Global patterns of C ₃ /C ₄ grass biomass allocation: how aridity mediates nitrogen‐induced divergent strategies

Summary 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 (C 3 vs C 4 ) 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 C 3 and C 4 plants. Increasing aridity promoted a convergent rise in RSR in both C 3 and C 4 grasses. However, soil N directly increased RSR in C 3 plants, but it enhanced net primary productivity in C 4 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 C 3 and C 4 grasslands, potentially exacerbating this functional divergence as atmospheric N deposition continues to rise.