A comprehensive, locally adapted soil quality indexing under different land uses in a typical watershed of the eastern Qinghai-Tibet Plateau

Modern human activities have resulted in the indiscriminate exploitation of land resources, which has exerted increasingly negative influences on soil quality. Critically, these effects are seen in already ecologically fragile areas. In this study, a total of 48 soil samples were obtained at 0–20 cm depth (surface soil) and 20–40 cm depth (subsurface soil) from five types of land uses in a typical watershed in northwest China, Qinghai-Tibet Plateau. A comprehensive fuzzy mathematical model combined with soil quality index (SQI) was used to evaluate and calculate soil quality in the study area. Results showed that the SQI value was highest in mountain cropland, with values of 0.520 and 0.421 for surface and subsurface areas, respectively. Comparatively, values were lowest in grassland, with 0.447 and 0.364 for surface and subsurface areas, respectively. Then we assessed the sensitivity index (SI) to understand the effects of different physicochemical soil properties on soil quality under different land use types, and the SI values of mountain cropland and grassland with a large variations of soil quality were analyzed. Results showed that among the 15 evaluating indices, SI values for alkali-hydrolyzed nitrogen (AN), available phosphorus (AP), and total nitrogen (TN) in the surface soil layer meanwhile AN and TN in subsurface soil were higher than 1.5, indicating the sensitivity of these properties increased in mountain cropland. Surface soil is more vulnerable to land use changes driven by a variety of human activities, given this, surface and subsurface soil samples had recognizable differences. We next calculated the stratification ratio (SR), with results showing that SR values for most soil properties were higher in mountain cropland relative to grassland soil, with AP in mountain cropland was a maximum at 1.94, indicating these properties were more important indicators of the effects of land use changes. These analyses, provide a better explanation of why the soil quality was better in mountain cropland with specific physical and chemical properties. Collectively, the outcomes of this study indicated the potential for assessing soil quality covered by five land use types along with soil stratifications in a region with a sensitive and fragile ecological environment.