Studies on soil organic matter (SOM) cycling in different climate zones are an important basis for further understanding of the feedback mechanism of terrestrial carbon storage to global climatic changes and are crucial for accurate projections of future concentrations of CO2 in the atmosphere. Using thin-layer methods, six soil profiles in the Dinghushan Biosphere Reserve (DHSBR), South China, and the Xiaoliang Ecological Station of the South China Institute of Botany, Chinese Academy of Sciences (CAS) were excavated and sampled for studies on the dynamics of SOM in the southern subtropical areas, based on SOM δ13C, Δ14C, soil grain size characteristics, and soil organic carbon (SOC) contents. Results indicate that the turnover of SOM occurs in three stages: (i) Rapid turnover of SOM occurs within 100 years, with SOC content decreasing sharply downwards from the ground surface and δ13C values becoming correspondingly enriched in 13C as a result of carbon isotope fractionation in the process of SOM turnover; maximum till is reached at about 260–270 years; (ii) from about 260–270 years to 800–1400 years, SOM turnover rates lessen, SOC content decreases slowly downwards, and δ13C values become gradually depleted in 13C due to the decomposition of SOM compartments with higher δ13C values; (iii) after about 1500 years, SOC content approaches the minimum, with slight fluctuations, and δ13C values become stable.
Comparison analyses suggest that soil clay materials control existing forms and turnover processes of SOM directly, SOM in the soil sections with high clay content at DHSBR are not easily decomposed and have longer turnover periods, and soil textures are an important factor controlling SOM dynamics. Fall leaf litter generally has more negative δ13C value than the topsoil samples, which may be a result of isotope fractionation caused by rapid carbon decay prior to the penetration of carbon from the litter into the topsoils. Above-ground vegetation species and composition impact SOM turnover processes directly; δ13C analysis may be an important tool for determination of the improvement in soil quality during the restoration of degraded ecosystems. Vegetation occupation history also influences the SOM dynamics of soil profiles at different sites in one area with similar vegetation species, as shown by the soil profiles at the Xiaoliang Ecological Station of the South China Institute of Botany, CAS.