group, and leaf form) and environmental factors (geographical location, climate, and soil properties) on the stoichiometric characteristics of non-leaf litter N and P. Results showed that (1) the marginal means of N and P concentrations and N:P ratio in non-leaf litter ranged from 0.36% to 1.49%, 0.05% to 0.13%, and 12.3 to 19.4, respectively, and the concentrations of N and P were significantly higher in functional tissues (e.g., flowers and fruits) than in structural tissues (e.g., stems and woody debris); (2) plant taxonomic group, leaf type, and mycorrhizal association significantly influenced N and P concentrations and N:P ratio, with angiosperms, broadleaf species, and arbuscular mycorrhizal (AM) plants exhibiting generally higher values than gymnosperms, conifers, and ectomycorrhizal (ECM) plants, respectively; and (3) among environmental drivers, soil total phosphorus (TP), total nitrogen (TN), and microbial biomass C-to-N ratio (MBC:MBN) emerged as the key factors controlling non-leaf litter N and P stoichiometric characteristics across or within different non-leaf litter types. These results are consistent with the global patterns found for the stoichiometric characteristics and driving factors of leaf litter N and P in the literature, while non-leaf litter exhibits a much wider range of variation. This greater variability reflects the combined influence of organ-specific functional traits, structural composition, and environmental context, which together confer non-leaf litter a distinct ecological role in nutrient release rates and their spatiotemporal dynamics. Overall, our study fills the knowledge gap that previous work largely focused on leaf litter, and the findings highlight the substantial heterogeneity in N and P composition of non-leaf litter and reveal multiple biotic and abiotic mechanisms underlying their variation, thereby extending the ecological stoichiometry framework and providing critical data support for nutrient modeling in terrestrial ecosystems.