Scaling and stoichiometry of seed nitrogen and phosphorus across Chinese grasslands

Nitrogen (N) and phosphorus (P) play crucial roles in early seedling establishment, influencing germination, survival, and overall performance. However, large-scale multispecies seed element survey data are extremely scarce, and studies on the N, P concentrations and N:P ratios as well as scaling relationships of N and P in seeds remain limited. To examine the N versus P scaling relationship and N:P stoichiometry in seeds, we conducted a large-scale survey and collected 1652 seed samples from 446 species in 51 families spanning 236 sites across the Chinese grasslands, including typical steppe, desert steppe, and alpine meadow. We found that the arithmetic means of seed N and P concentrations and N:P ratios were 28.95 ± 0.32 mg/g, 4.26 ± 0.05 mg/g, and 7.24 ± 0.07, respectively. Seed N, P concentrations and N:P ratios varied across different plant groups and grassland types. For example, N-fixers had the highest N, P concentrations and N:P ratios across plant groups. Among the grassland types, alpine meadow displayed the highest N and P concentrations but the lowest N:P ratios. Using a recent bootstrapping approach to resolve data imbalance, the overall seed N vs. P scaling exponent was 0.75, supporting the growth rate hypothesis. We also detected strong phylogenetic signals in these traits. After accounting for phylogeny, the scaling exponent in independently evolving lineages was 0.66. These findings highlight how both environment and evolutionary history regulate seed nutrient allocation, advancing our understanding of plant reproductive strategies and providing key parameters for modelling early plant growth.