Effects Of Wenchuan Earthquake On Soil Physical And Chemical Properties And Microbial Biodiversity Under Cupressus Chenginana Forest In Li County Of Sichuan Province

"5.12" Wenchuan earthquake occured in 2008 in the area where has an important significance for water source reserve and soil and water conservation in the upper Yangtze river. It is also one of the main forest distribution areas in China. The earthquake caused soil erosion, landslides, mud-rock flow and other secondary disasters, which has brought in continuing negative effects on stability and development of forest ecosystem, and the soil erosion and soil nutrient impoverishment in return caused reduction of forest ecological function. Therefore, typical Cupressus chengiana forests, damaged by earthquake, from two areas(Xionger Mountain and Puxi Gully) were chosen in this study. In order to explore the effects of earthquakes on forest soil environmental quality, and provide reliable theoretical basis for soil restoration in this area, a comparative study was conducted focusing on following three aspects, i.e. soil physical and chemical properties, by using modern instrument analysis methods and molecular biology techniques.The study results were included as follows:1. The earthquake caused dramatic changes of soil physical and chemical properties in studied Cupressus chengiana forests. The average soil clay content of five study sites in surface layer(0- 20 cm) decreased by 54.4% compared with the undamaged soil by earthquake. Average soil bulk density increased to 1.28 g·cm-3, it was significantly higher than that in contrast area. And the soil p H values showed significantly increases(an average value 11.5%) at different soil depths(0- 20 cm、20- 40 cm and 40- 60 cm). The extrusion and collapse of the earthquake led to a certain extent increase of soil bulk density, decrease of total porosity. Soil reunion structures were damaged by earthquake, which led to a lack of organic matter of soil particle cementation, and abated the soil erosion resistance ability, caused soil clay to be leached easily. The soil cation exchange capacity(CEC) was in range 1.74- 5.88 cmol·kg-1 in earthquake damaged forests, with a mean of 3.72 cmol·kg-1, which was about 1/4~1/5 of that in the undamaged forests, resulted in very low soil fertility. The loss of soil clay, reduction of soil organic matter content and increase of soil p H were mainly accounted for the reduction of CEC.The averages of soil total N, alkali-hydrolyzable N, available P and available K contents were lower than that of soils in the control soil organic matter(SOM) had significant correlations total N, alkali-hydrolyzable N,available P and CEC, indicating the SOM plays a very important role in enhancing and maintaining soil fertility. Soil C/N were mostly under 12 in earthquake damaged soils, by contrast, which were in the range of 19.5~34.9 in the controls,showing that earthquake could cause more C loss than N loss.2. Earthquake has caused significant impact on the soil microbial community structure and abundance. Six kinds of soil microbial abundance in the earthquake damaged forest soils were analyzed using Real-time quantitative PCR technique. A certain amount of Archaea was found in study soils except Bacteria and Fungi. The average gene copies of Bacteria, Fungi, Archaea, Crenarchaeota, Ammonia oxidizing archaea and Ammonia oxidizing bacteria were 4.73 × 107 copies·g –1(dry soil), 6.89 × 105 copies·g –1(dry soil), 2.88 x 106 copies·g –1(dry soil), 6.25 × 105 copies·g –1(dry soil), 5.36 × 104 copies·g –1(dry soil) and 4.77 × 104 copies·g –1(dry soil), respectively, which were significantly lower than in the controls. In earthquake damaged soils, the average gene copy numbers were in following order: Bacteria > Archaea > Fungi > Crenarchaeota > Ammonia oxidizing archaea > Ammonia oxidizing bacteria. Among which the Bacteria had the highest abundance, which was the dominant soil microbial community. correlation analysis showed that the relationships of soil microbial quantity and soil physical and chemical properties were not significant. It wan found by principal component analysis that Archaea, Crenarchaeota and Ammonia oxidizing archaea were closely linked, implying Ammonia oxidizing archaea has an important position in Archaea or Crenarchaeota in studied soils. It was also found that there was an large inconsistancy between the abundances of Ammonia oxidizing archaea and Ammonia oxidizing bacteria, showing a complementary relationship between them. The Bacteria and Fungi were closely positioned in the principal component analysis, implying the fact that both of them are closely related to the soil organic matter. The earthquake caused the sharp reduction in soil organic matter content, which would seriously affect the quantities of soil Bacteria and Fungi.3. Clay content in investigated soils had a profound signifcance in soil physical and chemical properties and maintaining soil fertility. The following results were achieved by using XRD, infrared spectrum and EDS analysis methods: The main soil clay minerals in both investigated areas(Xionger Mountain and Puxi Gully) are very similar, consisted of illite, kaolinite, chlorite and illite-montmorillonite, among which the illite content was the highest, accunted for 69.8% ~ 77.7%. The main chemical compositions of clay are Si O2, A12O3, Fe2O3, among which the Si O2 content accounts for about 50%. Specific surface area of clay are 56.8 ~ 46.5 m3·g-1, which mainly consists of mesoporous. The composition and structure features of the clay in the investigated soils have very important role in water retention and fertility maintenance.Research showed that the following soil constraints for restoration which caused by earthquake: reduction of soil organic matter, increase of soil alkalization, reduction of soil clay content and making the texture more coarse, reduction of soil microbial diversity. Therefore, the following suggestions were proposed:(1) Increasing soil organic matter. Application of organic manure or interplanting green manure can be options based on the concrete local situation.(2) Enhancing soil clay mineral content. Adding some clay minerals with high buffer capacity such as montmorillonite, vermiculite can help to enrich soil clay composition and increase soil clay content, which will result in enhancing soil cation exchange capacity, and promoting the formation of soil organ-mineral complex.(3) Improving the soil p H value. Some acid fertilizers or soil memdments such as organic-inorganic compound fertilizer(farmyard manure and ammonium sulfate compound fertilizer), acidic organic soil soil amendment( brown coal, weathered coal) and low temperature biochar with relative low p H value can be chosen to apply in the soils to improve soil p H and soil organic matter content.