Changes in cropland soil color in Northeast China's black soils region over the past 30 years

Cultivation has led to soil degradation, with soil color being the most direct indicator. This study examined the impact of nearly 30 years of cultivation on soil color. A total of 1273 topsoil samples were collected from cropland in Northeast China, resulting in seven Munsell soil colors. Combined with Landsat remote sensing images and soil formation factors, soil color predictions were made for every five-year period from 1990 to 2020. A stepwise classification method was introduced for modeling, with classification validation accuracy ranging from 0.61 to 0.72, and the overall average color difference was 1.60. Based on the definition of "black soils," the seven soil colors were classified into two groups: black soils color and non-black soils color, leading to four types of changes: consistently black soils color, positive change (non-black soils color transitioning to black soils color), negative change (black soils color transitioning to non-black soils color), and consistently non-black soils color. Using existing reclamation history and soil organic carbon (SOC) datasets, soil color change trends were analyzed. The experimental results show that: 1. After 30 years of continuous cultivation, 23.16 % of cropland in Northeast China experienced negative changes in soil color; 2. In areas of long-term negative soil color changes after 30 years of continuous cultivation, SOC changes ranged from −1.45 g kg⁻¹ to −3.38 g kg⁻¹, while in areas with long-term positive changes, SOC changes ranged from 7.4 g kg⁻¹ to 11.68 g kg⁻¹; 3. The proportion of black soils color decreased after the conversion of natural land to cropland, with the most significant reduction occurring when forests were converted. In areas that had been cultivated for 11 to 15 years, the proportion of black soils color decreased by an average of 8.09% every five years.4. Soil color changes were primarily in the Chroma dimension, followed by the Value dimension. This study shows that cropland reclamation has had a significant impact on soil color changes. To better protect black soils, appropriate agricultural policies and management measures are recommended. The findings provide an important reference for soil color changes in the black soils region of Northeast China and highlight the crucial role of soil color in black soils conservation. These results offer a scientific basis for formulating effective soil management strategies.