AGROGENIC TRANSFORMATION OF NITROGEN IN SOILS OF NORTHERN KAZAKHSTAN
The quantitative and qualitative monitoring of available nitrogen when the land is used for agricultural purpose for a long time is essential to understand the processes occurring in the soil and to develop practical measures to optimize the nitrogen status. The main zonal soils of the Northern Kazakhstan were investigated: chernozems, dark chestnut soils and chestnut soils. Permanent and semi-permanent plots were used to compare temporal changes of nitrogen. The total soil nitrogen was determined by the Kjeldahl method, humus content by the Tyurin method, readily hydrolysable nitrogen by the Tyurin and Konova method, and the fractional nitrogen composition by the Vorobyev method. In the Northern Kazakhstan, nitrogen makes about 5% per fraction, or 1/20 part of humus. The content of non-hydrolysable and low-hydrolysable nitrogen makes 80-96% of the total nitrogen. Readily hydrolysable nitrogen forms make less than 10%. Mineral forms make 1-2% only. At present the nitrogen requirements of plants are met by the soil reserves mainly. At the end of the 60-year period of the virgin and fallow lands development the soil nitrogen situation has changed for the worse. The decrease in humus content, total nitrogen and nitrogen mineral forms is observed everywhere. Nitrogen content should be increased to the medium or high level in every second hectare of tilled land. The C:N ratio in the main arable soils remains quite stable, except for chestnut sandy loamy soils. The rate of nitrates accumulation is significantly higher in arable soils than in their virgin compatibles. The cultivation of wheat as continuous crop for a number of years has significantly decreased the total and mobile nitrogen. To maintain the nitrogen status and increase cereal crop yields, mineral and organic fertilizers should be applied; legume crops should be used in crop rotation.