| It has been facing serious problems such as soil erosion, soil quality degradation and environmental deterioration in Loess Plateau, which it severely restricted the sustainable socio-economic development in this area. Soil degradation is the main and direct factor to make the soil quality decline. The method of returning cultivated land to forest or grass is the important measurement of rehabilitation and reconstruction for the degraded land ecosystems. It is important to improve the ecological environment in the Loess Plateau.In the study, taking 8 representatively typical small valleys as an example, the methods of a combined operation of historical data collection with field investigation and a combined field testing with laboratory analysis were used. The physical, chemical and biological indicators were measured to investigate the spatial and temporal variation and affecting factors and establish soil quality dataset and evaluation model. Quantitative evaluation was conducted to assess the soil quality of the grassland. The main conclusions of the study were as follows:1. In order to evaluate grassland soil quality of the 8 watersheds, 57 samplings of grassland soil were collected to analyze soil physical, chemical and biological indicators. In terms of physical indicators, the results showed that soil bulk density was in the range of 1.24 - 1.34 g/cm3 and in most cases the soil bulk density belonged to compaction or rather compaction. Soil texture was mainly the loam soil (The contents of <0.01 mm physical clay are mostly 30% - 40%). There was significant difference in the total aggregate content in different watersheds. In terms of chemical indicators, organic matter (OM), total N (TN), total P (TP), available N (AN), available K (AK), and CEC were higher in topsoil compared with the subsoil. There was no difference in the content and fluctuations of total K (TK) in the different soils. In study area, the content of soil nutrients was medium or low level (only in Guyuan and Wangjiagou, the content of AK was in higher range). The pH value was alkaline. In terms of biological indicators, there was significant difference in the activity of the urease, alkaline phosphatase and sucrase. However, the activities of these enzymes were highest in the top soil and decreased as the decline in profile.2. With the years of returning cultivated land to forest and grass increase, the indicators of soil showed the optimization. Biomass, coverage, abundance and height of the vegetation were increasing along with the length of the extension of abandonment. Soil bulk density was decreasing, but the content of physical clay was increasing. It had showed that physical characteristics of the soil had been developing constantly. As a whole, pH value and the soil salt were declining. OM, CEC, TN, AN, AP, AK were increasing year by year. The content of TP was relatively stable. The activities of urease, alkaline phosphatase and the sucrase showed the laws as increasing-decreasing-increasing.3. The correlation between the factors of the soil quality and the productivity of the vegetation (biomass and vegetation coverage) was analyzed. Correlation analysis showed that and the vegetation biomass and vegetation coverage was negatively correlated with soil bulk density (P≤0.05), was closely correlated with OM, TN, AN, TK, AK and CEC (P≤0.01). There was no significant correlation between the vegetation biomass and the urease, the alkaline phosphatase, and the sucrase, but there was significant correlation between the vegetation coverage and the three kinds of enzymes. In addition, the soil physical, chemical and biological properties were also relevant. The OM was closely correlated with CEC, TK, AK, TN, AN, and AP (P≤0.01). There was no significant correlation between the salinity of the soil, total aggregate content, TP and other properties, There was significant correlation between the urease, the alkaline phosphatase, and the sucrase (P≤0.01).4. In order to select soil quality evaluation index of the grassland, the 16 indicators was determined using the method of sensitivity analysis. It showed that TN was extremely sensitive. OM, TA, AP, AK, urease, alkaline phosphatase, and sucrase were medium sensitivity. The value of pH, the content of TP and soil bulk density were insensitive, while the others were slightly sensitive. The information of sensitivity and principal component analysis factor scores for each index was integrated. The OM, PC, CEC, AK, urease and the value of pH were selected to evaluate the soil quality and establish the integrated model for soil assessment.5. Integrated soil index was used to evaluate the soil quality of grassland. The results indicated the soil quality grade were below the medium level except Feima River Valley. The order of comprehensive quality index of the soil in the studied watersheds were: Feima River >Beigou>Zaozigou > Shanghuang in Guyuan > Gaoquan in Dingxi > Wangjiagou in Lishi > Zhifanggou in Ansai > Quanjiagou. The results showed the general trend under the 6-indicator-sysytem and 16-indicator-system was consistent by comparing the two systems of soil quality index, respectively. The results indicated that OM, PC, CEC, AK, urease and pH value may satisfy the evaluation of the soil quality.6. Paper innovations: 1) Six quantitative indicators related to soil property were chosen to evaluate the soil quality using integrated soil index. The results showed that six quantitative indicators were sufficient to meet the need of soil assessment and evaluation results provided a very useful reference and a basis. 2) Systematically study of the Loess Plateau grassland soil quality, and the soil comprehensive quality evaluation model for the soil assessment of grassland in the Loess Plateau was established to provide a guide for the development of returning cultivated land to forest and grass.
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