红壤区桉树人工林炼山后土壤肥力变化及其生态评价

炼山是我国南方林区清理林地的一种传统方法。以广西红壤区桉树人工林为研究区域，通过样地调查和采样，应用培养和非培养（PCR-DGGE）等传统与现代分析方法分别对火烧迹地土壤肥力演变及生态环境进行了评价。结果表明：与非炼山对照区相比，炼山1周后土壤剖面表层土壤（0-3 cm）中有机质、全氮、全磷、全钾以及碱解氮、速效磷和速效钾含量均不同程度地提高；同时，可培养土壤微生物数量和土壤微生物生物量碳、氮也显著地高于对照。虽然中层土（3-25 cm）中各理化性状和生物学性状指标也呈现出与表层土类似的变化趋势，但下层土（25 cm以下）却呈现出无规则的变化趋势；另一方面，炼山4个月后，除磷含量外，土壤剖面各土层土壤中理化性状指标均不同程度地低于非炼山对照区土壤。同时，炼山土壤中可培养微生物数量均低于对照区土壤，虽然表层土壤之间的差异均未达显著水平，但剖面各土层土壤微生物生物量碳、氮却显著低于对照区土壤。说明炼山无助于长效提高桉树人工林的土壤肥力。此外，虽然桉树人工林土壤细菌多样性指数、丰度和均与度指标在不同土层的变化不均一，但无论是炼山1周或4个月后，炼山方式均不同程度地导致了桉树人工林表层土壤细菌多样性指数、丰度和均与度指标的下降，说明炼山方式也不利于桉树人工林，尤其是表层土壤生态系统的持续稳定。

Variation of soil fertility in Eucalyptus robusta plantations after controlled burning in the red soil region and its ecological evaluation

Controlled burning is a traditional forest-clearing method used in Southern China. We analyzed the variability of soil fertility between controlled burning and non-burnt areas in a Eucalyptus robusta plantation forest of the red soil region, located in Hengxian County in Guangxi Zhuang Autonomous Region, China. Traditional incubation and culture-independent methods, such as the dilution-plate method, the polymerase chain reaction, and denaturing gradient gel electrophoresis (PCR-DGGE) were used. Possible effects on the soil ecological system were also evaluated. Soil organic matter (SOM), total and available nitrogen, phosphorus and potassium contents in the 0-3 cm soil layer only one week after burning were all higher than those in a non-burnt Eucalyptus forest. The number of soil culturable microbes, which includes bacteria, fungi and actinomycetes, and the biomass C and N in the 0-3 cm layer were also significantly higher than those in the non-burnt Eucalyptus forest. Indexes of chemical and biological properties in the middle layer (3-25cm) of the burnt soils also showed the same trends observed in the surface soil layer. However, irregular changes were found in these chemical and biological indexes in soil layers below 25cm. This indicates that the chemical and biological properties of soils, including soil organic matter (SOM), total and available nitrogen, phosphorus and potassium, as well as the number of soil culturable microbes, and the biomass C and N in layer below 25cm were not significantly impacted by controlled burning. Furthermore, four months after controlled burning, except for total and available phosphorus contents, nutrients such as total and available nitrogen, potassium and SOM contents did not show the same trends with burning as were found after one week, and were all lower than those in the non-burnt forest soil. Also, the number of soil culturable microbes, and the biomass C and N in each layer of the burnt soils were also lower than those in the non-burnt soil. Thus, controlled burning in the Eucalyptus plantations in the red soil region of southern China has only short-term beneficial effects on soil fertility, and is not helpful for improving long-term soil fertility of Eucalyptus plantations. Indexes of soil bacterial diversity (Shannon-Wiener, H), Richness and Evenness (E_H) in each layer after burning showed irregular changes. However, in the surface soil layers (0-3cm) of the burnt treatments these indexes were all lower than those in the non-burnt treatment and hardly recovered to the same level as in the non-burnt treatment 4 months after burning. These results suggest that the method of controlled burning in southern China might not only result in a decline in soil fertility with time, but also lead to a decrease in the indexes of soil bacterial diversity, Richness and Evenness, in surface soils in these two periods after burning. Thus, based on the above results, controlled burning is not helpful for improving the soil fertility and maintaining the development of a sustainable surface soil ecosystem in Eucalyptus plantations in the red soil region of southern China.