Plant diversity's positive effect on soil respiration diminishes with increasing productivity in global forests.

Plant diversity is essential for regulating ecosystem functions, yet its global-scale relationship with soil respiration, a critical component of the carbon cycle, remains unexplored. While plant productivity modulates ecosystem processes, understanding how plant species richness regulates soil respiration across net primary production gradients is key to predicting carbon-climate feedbacks. Here, we integrate two global plant species richness datasets (tree species richness and vascular plant species richness, including woody and non-woody plants) with a global soil respiration dataset estimated using a deep learning model trained on 6355 field observations. We demonstrate that plant diversity enhances soil respiration in low- to mid-productivity forests (<1300 g C m-2 yr-1), but this effect diminishes in high-productivity forests. By stratifying forests along a net primary production gradient and controlling for climatic, soil, and vegetation covariates via structural equation modeling, we reveal a context-dependent role of biodiversity. Greater plant species richness amplifies soil respiration in resource-limited systems but contributes minimally in high-productivity forests, where abiotic factors exert stronger influence. Our findings provide insights into how biodiversity influences soil carbon fluxes, revealing its dynamic role in shaping ecosystem carbon dynamics across productivity gradients.