The total area of black soils region in Song'nen Plain is 590×104 h m2. The content of soil organic carbon, its density and deposition were research ed from 102 soil samples in 34 soil sections at 9 cities. The results showed tha t the content of organic carbon in black soils in Song'nen Plain was 14.14 g·kg-1 on average, ranged from 11.29 g·kg-1 to 17.31 g·kg-1, the density of org anic carbon was 23.49 kg·m-2 on average, ranged from 17.24 kg·m-2 to 30.43 k g·m-2, which suggested that the content and density of organic carbon in black soils were both comparatively higher than in other types of soil, and organic m atter accumulated deeply in black soils. It was estimated that this region depos ited a black soil organic carbon (SOC) pool of 1.412 billion tons that contribut ed to 1.53%of that of the whole country, though the area of the black soils reg ion in Song'nen Plain occupied only 0.67%of the total area of our country. Fact ors such as moisture, temperature, terrain affected the density of organic carbo n in different areas. A very significant correlation was found between the densi ty of organic carbon and temperature in black soil region, and the density incre ased with the latitude ascending, but decreased with accumulated temperature (≥ 10 ℃) increasing.
The decomposition speed of organic matter was slow in high al titude area since the decreased annual average temperature and accumulated tempe rature and the limited activities of microbe, but the organic matter accumulated largely for its short and hot summer and the thickest plant with developed root system that benefited the accumulation of organic carbon in black soils. No sig nificant correlation was obtained between the density of SOC and rainfall. The m oderate rainfall there advanced the development of plant and its root system and was helpful for the return of litter and root to soils, so the organic matter c ould form and then accumulate in soils. The effect of rainfall on density of org anic carbon was not observed within 400～600 mm that concentrated during April-September and could not restrict the development of plant.