Spatial Pattern and Influencing Factors of Topsoil Organic Carbon Content in Hilly Region of Eastern Sichuan

Understanding the spatial distribution of soil organic carbon (SOC) and the interaction of its influencing factors is crucial for improving soil properties and promoting sustainable land use. The hilly area of eastern Sichuan is significant both for food security and ecological protection. However, improper agricultural practices, such as over-cultivation and excessive use of fertilizers, have led to a decline in SOC content. This poses increasing risks to soil health and the long-term sustainability of agriculture in the region. This paper aims to explore the SOC distribution in the hilly areas of eastern Sichuan, taking into account the unique terrain and soil texture characteristics of the region. The research is based on 1,301 soil samples collected from five different fields, spread across various districts and counties, ensuring a representative coverage of all secondary cultivated land categories. The soil samples were taken at a depth of 15–30 cm, which is critical for understanding the distribution of organic carbon in agricultural soils. To analyze the factors influencing SOC content, the study classifies them into two main categories: external environmental random factors and soil endowment factors. External environmental factors include meteorological data, such as annual mean temperature and precipitation, while soil endowment factors encompass the inherent properties of the soil, including texture, parent material, nutrient content, and other physicochemical characteristics. Topographical data, such as slope and elevation, were derived from DEM (Digital Elevation Model) calculations, providing a detailed understanding of the terrain's impact on SOC distribution. The study employs a combination of statistical methods, spatial analysis, and random forest models to assess the correlation between these factors and the SOC distribution. Results indicate that: 1) SOC content in the hilly areas of eastern Sichuan ranges from 4.91 to 15.26 g/kg, with an average of 7.39 g/kg and a coefficient of variation of 21.7%. This suggests moderate spatial variability across the region. 2)Further analysis using the Gaussian model revealed a nugget effect of 0.173, with a nugget ratio of 61.5%, indicating moderate spatial variation at small scales. 3) The Kriging interpolation method predicted a spatial pattern of SOC distribution, showing a "north high, south low" trend, with SOC levels gradually increasing from the southern to northern regions. This spatial variability is influenced by both external environmental random factors and soil endowment factors. 4) Climate and topography were found to have significant correlations with SOC content (P < 0.01). Among these, annual mean temperature displayed a strong negative correlation (−0.72) with SOC content, while the correlations with precipitation (0.34) and slope (0.30) were relatively weaker. 5) Soil texture and physicochemical properties, on the other hand, were the most significant factors affecting SOC content, overshadowing the influence of topographical and climatic conditions. The findings emphasize that soil management practices, particularly those focusing on soil texture and nutrient levels, have the greatest impact on SOC distribution in the region. 6) Among the various models tested, the random forest regression model showed the best performance in predicting SOC distribution. This model identified nutrient content as the most critical factor influencing SOC, accounting for 61.3% of the spatial variation. In contrast, topographical factors contributed only 1.7%, highlighting the relatively minor role of terrain in determining SOC levels. These findings suggest that improving soil nutrient management is essential for enhancing SOC content and promoting sustainable agricultural practices in the hilly areas of eastern Sichuan. This research provides a comprehensive understanding of the factors affecting SOC distribution and highlights the importance of targeted land management practices to enhance soil health and ensure sustainable agricultural productivity.