Grasslands play a pivotal role in regulating the global carbon cycle, hydrological processes, and biodiversity, yet the mechanisms controlling biomass production vary widely across biomes. Here, we compiled 842 site-level observations from desert, montane, and typical temperate grasslands worldwide and combined threshold regressions, machine learning-based variable importance rankings, and piecewise structural equation models to disentangle the relative contributions of climatic variables (mean annual precipitation, mean annual temperature, and aridity index), edaphic properties (soil carbon, nitrogen, phosphorus, and pH), and biotic factors (species richness, leaf functional traits, and vegetation indices). We show that the dominant controls of biomass differ systematically among grassland types: biotic factors explained 74% of biomass variation in desert grasslands, climatic variables accounted for 54% in montane grasslands, and soil factors explained 31% in typical grasslands. These divergent control regimes reveal a global regulatory gradient in which biomass formation transitions from being primarily driven by biotic factors in desert grasslands, to being governed by climatic factors in montane grasslands, and ultimately limited by edaphic factors in typical grasslands. Our findings provide a mechanistic basis for improving predictions of grassland productivity, refining Earth system model parameterizations, and designing targeted conservation and restoration strategies under ongoing climate change.