Content of N and C in soil were investigated in a long-term field experiment under different systems of N fertilization. Chernozem and Cambisol were extracted using hot water (N<sub>hws</sub>, C<sub>hws</sub>) and 0.01M CaCl<sub>2</sub> (N<sub>CaCl2</sub>, C<sub>DOC</sub>). The C<sub>t</sub>/N<sub>t</sub> ratio in Chernozem was 9.6:1 and in Cambisol 6.1:1. The lowest C<sub>t</sub>/N<sub>t</sub> ratio in both experiments was found in the control treatment. Results showed that C and N compounds are less stable in Cambisol, which leads to a higher rate of mineralization. In the Chernozem, N<sub>hws</sub> formed 3.66% from the total N content in the soil whereas N<sub>CaCl2</sub> formed only 0.82%. C<sub>hws</sub> formed 2.98% and C<sub>DOC</sub> 0.34% from total C content. Cambisol contains 4.81% of N<sub>hws</sub> and 0.84% of N<sub>CaCl2</sub> from the total N amount and 5.76% of C<sub>hws</sub> and 0.70% of C<sub>DOC</sub> from the total C content, respectively. Nitrogen extracted by 0.01M CaCl2 formed only 22.4% of N extractable by hot water in Chernozem and 17.5% in Cambisol. The lowest C/N ratios were obtained after the CaCl<sub>2</sub> extraction (3.0–6.2:1). The application of manure increased the content of soil organic N and C compared to the sewage sludge treatments.
The changes of soil nitrogen and carbon contents in a long-term field experiment under different systems of nitrogen fertilization