人类活动干扰导致地带性森林退化成次生林、人工林后，通常引起表层土壤有机质和营养元素的损失，尤其是土壤轻组有机碳的淋失。本研究以相对原生阔叶林和与其毗邻的次生林和杉木人工林为对象，测定土壤（0~10、10~20和20~30 cm） pH值、总有机碳（TOC）、轻组有机碳（LFOC）和重组有机碳（HFOC）以及交换性K+、Ca2+的含量，旨在阐明林地转化后土壤有机碳及K+、Ca2+养分的动态变化。研究表明：原生阔叶林向次生林转变后，0~30 cm土层中土壤pH值升高（4.4%），交换性K+、Ca2+含量增加（22.5%、8.2%），其储量增加尤其明显（60.2%、55.1%）；而TOC、LFOC和HFOC含量明显下降（26.7%、40.8%和11.3%），除LFOC储量减少22.2%外TOC和HFOC储量变化不显著。次生林转变成杉木林后，0-30 cm土层中交换性K+、Ca2+含量微弱增加（6.3%、2.9%），HFOC储量减少（13.8%），其它指标的变化则均不显著。原生林土壤pH值与TOC、LFOC、HFOC及交换性K+、Ca2+含量之间呈显著负相关关系，但在次生林和杉木人工林中这种相关性的显著水平被削弱，表明林型转变对土壤pH值、有机碳和养分产生了不同程度的影响。综合而言，原生阔叶林向次生林或次生林向杉木人工林转变后导致土壤有机碳大幅下降，但土壤酸度得到改良和矿质养分有效性增加，为植被快速恢复提供了有利条件。因此，从森林碳汇的角度优先保护表层土壤，选择速生、高效固碳的目标树种对退化次生林进行更新改造，是提高森林生物量碳和土壤净碳积累，扭转因林地利用转变而引起土壤碳流失的一个重要途径。%Human-inducedland-use changes from primary climax forests to degraded secondary or plantation forests usually caused the leaching losses of surface soil organic carbon especially the light organic carbon fractions and associated nutrients. In this study, soil samples (0~10, 10~20 and 20~30 cm depth) were collected from a Primary forest (PF), an adjacent Secondary forest (SF) and a Chinese fir plantation (CFP). We determined soil pH value, concentrations of soil total organic carbon (TOC), light fraction organic carbon (LFOC) and heavy fraction organic carbon (HFOC), exchangeable K+ and Ca2+, with the aim of understanding the impacts of this land-use change on these variables. After the conversion from PF to SF, in 0~30 cm depth, the soil pH value increased slightly by 4.4% while the concentrations of exchangeable K+, Ca2+ increased by 22.5% and 8.2%, and their storage increased correspondingly by 60.2% and 55.1%, respectively. By contrast, the concentrations of TOC, LFOC and HFOC significantly decreased by 26.7%, 40.8% and 11.3%, respectively. The storage of LFOC decreased by 22.2% while no significant changes were found for TOC and HFOC. After the conversion from SF to CFP, the concentrations of exchangeable K+, Ca2+ increased slightly by 6.3% and 2.9%, respectively. All other variables did not show any significant changes except for the HFOC’s storage which decreased by 13.8%. Among the three types of forests, the PF showed significant negative correlations between soil pH value and the concentrations of TOC, LFOC and HFOC, as well as exchangeable K+, Ca2+. However, the significance of these correlations were generally weakened in SF and CFP, suggesting that forest conversion had posed disproportionate impact on soil pH, organic carbon and nutrients. In summary, this forest land conversion generally led to a dramatic reduction in soil organic carbon, while the acidity of the soil and the availability of soil mineral nutrients were improved, which was presumably favorite for the quick recovery of vegetation. From the perspective of carbon sequestration, we highlighted that priority protection of surface soils for the renewal of the degraded forests by planting target species with fast growth and high efficiency in carbon gain was crucial to increase the forest biomass carbon storage and soil carbon sequestration, and consequently reverse the loss of soil carbon resulted from forest land use change.
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