Exploring the Impact Path of Cultivated Land Use and Its Environmental Factors on Soil Nutrients in Southeastern Tibet

The rapid expansion of agricultural activities on the Tibetan Plateau has raised concerns about soil degradation and nutrient dynamics in this ecologically fragile alpine region. However, existing studies lack a systematic quantification of the vertical influence depth of farmland utilization on soil nutrients and the relative importance of multiple environmental factors along their impact pathways. To address this gap, this study conducted extensive field soil sampling across southeastern Tibet and employed one-way ANOVA and structural equation modeling (SEM) to investigate the effects of farmland utilization (farm cropland vs. facility agriculture) and environmental factors (terrain, climate, soil properties, and human activities) on soil nutrients. The results showed that cultivated land use, particularly facility agriculture, significantly enhanced soil organic carbon, total nitrogen, total phosphorus, and the comprehensive nutrient index compared with natural vegetation, with values ranked as facility agriculture > farm cropland > natural vegetation, while total potassium remained insensitive to land use change. The influence of farmland utilization on soil nutrients was concentrated in the 0–30 cm plow layer, with the most pronounced effects in the 0–10 cm surface layer, whereas detectable but weaker effects extended to approximately 50 cm depth. Among environmental factors, elevation, land use intensity, mean annual temperature, and soil moisture were the dominant influencing factors. SEM further revealed that terrain and human activities had the strongest direct effects on soil nutrients (path coefficients of 0.54 and 0.41, respectively), whereas climate factors exhibited a negative direct effect (−0.31). Elevation-driven hydrothermal differentiation serves as the fundamental driver of soil nutrient variation, and land use patterns, especially facility agriculture, modulate this influence by altering local climate and soil properties. These findings provide a scientific basis for optimizing agricultural land management and sustaining soil fertility in high-altitude regions and offer quantitative insights into the pathways through which human activities and environmental factors interact to shape soil nutrient dynamics in alpine agroecosystems.