The black soil region in northeastern China is one of the most important grain production areas in the country, and plays a key role in food security. Large lands have been brought into cultivation in the last 100 years. Because these lands have lesser slope gradients compared to those in the loess plateau and other regions in China, soil erosion has been ignored in this area for a long time. Few soil conservation practices have been implemented. Serious soil erosion problems due to long slope lengths have been identified based on recent research and observations in the study area. The government has realized the necessity and urgency of soil conservation. However, to create policies and plans for black soil conservation, questions such as how much of the black soil region, and where in the region, erosion is a problem must be addressed. Currently, the black soil region of northeastern China is regarded as having a total area of 1.09 million square kilometers, including Heilongjiang, Jilin, the north part of Liaoning, and the east part of Inner Mongolia, in which there are also many non-black soils in mountain regions of Daxingan, Xiaoxingan, and Changbaishan. Therefore, it is difficult to determine precisely the conservation priorities for this large area. In this study, the black soils were defined and their concentrated area was delineated in northeastern China. The black soils were defined as soils with the “mollic epipedon”, which were classified as Isohumosols in Chinese Soil Taxonomy or Mollisols in the USDA Soil Taxonomy, and classified as Phaezoems, Chernozems, Kastanozems and Grayzems soils according to the Genetic Soil Classification of China (GSCC). The region dominated by these four soils in GSCC was defined as the black soil region. Because Phaezoems and Chernozems have higher organic matter contents and deeper mollic epipedon depths, and are distributed in the center of the study area, these two soils were defined as the typical black soil, and their dominate area was defined as the typical black soil region. A soil map in the scale of 1:1000000, made from the second national soil survey, was used with ArcGIS 10.2 software to delineate both black soil and typical black soil regions. A method called “central gravitational agglomeration” was developed. All of the soil polygons were selected, and the farthest and the smallest polygon were merged into its closest adjacent one if its radius was larger than the distance between it and its closest one. Otherwise it was abandoned. The same process was repeated until only a few small polygons were left and a regional outline appeared. Four soils were used to determine the exact boundaries of the black soil region, and two soils were used to determine the exact boundaries of the typical black soil region. The results showed that the area of the black soil region was 556000 km², including 146 counties in which 33.0% was cultivated land. The area of the typical soil region was 333000 km², including 138 counties in which 50.2% was cultivated land. Due to the long slopes, the soil erosion rate from the cropland with slope greater than 0.25° is estimated to be larger than soil loss tolerance of 200 t/km². The cultivated lands with slope greater than 0.25° were regarded as sloped cultivated land or sloped cropland, which totaled 89000 km², or 16.0% of the total land for the black soil region, and 78000 km², or 23.4% of the total land for the typical black soil region. The sloped cultivated land in both the black soil region and typical black soil region should be targeted as key areas for soil erosion control in northeastern China.

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