Soil organic carbon (SOC) plays an essential role in determining the physical and chemical characteristics of soil and therefore in determining its fertility.Currently, there has been an additional interest in the role of SOC as a potential sink for atmospheric CO₂.Meanwhile, SOC is recognized to consist of various fractions varying in degree of decomposition, recalcitrance, and turnover rate.For example, dissolved organic carbon (DOC) is biologically available and a source of C and energy for soil microorganisms.Although the absolute C amount in microbial biomass (MBC) is small, the microbes are the most important labile C pool since they are vital for SOC dynamics and nutrient cycling.As SOC is a heterogeneous mixture of organic substances, the different forms or fractions of SOC might have different effects on soil fertility and quality.
Forest succession is a fundamental ecological process which can modify biogeochemical cycles, ameliorate stand conditions and microclimate factors and change in species composition and abundance.Soil carbon, nitrogen and microbe in the secondary forest succession have been studied.However, changes of soil labile fractions along with the secondary successional gradient of evergreen broadleaved forests, have not been reported.The objective of this study was to evaluate the effects of forest succession on soil labile fractions (MBC and DOC) and soil microbial quotient (SMQ).Soil samples were collected from a secondary successional sequence of evergreen broadleaved forest in Wangmulin Nature Reserve in Fujian Province.This successional series included three sites (one early successional site, one midsuccessional site and one late successional site) that were closely located.According to the age and vegetation type, we categorized the 15 year-old site as early successional; the 47 year-old site as midsuccessional; and the 110 year-old site as late successional.These successional sites are similar in topography, regional climate and soil type.The results showed that SOC, MBC, DOC and SMQ in 47-year-old and 110 year-old forests were significantly higher than those in 15 year-old forest (P<0.05).MBC and DOC in 47-year-old forest were higher than those in 110 year-old forest, but there were no significant differences in SOC and SMQ between these two forests.SOC, DOC and MBC decreased with soil depth in three successional stages.Also, succession promoted accumulation of SOC, MBC and DOC in humus layer.We also found that MBC, DOC and SMQ had significant seasonal dynamics at three succession stages, the lowest values occurred in autumn, and the highest values transferred from winter to summer with the succession.Seasonal fluctuation of soil MBC content at 0-40cm soil depth ranged from 216.5 to 411.2 mg/kg, 471.9 to 964.1 mg/kg and 374.5 to 903.0 mg/kg in early-, mid-and late successional forests, respectively.The corresponding DOC content ranged from 486.5 to 667.9 mg/kg, 987.1 to 1 318.2 mg/kg and 828.4 to 1028.9 mg/kg in early-, mid-and late successional forests, respectively.Correlation analysis showed that SOC significantly correlated with MBC and DOC (P<0.01).MBC, DOC and SMQ were more sensitive to the changes of soil carbon storage than SOC.It suggested that forest succession significantly affects seasonal variations and contents of soil labile organic C, which may be attributed to a combination of factors including quantity of litter materials, microbial activity etc., which would change greatly with the forest succession.