Extreme terrain modification and urban anthropogenic pressure jointly reshape soil chemical properties: A case study of the MECC project in the Loess Plateau

Abstract

Urban expansion is a major driver of soil degradation, particularly in ecologically fragile regions where large-scale landform modification is employed to accommodate urban growth. While many studies have examined soil changes under conventional urbanization, the combined effects of landform reshaping and human activity intensity remain underexplored. The Mountain Excavation and City Construction (MECC) project in Yan’an, China, represents an extreme case of engineered urban expansion, involving deep excavation and extensive gully filling to create buildable land. This study examined how such engineered urban expansion and associated anthropogenic pressures reshape urban soil chemical properties. We combined high-density soil sampling, remote sensing interpretation, and machine learning modeling. The respective influences of terrain engineering (excavation and fill), land use cover, and urban anthropogenic pressures (population density and nighttime light intensity) were evaluated. Results showed that soil carbon was particularly sensitive to engineering type, with excavation suppressing accumulation while fill promoted it. Soil pH responded more strongly to land use, where construction and bare land were linked to lower carbon levels and forest and grass land supported higher levels. Urban anthropogenic pressures showed significant negative correlations with carbon stock, and population density exhibited a power function relationship, indicating a non-linear response with a critical point beyond which changes in soil function decelerate. Random forest analysis identified human activity intensity as the dominant factor. Logistic regression further showed that these single-factor effects persisted in combination, with fill–vegetated areas under low anthropogenic pressure most favorable for carbon accumulation. These findings offer new insight into how large-scale terrain reshaping and urban intensification jointly alter soil systems, highlighting the need to account for their compound effects when designing urban land use strategies.