No abstract

The vegetation restoration sequence of ''grass-shrub-tree'' has been successfully employed in many degraded areas; however, its applicability in desertified area reclamation is questionable. In this study, soil properties of a desertified land in the northern Shaanxi province of China were determined to assess the performance of this restoration sequence. Soil samples were collected from a contiguous area consisting of a control area of original desertified land (bare control) and three vegetation restoration sequence communities (continuous grass, grass-shrubs, and grass-shrub-trees) for 30 years. Results indicate that revegetation on the desertified area decreased soil bulk density (BD) and increased soil organic matter (SOM), cation exchange capacity (CEC), available nutrients (N, P, and K), and readily oxidizable carbon (ROC). Nonactive organic carbon (NAOC) and carbon pool management index (CMI) also improved in the top soil layer but not in the lower layer. Soil texture as well as total potassium (TK) and phosphorus (TP) did not change significantly. Comparing the three vegetation restoration communities, soil physical properties, SOM, and available nutrient content improved in grassland and shrubland, but declined in treeland, lability of C (L) was higher in the top layer of restored area than in bare control. L was 0.35-0.54 in grassland, 0.49-0.57 in shrubland, 0.43-0.52 in treeland, and 0.24 to 0.26 in bare control. Results of this study indicate that vegetation restoration on desertified land can improve soil properties. However, the popular restoration sequence of ''grass-shrub-tree'' is not appropriate for the restoration of semi-arid study area with low precipitation.

The agricultural soil alongside highways has experienced multiple potential ecological risks from human activities. In this study, 100 soil samples near the highways were collected in Lishui District, Nanjing City. Using the single-factor pollution index, the Nemerow comprehensive pollution index, and the potential ecological risk index, the study investigated the heavy metal contents and distribution in roadside agricultural soil. PCA and a multiple regression model were applied to quantitatively analyze the spatial relationships between sampling soil heavy metal accumulation and the surrounding man-made landscape. The mean contents of Cu and Pb exceeded the background, while Cd, Cr, and Zn were lower than that. The potential ecological risk index exhibited a very low ecological hazard and only Cr in soils rarely showed moderate risk. Furthermore, quantitative analysis for the sources of contamination revealed that agricultural practices were the dominant contributors to the heavy metals, including Cd, Cu, and Zn, while road and heavy industrial practices contributed to Cr and Pb. The study provides sources of heavy metal pollution from human activities in roadside agricultural land and serves as a reference for ecological restoration.