Climate reverses nutrient controls on forest ecosystem carbon allocation

Understanding how soil nutrients and climate jointly regulate carbon allocation between tree and soil is crucial for predicting terrestrial carbon dynamics under global change. Using a nationwide forest dataset across China, we quantified the effects of soil nitrogen (N) and phosphorus (P) on carbon allocation between tree and soil stocks and examined how these relationships vary with stand stages and climate. In the early stand stages, rapid tree growth led to a sharp increase in carbon accumulation in trees, yet soil N exerted a negative influence on carbon allocation, indicating that elevated N favored carbon transfer to soils, yielding greater soil carbon accumulation than in trees. In the subsequent stand stages, both N and P exerted negative effects on carbon allocation, shifting from N-control to coupled N–P regulation. However, climate reversed these negative effects, favoring greater carbon retention in trees rather than in soils. Together, these findings uncover a previously unrecognized mechanism by which nutrient enrichment channels more carbon from trees into soils, rather than merely increasing biomass carbon stock. This finding challenges the traditional view that nutrient enrichment mainly boosts tree carbon and highlights that forests may sustain carbon sequestration through enhanced soil carbon accumulation under changing climatic conditions.