Abstract: Increasing soil carbon (C) and nitrogen (N) storage can help mitigate climate change and sustain soil fertility. A large number of biodiversity manipulation experiments collectively suggest that high plant diversity increases soil C and N stocks. It remains debated, however, whether such conclusions hold in natural ecosystems. Here we analysed the First (2000-2006) and Second (2008-2017) censuses data from Canada’s National Forest Inventory with the help of structural equation modelling to explore the relationship between tree diversity and the changes in soil C and N stocks of natural forests. The dataset comprises 406 sites that cover much of the range of Canadian temperate and boreal forests with a variety of forest types, including deciduous, coniferous and mixed forests. We found that higher tree functional diversity was associated with greater soil C and N accumulation in the mineral horizon, while forests with higher species evenness had larger soil C and N accumulation in the organic horizon. Specifically, on a decadal scale, increasing species evenness from its minimum to maximum value significantly increases soil C and N in the organic horizon by 30% and 42%, while increasing functional diversity significantly enhances soil C and N in the mineral horizon by 32% and 50%, respectively. Our results indicate that greater tree diversity is significantly associated with higher accumulations in soil C and N stocks, validating inferences from biodiversity manipulation experiments. Our findings highlight that conserving and promoting functionally diverse forests could promote soil C and N storage, enhancing both C sink capacity and soil N fertility.