Background Soil quality is defined as the capacity of soil performing its functions within ecosystem boundaries, including sustaining biological productivity, maintaining environmental quality, and promoting plants and animals' growths. Existing studies suggested that serious land degradation resulted in the reduction of vegetation and productivity in the dry and warm valley of Dadu River, thus it is urgent to restore the soil quality. This study evaluated the soil quality in the dry and warm valley of Dadu River, and the results may provide scientific support for establishing restoration measures in this region.

Methods Nevertheless, soil quality cannot be evaluated by individual soil properties, because many of these properties are interdependent and their responses to environmental changes are difficult to interpret. Thus, to comprehensively assess soil quality, a soil quality index (SQI) that integrated soil properties into an overall index was established and has been widely used. The minimum data set (MDS) method was chosen to calculate the soil quality indexes of 5 typical vegetation types including shrub, grassland, coniferous forest, broadleaf forest, and mixed broadleaf-conifer forest along the dry and warm valley of the Dadu River. The soil quality indexes were graded from low to high: ⅰ, ⅱ, ⅲ and ⅳ.

Results 1) Based on correlation analysis and principal component analysis, 7 of 16 soil physical and chemical indicators had been confirmed to be the MDS, including capillary porosity, non-capillary porosity, organic matter, total nitrogen, total phosphorus, available phosphorus and available potassium. According to the load of each index factor, the weight of each index was calculated and then the soil quality index was obtained by summing. The top three weighted indicators were capillary porosity (18.53%), total phosphorus (17.36%) and organic matter (15.88%). The soil quality indexes of those 5 vegetation types ranked in the order of shrub (0.558), grassland (0.482), broad-leaved forest (0.392), mixed broadleaf-conifer forest (0.387), and coniferous forest (0.357). Meanwhile, the soil qualities of the 5 typical vegetation types were also assessed by "soil fertility scoring method " referred to the results of the Second National Soil Survey. As expected, the results were coincided with "the minimum data set method ". The classification of soil quality indexes showed that shrub and grassland were dominated by grade ⅲ and ⅳ plots, while the other three vegetation types were dominated by grade ⅰ and ⅱ plots.

Conculsions In short, this study demonstrates that the effect of shrub on soil quality improvement is better than other vegetation types and should be the direction of vegetation restoration in the dry and warm valley of the Dadu River, and grassland has great potential for vegetation restoration due to its quite good soil quality.