Root asynchrony stabilizes grassland belowground productivity under global change

Stabilizing ecosystem productivity under increasing climatic variability is a central challenge in ecology. While species asynchrony is a well-established mechanism buffering fluctuations in aboveground productivity, whether an analogous stabilizing process operates belowground remains unclear. Here we introduce root asynchrony, the temporal desynchronization of root productivity among soil layers, as a vertical analogue of species asynchrony. Using a multi-year precipitation-manipulation experiment in a temperate grassland and a global synthesis of grassland precipitation and nitrogen manipulations, we show that root asynchrony consistently increases the temporal stability of depth-integrated belowground net primary productivity (BNPP). Across drivers and sites, root asynchrony predicted whole-profile BNPP stability but had little explanatory power for stability within individual soil layers, indicating that stabilization emerges through cross-depth compensatory dynamics rather than local buffering. Shallow-layer stability also independently and positively covaried with whole-profile stability, indicating that profile stability arises not only from cross-depth compensation but also from variance reduction within the shallow layer. Environmental controls on root asynchrony were driver-specific, with hydroclimatic conditions structuring responses to precipitation change and treatment intensity dominating responses to nitrogen addition. Structural equation models further revealed root asynchrony and shallow-layer stability as key mediators linking environmental variation to belowground stability. Together, our results establish root asynchrony as a general, depth-integrated stabilizing mechanism in grasslands, extending biodiversity-stability theory into the belowground domain and improving predictions of ecosystem stability under global change.