Key role of microbes in promoting soil carbon accumulation following afforestation in a subtropical region

Afforestation is an efficient strategy used to promote soil organic carbon (SOC) sequestration. Though the effects of afforestation on SOC sequestration have been investigated worldwide, the underlying mechanisms remain to be further explored. We adopted a space-for-time approach by comparing maize field and plantation forest at a regional scale in subtropical China, and explored the mechanisms underlying the effects of afforestation on SOC sequestration. Amino sugars and lignin phenols were used as biomarkers to indicate soil microbial and plant residual carbon. SOC stock was significantly promoted 20 years after afforestation with an accrual rate of 301.7 ± 43.3 g C m^-2 yr^-1. According to the ¹³C mass balance method, new carbon contributed 28.9 ± 2.8% of SOC pool in the plantation forest with the contribution much higher in the topsoil than in the subsoil horizon. The turnover time of SOC was comparable across the soil profile from 0 to 45 cm depth with the average being 130.8 ± 26.6 years. Afforestation promoted particulate organic carbon (POC) content more pronouncedly in the topsoil than in the subsoil horizon, but promoted bulk SOC and mineral-associated organic carbon (MAOC) content similarly across the soil profile from 0 to 45 cm depth. In the topsoil, microbial residue played a key role in stimulating SOC accumulation following afforestation, with the roles of lignin and mineral protection being much minor. In the subsoil, the roles of microbial residue and mineral protection in stimulating SOC accumulation were comparable, with the role of lignin being minor. For both POC and MAOC accumulation following afforestation, the role of microbial residue was much higher than that of lignin or mineral protection. The findings demonstrate that microbes may play a key role in promoting soil carbon accumulation following afforestation in the subtropical region.