Satellite images reveal soil color changes in typical black soil region of China: brighter, redder, and yellower

The soil quality of cropland in the black soil region of China has been declining, and soil color is the most intuitive soil parameter reflecting soil degradation. While there is substantial research on changes in soil organic carbon (SOC) in this region, studies on soil color changes are relatively scarce. This study quantitatively investigated the soil color changes in a typical black soil region of China (Black soils and Chernozems) over the past 32 years based on conversions of CIELAB color space and Munsell color system from remote sensing data. Landsat images of 1990 and 2022 were acquired through Google Earth Engine, and Synthetic Soil Image (SYSI) system were applied to obtain the barest soil images before converting to soil color. SHapley Additive exPlanations (SHAP) values were calculated to analyze the contributions of environmental factors. Our study indicated that: (1) The lightness values, red-green and blue-yellow components of cropland in the typical black soil region have shown an increasing trend from 1990 to 2022, becoming brighter, redder and yellower; (2) Areas with hues G (Green), GY (Green-Yellow) and Y (Yellow) have gradually decreased, with 10GY 1/2 showing the largest reduction of 9,500 km2, while the area with hue YR (Yellow-Red) has been gradually increasing with 10YR 1/2 showing the largest increase of 28808 km² for Munsell soil colors in the typical black soil region; (3) The contribution of precipitation to soil color changes is the largest, leading to increases of 0.1945 and 0.5978 for lightness in the typical black soil region and Chernozems, respectively, and the effect of topographic factors on Chernozems is greater than on Black soils. Our results provide quantitative evidence for understanding soil degradation in the typical black soil region and demonstrate that soil color can serve as an effective parameter for monitoring soil changes.