Global patterns of C₃/C₄ grass biomass allocation: how aridity mediates nitrogen-induced divergent strategies

Plant biomass allocation is dynamically shaped by drought and nitrogen (N) availability, yet how aridity modulates N effects and whether this interplay is conserved across major photosynthetic pathways (C₃ vs C₄) remain mechanistically unresolved. Using a global dataset of 2747 root-to-shoot ratio (RSR) observations from 631 herbaceous species, we applied a multi-model framework (Random Forest, linear mixed-effects models, and Bayesian structural equation modeling) to investigate: how aridity modulates the effect of soil N on the root-to-shoot ratio (RSR); and whether this modulatory effect differs between C₃ and C₄ plants. Increasing aridity promoted a convergent rise in RSR in both C₃ and C₄ grasses. However, soil N directly increased RSR in C₃ plants, but it enhanced net primary productivity in C₄ plants, which indirectly reduced RSR. This decoupling between drought-response mechanisms and N-acquisition strategies reveals that while the two functional types followed a shared hydraulic rule, they adhere to distinct N economies. Our results reveal that increased soil N availability will trigger divergent carbon storage responses in C₃ and C₄ grasslands, potentially exacerbating this functional divergence as atmospheric N deposition continues to rise.