Shifting cultivation and logging change soil organic carbon functional groups in tropical lowland rainforests on Hainan Island in China

•SOC stock after logging and shifting cultivation recovered rapidly in rainforests.•SOC groups were largely labile O-alkyl C and aromaticity after forest disturbances.•Litter C, microbial diversity and soil nutrient related to SOC functional groups.•SOC has greater stability in terms of decomposition degree in old-growth rainforests.