The formation of the trace gases NO and N2O in soils by the coupled processes of nitrification and denitrification: results of kinetic 15N tracer investigations

Biogenic production in soils, especially when fertilized with high levels of nitrogen, is one of the main sources of nitrous oxide (N₂O) and may be a significant source of nitric oxide (NO). NO and N₂O are formed by the microbial processes of nitrification and denitrification but in soils these processes are not fully understood in detail. The kinetic ¹⁵N isotope method, is very useful for investigating the mechanism of such complex N transformation processes in soils. We used this method with ${}^{15}\text{NH}{}_4{}^{\mathrm{+}}$, ${}^{15}\text{NO}{}_2{}^{\mathrm{-}}$, and ${}^{15}\text{NO}{}_3{}^{\mathrm{-}}$ as tracers to study the processes of NO and N₂O formation in black earth soil (Haplic Phaeozem). The experiments were conducted using a special analytical set-up in which the incubation vessels were directly connected to a gas chromatograph quadrupole mass spectrometer system. This system allows simultaneous determination of the concentration of N₂O, NO and N₂ along with their ¹⁵N abundance in the gas phase. The results of these investigations are discussed and combined in a diagram, termed the model of coupled nitrification and denitrification, illustrating the formation of NO and N₂O. Nitrification and denitrification proceed under both water-unsaturated (nearly aerobic) and water-saturated (nearly anaerobic) conditions. The two processes are linked by a “common” nitrate pool. In contrast the nitrite, an important intermediate of the two processes, evidently exists within two separate pools. NO is mainly produced by nitrification as a by-product of the oxidation of ammonium to nitrite or directly by nitrite decomposition. If NH₄⁺ contents are high under aerobic conditions NO emission can markedly exceed that of N₂O. Nitrous oxide is mainly formed by denitrification of nitrate. Therefore increasing water saturation promotes N₂O emission. NO could not be confirmed as a free precursor of N₂O formation via denitrification.