Assessment And Response Of Soil Quality To Vegetation Natural Restoration Processes In The Loessial Hilly-Gully Region

The effect of vegetation natural restoration on soil quality is an important issue of environmental effect assessment on"the Grain for Green Project". This doctoral dissertation took two research areas of the typical grassland zone (Guyuan, Ningxia province) and the transition area from grassland to forest zone (Yangou catchment, shaanxi province) as research sites to study soil quality responses to vegetation natural restoration. According to research methods of the field investigation, experiments of soil seed bank germination, soil property analysis, and statistic analysis, we studied the characteristics of vegetation succession and soil seed bank in the process of vegetation natural restoration; analyzed the response of soil physical, chemical, enzyme activity, microbiological properties to vegetation natural restoration processes; selected soil quality assessment indicators based on correlation analysis, sensitivity analysis, and principal component analysis; and also quantitatively evaluated response of soil quality to vegetation natural restoration processes through soil synthetical index method. The main results are as follows:1) Soil sampling depths during vegetation natural restoration was proposed. LSD analysis showed that there were significant differences of soil quality factors among 0-5, 5-10, and 10-20 cm layers of soil profile, the order was 0-5 cm>5-10 cm>10-20 cm. It showed that the differences between the soil layers increased with the increase of vegetation restoration years. The response of soil properties in the top layer (0-5cm) to vegetation restoration was the most sensitive, the sensitivity of other two layers (5-10cm and 10-20cm) decreased, compared to the top layer. Based on the principal component analysis, the soil property information in top layer could cover more than 95% of the information in the 0-20 cm soil layer. Hence, 0-5 cm soil layer could be best the soil layer to study the response of soil quality to vegetation natural restoration, this sample method could show sensitivity of response of soil quality to vegetation natural restoration and reduced sampling workload.2) Vegetation succession during natural restoration was clarified. At the grassland zone, plant community began with annual herb community mainly composed of Artemisia scoparia and Corispermum hyssopifolium, then succeeded to perennial herb community, at last formed climax community mainly composed of Stipa bungeana and Stipa grandis. Species diversity was volatility increased with the increase of restoration years. At the last succession stage, the dominant species tended to diversification, plant community was stable state. Density of soil seed bank was 3542.5 seeds/m2, main dominant species were Artemisia lavandulaefolia and Roegneria kamojii; the species of above-ground vegetation was 1.14 times than of soil seed bank; while the relative coefficient of species composition between above-ground plants and seed bank was varied from 0.167 to 0.276. At the transition area from grassland to forest zone, the process of vegetation succession began with annual herb community, succession followed shrub community, and lastly became tree community (Pyrus betulaefolia). With the increase of vegetation natural restoration years, species diversity increased distinctly, shrub and tree community occupied the dominant ecological niche at the last period of vegetation succession, plant community was stability. Density of soil seed bank was 5889.29 seeds/m2, main dominant species were Artemisia lavandulaefolia and Setaria viridis, the species of aboveground vegetation were 1.69 times than of soil seed bank; the relative coefficient of species composition between ground plants and soil seed bank was varied between 0.235 and 0.421. It explained that contribution of soil seed bank to species composition of above-ground was relatively small in the loessial hilly-gully region.3) Response of soil structure to vegetation natural restoration process was analyzed. Using Le Bissonnais (LB) methods, we measured soil water-stable aggregate content during vegetation natural restoration in the loessial hilly-gully region. It showed that the primary broken mechanism of soil aggregate was dissipation by"air explosion"in the soil. Comparing with the traditional method (Yoder Method), >0.5 mm aggregate content and MWD all showed FW0.5 mm aggregate content increased from 45.71% to 88.92%, MWD increased from 1.08 mm to 2.98 mm; at transition area from grassland to forest zone, during 100a vegetation natural restoration, >0.5 mm aggregate content increased from 21.03% to 80.89%, MWD increased from 0.49 mm to 2.66 mm.4) Response of soil chemical properties and biological properties to vegetation natural restoration process was researched. At the grassland zone, soil organic carbon density, nitrogen density, soil alkaline nitrogen, soil enzyme activity, soil respiration rate, soil microbial biomass C and soil microbial biomass N increased greatly within vegetation restoration of 23 years, then tended steady from 23a to 75a of vegetation restoration. Soil available phosphorus content decreased first and tended steady later. Soil C/N varied from 9.11 to 10.44. At the transition area from grassland to forest zone, soil chemical properties and biological properties increased greatly within vegetation restoration of 29 years, decreased from 29a to 55a of vegetation restoration, went up again from 55a to 100a of vegetation restoration; Soil available phosphorus content decreased first and increased later. Soil C/N change from 8.70 to 11.79.5) Soil quality indicators were selected. Based on correlation analysis, sensitivity analysis and principal component analysis, we chose MWD, soil organic carbon density, nitrogen density, soil urease, soil alkaline phosphatase, soil microbial biomass C, soil microbial biomass N and microbial respiration quotient as soil quality evaluation index during vegetation natural restoration in the loessial hilly-gully region. Contrasting analysis two way of SQI calculation which based on the 8 soil quality indexes and the 20 soil quality factors, we found that both of them had same change trend with vegetation restoration year. It tested and verified that the 8 soil quality evaluation indexes were highly representative and practicability.6) Response of soil quality to vegetation natural restoration process was assessed. At the grassland zone, during vegetation natural restoration, the change of soil quality had two stages, the first stage was rapidly growing stage of soil quality, which occurred within vegetation restoration of 23 years, SQI was from 0.052 to 0.742, soil quality transformed from the low level to the middle level; the second stage was slowly changing stage of soil quality, which occurred from 23a to 75a of vegetation restoration, SQI was from 0.615 to 0.722, soil quality was in middle level. At the transition area from grassland to forest zone, during vegetation natural restoration, the change of soil quality included three stages, the first stage was increasing significantly stage of soil quality, which occurred within vegetation restoration of 29 years, SQI increased from 0.107 to 0.454, soil quality transformed from low level to relatively lower level; the second stage was decreasing stage of soil quality, which occurred from 29a to 55a of vegetation restoration, during this period, the herbosa community exited dominant position, and then the shrub community invaded reducing vegetation coverage, and soil nutrient and moisture were consumed more than accumulated by plant, SQI decreased from 0.242 to 0.454, soil quality was in the lower level; the third stage was increasing stage of soil quality again, which occurred from 55a to 100a of vegetation restoration, SQI increased from 0.242 to 0.823, soil quality transformed from the relatively lower level to the high level.