Objectives The alluvial fans are precious land resources in the Lhasa River Basin. However, the extensive management of alluvial fan land in Tibet, such as excessive grazing, unreasonable reclamation, and other human activities, has degraded the fertility of some land resources. We assessed the soil fertility grades in the middle and lower reaches of the Lhasa River Basin for its effective management in the future.

Methods Three hundred and twenty-one (321) soil samples were collected from 20 typical alluvial fans from July to August 2019 and 2020. Soil organic matter (OM), total nitrogen (TN), total phosphorus (TP), total potassium (TK), available nitrogen (AN), available phosphorus (AP), and available potassium (AK) were measured. We used principal component analysis to determine the weight of nutrient indicators and comprehensively evaluate soil nutrients by matter-element model based on the nutrient classification standard of the second national soil nutrient census.

Results The contents of soil OM, TN, TP, TK, AN, AP and AK in the alluvial fans ranged from 2.65–314.57 g/kg, 0.14–17.55 g/kg, 0.04–2.06 g/kg , 7.71–25.57 g/kg, 7.70–358.56 mg/kg, 0.12–342.50 mg/kg, and 17–1350 mg/kg, respectively. Among the 321 sample points, soil TN and TP below grade Ⅳ accounted for 9.97% and 15.89%, respectively. In contrast, soil AN and AP below grade Ⅳ accounted for 46.42% and 48.60%, respectively, indicating that soil AN and AP were deficient. 61.99% of sample points were rated above grade Ⅳ, indicating sufficient soil nutrients. 63.84%, 55.77%, and 60% of the sample points were above grade Ⅳ in the grassland, shrubland, and cropland of the alluvial fans. Soil TN, OM, and AN were the most critical indicators affecting soil nutrient comprehensive levels. The comprehensive evaluation of soil nutrients showed that one alluvial fan was grade Ⅰ, indicating sufficient soil nutrient content. Nine alluvial fans were grade Ⅲ and grade Ⅳ respectively, with relatively moderate to rich soil nutrients. After a comprehensive evaluation, one alluvial fan had grade Ⅵ soil nutrients, indicating extremely poor soil nutrients. The altitude and mean annual precipitation of grade Ⅲ, Ⅳ, and Ⅵ alluvial fans gradually decreased, and the mean annual temperature steadily increased.

Conclusions In the middle and lower reaches of Lhasa River Basin, the lower the altitude, the lower the soil nutrient level. In the 20 alluvial fans, the sample points with poor to very poor soil AN and AP increased by 36.45% and 32.71% respectively, compared with soil TN and TP content, and this phenomenon was found in different vegetation types.