Meta-analysis reveals asymmetric root and microbial phenology shifts under global change

Global environmental change is reshaping phenological patterns across ecosystems, yet belowground phenological responses remain poorly characterized. Using a flux-based framework, we derived phenological metrics from 2100 site-years of paired root and microbial respiration time series across 163 sites worldwide. Our meta-analysis shows that root and microbial phenology typically shift asymmetrically in direction and magnitude under global change, except for a warming-induced symmetric advance in season onset. Warming and nitrogen addition extended root growing season, whereas microbial season lengthened under warming and reduced precipitation but shortened following partial vegetation removal. These phenological shifts were largely decoupled from changes in respiration fluxes, indicating a distinct dimension of ecosystem responses to global change. Such phenological adjustments critically influence carbon dynamics, as warming-driven extensions of the microbial season alone could increase global heterotrophic respiration by 2.5 ± 0.8 Pg C yr^-1 (mean ± SE). Our results provide flux-based empirical guidance for representing root and microbial phenology as distinct, driver-sensitive processes in Earth system models, thereby improving projections of ecosystem carbon dynamics.