Ecological Restoration And Environmental Quality Assessment In The Composite Zone Of Mining And Fragile Ecology In Northern Shaanxi Province

More and more issues on ecology and environment have been drawn attentions of the world in the composite zone of intensive human activities and fragile ecological regions. The composite zone of mining and fragile ecology in northern Shaanxi province is one of the most serious desertification areas, as well as is also an important energy and mineral bases in China. It has reported that desertification has been reversed and soil quality has been improved after several generations of vegetation restoration and reconstruction projects in this area. However, the effects of vegetation restoration is not very good in some area even has experienced a long time revegetation projects. On this point, we have to rethink that what is the most reasonable revegetation modes in the desertification area. In this paper, soil quality responses effected by the vegetation restoration in the seriously degraded and fragile area and soil heavy metal accumulation characteristics in the high risk pollution area based on the field survey and lab analysis and soil samples selection in the composite zone of mining and fragile ecology in northern Shaanxi province. Meanwhile, soil quality effects of revegetation were assessed in the desertification area. The main results are as follows:(1) Soil quality has improved after 30 years of revegetation under 10 plant species of 3 vegetation communities(woodland, shrubs and grassland) Soil bulk density has reduced significantly, soil total porosity has increased, and soil texture has become refined. Soil nutrients and cation exchangeable capacity(CEC) have increased, soil p H decreased slightly, the content and quality of soil total organic carbon(TOC), soil readily oxidizable carbon(ROC), non-active carbon(NOAC) and carbon pool management index(CPMI) were increased significantly compared with barely sandy land after 30 years vegetation restoration. However, significant differences were found on soil quality among different plant species. The content of soil organic matter, total nitrogen, available nitrogen, phosphorus and potassium, CEC, ROC, NOAC and ROC/TOC have the highest increase under the Salix and Caragana. ROC/TOC can be thought as a sensitive indicator on soil quality effects of vegetation restoration.(2) Vegetation restoration modes were important factor on soil quality and significant differences were found on soil quality efficiency under different vegetation restoration modes. Soil bulk density, total porosity, organic matter, CEC and nutrient content were found higher in shrub land than in woodland. The order of soil quality efficiency for the pure shrub land> mingled shrub land>trees mingled shrubs>mingled wood>pure woodland.(3) With the increasing of restoration years, soil environmental quality gradually improved, but after 30 years then it decrease. In the first 30 years of vegetation recovery, with the increase of vegetation restoration years, soil bulk density decreased, increased soil porosity, soil texture showed the refined tendency. Meanwhile, soil organic matter, CEC and soil have increased significantly with the increase of restoration years. However,after 30 years of vegetation restoration, soil bulk density increased, soil organic matter, CEC and nutrients decreased with the increase of restoration years, and soil surface became more dry, vegetation were degraded and the fixed dune were reversed, soil characteristics were degraded.(4) Compared with the mobile dune, the vegetation and soil characteristics have been improved after 15 years revegetation of all 4 management modes(i.e. unenclosed natural management, enclosed natural management, unenclosed artificial management and enclosed artificial management). Vegetation height, coverage and biomass have increased significantly by 0.12~1.03 m, 12.70%~53.18 % and 7.27~29.08 g·m-2 separately, soil bulk density have decreased 0.12~0.27 g·cm-3 and soil total property have increased 6.80%~17.01%, meanwhile soil organic matter, cation exchange capacity(CEC), total N, and available N, P and K have improved significantly. The quality of vegetation and soil characteristics are better in enclosed management mode than in unenclosed mode, meanwhile they are also better in artificial management mode than in natural mode. By the enclosed and artificial revegetation measures, the plant and soil gradually have formed an interaction system of the degraded sandy grassland.(5) Soil organic matter, available phosphorus, readily oxidizable carbon, bulk density, clay and available nitrogen were selected as the indicators, which have composed the minimum data system(MDS) based on the principle component analysis(PCA) method. The weights of the indicators were calculated by the factor analysis(FA) method. The model on soil quality efficiency evaluation were established shown as IQI =Σ(Wi(Ii-I0))?100. And soil quality efficiency were evaluated in the 10 vegetation species after 30 years of restoration with this model.(6) Significant contribution was achieved of coal mining on the soil heavy metal accumulation in the farmland around coal mining area. The content of heavy metals was 10% to 50% higher in the toper layer than that in the lower layer, except Zn. The enrichment factors(EF) of the heavy metals were 1.7-3.3. The content of the heavy metals were higher than the regional background, but were lower than the national II standard significantly. The soil is still clean. However, the single indicator evaluation showed that some of the heavy metals have near the slightly pollution grade, and more attention should be paid on those metals.(7) The heavy metals of Cd and Cu have accumulated in the soil around the chemical enterprise caused by the sewage discharge. The content of Cd have higher than the national II standard, its single index is 1.21, which classified as the slightly pollution grade. The Nemero index of the Cu, Zn, Pb and Cd is 0.74, which is classed as the alert grade. All the heavy metals have been accumulated in the top 10 cm. The content of Cu, Zn and Cd were gradually decreased from the outflow of the sewage in the 100 m, but the distance is 200 m for the Pb. The average content of Cu, Pb and Cd in corn grain were 4.74 mg/kg, 0.129 mg/kg and 0.036 mg/kg respectively, which is significantly higher than that in control area. All the single index of the heavy metal is less than 1 in the corn grain, and the Nemero index is 0.53, which is classed as the safe and clean grade.