Forms of soil organic nitrogen in a Pactola soil as seen by pyrolysis/gas chromatography/atomic emission detection (Py–GC/AED)

In this study, soil organic nitrogen (SON) forms from a Pactola forest soil were investigated by using pyrolysis–cryogenic gas chromatography/atomic emission (Py–GC/AED). The samples were taken at different soil depths of 0–12 cm, 12–25 cm and 25–38 cm and separated into particle-size fractions. Each fraction was analyzed by Py–GC/AED at 300, 400 and 500 °C consecutively. The main N-structures found in pyrolysates were ammonia, acetonitrile, hydrogen cyanide, pyridine and pyrrole. Ratios of acetonitrile and hydrogen cyanide to pyridine and pyrrole decreased with soil depth. Acetonitrile and hydrogen cyanide had positive correlations with total organic N (TON) down the soil profile. A decrease of the optimal pyrolysis temperatures of soil samples with increasing soil depth was observed for acetonitrile and hydrogen cyanide. In the surface soil samples, there was no correlation found for both pyridine and pyrrole with TON. But with depth, the correlation coefficient increased and reached to 0.912 for pyridine, and 0.875 for pyrrole at the depth of 25–38 cm. The increased correlation coefficients of pyridine and pyrrole with TON in combination with the low shifts of the optimal pyrolysis temperatures of the soil samples down the soil profile for acetonitrile and hydrogen cyanide indicated clear differences in N-structures between different soil layers.