Organic carbon content and mineralization characteristics of soil in a subtropical Pinus massoniana forest

Soils were collected from a broad leaf-Pinus massoniana mixed forest, a Chinese fir (Cunninghamialan ceolata)-Pinus massoniana mixed forest, and a pure Pinus massoniana subtropical forest for subsequent analyses to compare characteristics of soil organic carbon and microbial biomass carbon content. Soil organic carbon mineralization characteristics were further modeled using a double exponential equation. The soil organic carbon and active carbon content displayed obvious surface accumulation and decreased with increasing soil depth in all three forest types. Soil microbial carbon in the two mixed forests was primarily concentrated at depths of 0-20 cm, and significant differences were found between the topsoil and subsoil. However, soil microbial carbon in the pure forest exhibited a gradual vertical change without significant differences between soil layers. Soil organic carbon mineralization characteristics in each layer could be well represented using a double exponential equation, with similar fitting results displaying a strong initial reaction intensity followed by a gradual decrease at later stages in all three forest types. Soil organic carbon mineralization reactions at depths of 0-10 cm were always strong due to high levels of soil organic carbon, active carbon content, and greater microbial activity; however, there were no significant differences among layers below 10 cm. More intensive soil organic carbon mineralization processes were observed in mixed forests compared with the pure forest.