Tree species mixing promotes surface soil organic carbon accumulation in mid-age and stability in old-growth forests

Afforestation or reforestation with mixed tree species is a promising strategy for carbon sequestration and climate change mitigation. However, whether soil organic carbon (SOC) stability in mixed-species forests (MF) is higher than that of monoculture forests (PF) is unclear.

The origin and stability of SOC were investigated using soil samples (1 m in depth) obtained from 12 pairs of monoculture and polyculture plots, which included four age classes.

We found that the contents of SOC and its components were higher in mixed-species forests than in monoculture forests, particularly in the 0–40 cm soil layer and middle-aged (33–45 years) PF. In middle-aged forests, the MF had a smaller percentage of microbial-derived C and mineral-associated organic carbon (MAOC) to total SOC ratio than those of PF. However, the ratio of MAOC to total SOC in the MF was considerably higher than that in old-aged PF (62 years), indicating that transforming old-aged PF into MF aids SOC stabilization. Mineral protection played a major role in influencing the accumulation of plant-derived C, microbial-derived C, and SOC fractions in the PF, whereas total nitrogen and ammonium nitrogen played a dominant role in the MF.

This study provides new evidence that transforming single-species monocultures into mixed-species polycultures is beneficial for the accumulation and stabilization of SOC in subtropical forests and highlights that these effects depend on the influence of forest age and soil depth on organic carbon fractions.