Comparing soil organic carbon stock and fractions under natural secondary forest and Pinus massoniana plantation in subtropical China

Comprehensive comparison of soil organic carbon (SOC) stock and its fractions in different forest types with same age is scantily considered, especially between natural secondary forest (NSF) and Pinus massoniana plantation (PMP), which are widely distributed in subtropical China. Thus, we evaluated the differences of C stocks in stand litter and 0–40 cm soil profile of an Andosols and the labile fractions [dissolved organic carbon (DOC), microbial biomass carbon (MBC), coarse particulate organic carbon (cPOC, 250–2000 μm), fine particulate organic carbon (fPOC, 53–250 μm) and easily oxidizable carbon (EOC)] and heavy fraction organic carbon (HFOC) in SOC between NSF and adjacent 70-year-old PMP. We found that NSF enhanced the C stocks in stand litter and 0–40 cm soil epipedon by 15.1% and 37.9% higher those of PMP. Compared with PMP, the stocks of MBC, cPOC, fPOC, EOC and HFOC in 0–40 cm soil epipedon under NSF were increased by 68.2%, 59.8%, 51.6%, 31.7% and 47.18% respectively, but the DOC stock was reduced by 9.9%. The impact of forest type on the SOC pool in surface soil (0–20 cm) was stronger than that in subsoil (20–40 cm). Our results suggested that NSF is more suitable for SOC sequestration and soil C sink in the study region because significantly higher SOC is incorporated as HFOC in the soil rather than stand litter under the naturally generated secondary forest.