Forests are recognized as the largest natural source of nitrous oxide (N₂O) emissions on land, with deforestation drastically reducing the cover and biodiversity of native forests worldwide. Yet, how losses in forest biodiversity affect soil N₂O fluxes remains poorly understood. Here, we combined a global tree diversity-forest soil N₂O dataset, including 201 paired comparable observations from global forests, with a three years field survey of in-situ fluxes data gathered from a long-term plant diversity field experiment. Our analyses reveal that tree diversity has a significant negative effect on soil N₂O emissions, primarily driven by a decrease in N₂O production associated with denitrification. More specifically, we showed that reductions in N₂O emissions with tree diversity can be attributed to a decrease in the availability of soil inorganic nitrogen. Predictive modeling further shows that compared to single tree species, forests with two tree species can reduce global N₂O emissions by 10.39%, while those with 24 tree species achieve the maximum mitigation effect, reducing emissions by 56.30%. Taken together, our work highlights the contribution of tree diversity for mitigating N₂O emissions, highlighting the importance of accounting for biodiversity when reforesting old forests, and while supporting new afforestation processes.

Forests are recognized as the largest natural source of nitrous oxide (N₂O) emissions on land, with deforestation drastically reducing the cover and biodiversity of native forests worldwide. Yet, how losses in forest biodiversity affect soil N₂O fluxes remains poorly understood. Here, we combined a global tree diversity-forest soil N₂O dataset, including 201 paired comparable observations from global forests, with a three years field survey of in-situ fluxes data gathered from a long-term plant diversity field experiment. Our analyses reveal that tree diversity has a significant negative effect on soil N₂O emissions, primarily driven by a decrease in N₂O production associated with denitrification. More specifically, we showed that reductions in N₂O emissions with tree diversity can be attributed to a decrease in the availability of soil inorganic nitrogen. Predictive modeling further shows that compared to single tree species, forests with two tree species can reduce global N₂O emissions by 10.39%, while those with 24 tree species achieve the maximum mitigation effect, reducing emissions by 56.30%. Taken together, our work highlights the contribution of tree diversity for mitigating N₂O emissions, highlighting the importance of accounting for biodiversity when reforesting old forests, and while supporting new afforestation processes.