Effects of different restored vegetation on soil organic carbon pools in subtropic erosive lands: Insights from stable carbon isotopes

Forest soil organic carbon (SOC) represents an important component of the Earth's terrestrial carbon sinks, and its accumulation and turnover in deforested areas after revegetation are still largely unknown. This work investigated the plant and litter inputs and SOC pools of coniferous forest (conifer), coniferous and broad-leaved mixed forest (mixed), broad-leaved forest (broad-leaved), and barren land (bare) from different restoration stages in subtropical China. Both 13C stable isotope and 13C nuclear magnetic resonance techniques were used to assess changes in C storage and turnover rates during forest restoration. The results show that afforestation effectively increased SOC content and storage. The C, N and P nutrients that plants provided into soil through litter not only promoted the accumulation of SOC, but also stimulated the accumulations of soil microbial biomass and enzymatic activities. The stoichiometric ratios of enzymes showed that the C limitation of the soil was alleviated, and the N limitation was transitioned into the P limitation after vegetation restoration. After afforestation, the recalcitrant organic carbon ratio in SOC decreased while the O-alkyl C proportion increased, but the A/O-A ratio and the proportion of aromatic C and alkyl C in soil increased along afforestation, indicating that short-term afforestation may promote the activities of SOC pool while the long-term afforestation can form a more stable SOC pool. On this basis, a comprehensive C accumulation and conversion scheme was proposed for the conversion of plant carbon to soil carbon and the change of organic carbon composition in the process of vegetation restoration. Furthermore, a comprehensive C accumulation and turnover scheme was proposed to describe C transformations from plants to soil and changes in SOC chemical compositions during vegetation restoration. Microorganisms were the dominant driving force in SOC pool as indicated by structural equation model. This study provides a theoretical basis for vegetation restoration in subtropical soil erosion areas, and is of great significance for understanding the long-term effects of vegetation restoration on soil carbon sequestration.