Tree species richness reduces soil carbon loss via suppressed priming effects

Global biodiversity manipulative experiments report positive effects of plant diversity on ecosystem productivity. Yet, there is lower confidence in predicting a positive plant diversity effect on soil carbon (C) sequestration, largely due to limited understanding of how the decomposition of native soil C responds to diversity-promoted fresh C inputs, the so-called priming effect. Combining a large-scale biodiversity manipulative experiment with stable isotope (13C-glucose) labeling, we found that the priming effect decreased with increasing tree species richness. This reduction was characterized by decreased positive priming (i.e., stimulating native soil organic C decomposition) alongside enhanced negative priming. The variation in the priming effect with increasing tree diversity was associated with increased soil phosphorus availability, enhanced C stability (characterized by physical protection and chemical recalcitrance) and improved microbial network complexity. Our findings reveal a novel mechanism by which tree species diversity promotes soil C storage through dampening microbial decomposition triggered by fresh C inputs. This suppression of the priming effect suggests that diverse forests are better able to stabilize soil organic matter, highlighting the potential of biodiversity-based afforestation strategies to strengthen nature-based climate solutions.