| Forest soil C pool plays an important role in regulating the global carbon cycle. In concerned of the complexity of soil organic C (SOC), the study of different soil organic carbon fractions may help to understand the dynamics and the control of soil C. In South China where high rainfall, steep slopes and fragile soil are characteristic, native broadleaved forests have been cleared for the last several decades. The successive monoculture plantations of economical conifers are established following forest land clear cutting, slash burning and soil preparation and are commonly found subject to severe soil erosion in the early stage before canopy closure. In addition to soil degradation and yield decline, there are considerable concerns about the changes of SOC pool following the conversion of natural forests into plantation forests. We chose a natural forest of Castanopsis kawakamii (NF), a C. kawakamii plantation forest (CK), and a Chinese fir (Cwninghamia lanceolata) (CF) plantation forest in Sanming nature reserve, Fujian, China, to assess these changes through an effort to measure the C reserves of the ecosystems and the various SOC fractions in these forests.The following results can be indicated,(1) The ecosystem C reverse was 475.419 t-hm"2 in the NF and was respectively 1.49 and 2.10 times as much as that in the CK and the CF. The SOC reverses deep to 100 cm in the NF was respective 1.17 and 1.35 times greater than that in the CK and the CF. The maximum difference occurred in the soil layer of 0-10 cm, where the NF had a SOC reserves respectively 1.40 and 1.45 times higher than that in the CK and the CF.(2) The C storage of the light fraction of SOC (LFOC) in the NF was respective 1.64 and 2.16 times as much as that in the CK and in the CF in the 0-100 cm soil depth, and the proportion of SOC as LF-SOC in the NF was significantly higher than that in the CK and the CF. In the 0-10 cm layer, the C concentration and the C storage of the LF-SOC in the NF were respectively 1.70 and 1.56 times as high as that in the CK, and were 2.14 and 1.92 times as high as that in the CF.(3) The C concentration, C storage of the soil particulate organic C (POC), and the proportion of SOC as POC in the soil layer of 0-100 cm in the NF were respectively 1.65,1.60 and 1.37 times as much as that in the CK, and were 3.59, 3.43 and 2.55 times as much as that in the CF. The C concentration of POC in the 0-10 cm soil layer in the NF was respectively 2.18 and 4.85 times higher than that in the CK and in the CF. Responsively, the storage of soil POC was 2.00 and 4.34 times and the proportion of soil POC was 1.43 and 2.99 times respectively.(4) The NF had a proportion of SOC as unprotected organic C (UOC) 1.38 and 2.10 times respectively as much as the CK and the CF did in the 0-100 cm soil layer, and had a proportionin the 0-10 cm soil layer 1.28 and 1.94 times as high as the CK and the CF had.(5) The concentration of soil readily oxidizable C (OXC) and the proportion of SOC as OXC decreased with the increase of soil depth in different forests and showed a maximum difference among forests in the 0-10 cm soil layer, where the concentration of OXC in the NF was 1.87 and 2.25 times as much as that in the CK and the CF, and its proportion in the NF was 1.23 and 1.39 times as high as that in the CK and the CF, respectively.(6) The concentration of soil microbial biomass C (MBC) and the proportion of SOC as MBC decreased with the increase of soil depth in different forests. In the 0-10 cm soil layer, the MBC proportion showed no significant difference among forests and the concentration of MBC in the NF was 1.65 and 1.89 times as high as that in the CK and the CF. The concentration of MBC in the rhizospheric soil of the 0-10 cm layer in the NF was 2.15 and 2.77 times respectively as much as that in the CK and the CF, and the ratio of rhizospheric / bulk soil (R/S) in the NF (2.00) was higher than that in the CK (1.54) and in the CF (1.37).(7) The concentration of soil mineralizable C (MC) in the NF was respec...
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