Constructing and Spatially Differentiating Soil Quality Indices in Qiqihar’s Typical Black Soil Zone: A Case Study of Tailai, Longjiang, and Gannan Counties, China

Black soils in Qiqihar City are comprised primarily of black soil. They have been extensively exploited for agriculture. To investigate the spatial distribution of soils in this region, we analyze 72 samples collected from Tailai, Longjiang, and Gannan counties. A soil quality index (SQI) based on a subset of measured soil indicators is constructed to comprehensively evaluate black soil quality. We report an average soil bulk density in these black soil areas of 1.42 g/cm3, indicating relatively high compaction and density. The average soil moisture content (19%) is relatively low. In some areas, soil electrical conductivity reaches 2.92 μS/cm, indicating mild salinization (<4 μS/cm). Overall soil nutrient levels are relatively high, but in some areas they are poor. Principal components and correlation analyses identify five of nine measured indicators (soil bulk density, pH, moisture, nitrate nitrogen, and organic matter contents) that adequately characterize soil quality. The SQI values reveal soil quality to decrease along a north–south gradient, sand to be highest in Gannan County and lowest in Tailai County. Overall, black soil quality in Qiqihar City is relatively low. These results provide a scientific foundation and data support for soil restoration and ecological construction efforts in these areas.