Complementarity in resource use leading to increased resource partitioning is the most commonly proposed mechanism for explaining the positive relationship between plant diversity and productivity. However, we still have a poor understanding of the relationship between plant diversity and root biomass. We used molecular method to identify tree species and to estimate the biomass of fine root (≤ 2 mm in diameter) for each tree species in soil cores sampled from the plots along a tree species gradient elaborated in subtropical forests. Our objectives were to examine whether spatial resource partitioning and symmetric proliferation are responsible for the relationship between aboveground tree species richness (SRA) and fine root biomass. We found that increasing SRA led to higher fine root biomass and a support for symmetric proliferation strategies, but this pattern only appeared in nutrient-rich upper soil layer. Structural equation modelling (SEM) indicated that stand density was the dominant factor to mediate SRA effects on fine root biomass. Specifically, fine root biomass depended on the SRA × stand density interaction, with lower biomass at lower density and low richness, and this effect disappeared in higher density forests. Overall, we found inconsistent support for the vertical niche partitioning, indicating that greater soil volume filling is not the reason for belowground overyielding pattern. Alternatively, density-dependent biotic interactions affecting tree recruitment are an important driver affecting productivity in diverse subtropical forests but the usual root distribution patterns in line with the resource partitioning hypothesis are unrealistic in contexts where soil nutrients are heterogeneously distributed.