The quadratic relationship between tree species richness and topsoil organic carbon stock in a subtropical mixed-species planted forest

1151–1161

200210295

Despite recent evidence from inventories and experimental forests indicating that high species richness among trees increases productivity and further improves soil organic carbon (SOC) storage, the mechanisms controlling the richness–SOC relationship are inconsistent. Hence, the optimal tree species richness required to increase SOC stocks needs to be further explored. Based on field observations from a subtropical mixed-species planted forest, which was established 40 years ago through a mosaic pattern of afforestation and natural regeneration, we examined the effects of tree species richness on the SOC stock and the chemical composition of SOC in the topsoil (top 10 cm layer). We found a quadratic relationship between tree species richness and SOC stocks. The threshold of tree species richness appeared between four and five. Tree species richness affected the SOC stocks primarily by increasing the quantity of leaf litter, and while decreasing the quality of leaf litter and fine root. In addition, tree species richness changed the chemical composition of SOC by affecting the chemical composition of the plant carbon, and soil bacterial α-diversity, thereby affecting the SOC stock. The trade-off between leaf litter and fine root C sources contributed to the quadratic relationship between tree species richness and SOC stocks. These results show that the mixing of 4–5 tree species can achieve a higher SOC level than having fewer or more tree species in a planted subtropical forest.