NITROGEN LOSS DEGREE AND DISTRIBUTION CHARACTERISTICS OF BLACK SOIL AREA IN SOUTH-CENTRAL SONGNEN PLAIN

The analysis of nitrogen content data in the black soil area of south-central Songnen Plain in 1980s and at present shows that the soil nitrogen loss rate is up to 5.02% over the past 40 years, generally with the grades 1 and 2 land area of rich-relatively rich soil nitrogen content greatly reducing and the grades 3, 4, and 5 land area of moderate-deficient nitrogen content gradually increasing. The statistics of variation characteristics of soil nitrogen content by land use types indicate the proportions of paddy field and upland field strongly affected by human activities with high nitrogen content have decreased by 20.9 and 6.1 percentage points respectively, while the areas of marsh wetland, grassland and forest less affected by human activities rich in soil nitrogen content have increased. The spatial analysis on variation of soil nitrogen content in different administrative regions reflects the soil nitrogen loss in 6 cities and counties has reached severe degree, which should be the key areas of black soil nitrogen loss control as suggested. The soil nitrogen loss in other 8 cities and counties/districts is moderate, regarded as the key areas for both prevention and control of soil nitrogen loss.