Differential responses of soil quality in revegetation types to precipitation gradients on the Loess Plateau

Vegetation restoration can control soil erosion and improve soil quality; however, these positive effects on soil quality can be challenged with inceasing aridity. It is vital to understand the mechanisms affecting soil quality along aridity gradients and to determine the appropriate revegetation types for soil restoration. In this study, we selected three revegetation types (forest, shrub, and grass) along four precipitation gradients (365, 452, 507, and 600 mm mean annual precipitation [MAP]) to analyze the changes in soil properties from two soil layers (0-10 and 10-20 cm). To quantify the variations in soil quality, a soil quality index (SQI) combining the chemical, physical, and biological properties was developed. The results showed that the SQI was formed based on total nitrogen, clay, urease, and pH. In both soil layers, SQI values increased significantly (P < 0.05), and the stage-dependent increase in rates of SQI were 0.24%-0.38%, 0.18%-0.26%, and 3.04%-5.23% for 452 mm, 507 mm, and 600 mm of MAP, respectively. Significant differences in SQI values were observed among the three revegetation types under the same precipitation gradients (P < 0.05), and Caragana korshinskii Kom (CK) and abandoned farmland (AL) had similar patterns of SQI response to altered precipitation, which were different from those of Pinus tabuliformis (PT). The increased rates of SQI at PT (0.67%-3.40%) sites were higher than those at CK (0%-2.74%) and AL (0.05%-3.01%) sites, suggesting that the magnitude of soil quality reduction in forestlands was larger than that in shrub/grasslands with a decrease in precipitation.