The biodiversity-ecosystem function relationships are foundational to our understanding of the global extinction crisis and its ecological impacts. As the second largest carbon flux, soil respiration greatly influences the global carbon cycle, especially in carbon-rich forests. However, the tree species richness-soil respiration relationship remains unknown. Here, soil respiration was monitored monthly over 3 years in a large biodiversity experiment in subtropical forests. We found a unimodal relationship between soil respiration and richness ranging from 1, 4, 8, 16, to 32, with the highest emission appearing in approximately six-species mixtures. This nonlinear pattern was strongly confirmed by a meta-analysis of forests worldwide. Productivity was the first-order factor which could directly and indirectly promote soil respiration hrough increasing soil water contrnt and fungal community diversity. Incorporating the identified nonlinear relationship and ’tree species richness–productivity–soil moisture/fungal diversity-soil respiration’ pathway into carbon cycle models would improve the prediction of soil carbon dynamics in the context of climate change, deforestation and afforestation in forests.

Water use efficiency (WUE) is central to the global cycles of water and carbon. However, whether increasing tree diversity in plantation can increase WUE remain poorly understood. Here, we conduct a forest biodiversity experiment with 32 tree species spread in 14 ha in subtropical China to assess the effects of neighboring tree diversity on foliar WUE of Cunninghamia lanceolata, a widespread tree plantation species in China. We measure foliar δ13C as the proxies of changes in intrinsic WUE. Folia P concentrations of focal trees increase with trait dissimilarities between focal trees and neighbors, and the increased foliar P concentrations improve foliar WUE of focal trees. This neighborhood complementarity effect on WUE is stronger under more shaded neighborhood. However, neighborhood biodiversity did not significantly affect foliar δ18O, a surrogate for stomatal conductance. These findings suggest that tree biodiversity increases WUE through the complementary usages of soil P between neighboring tree species.