Application of a Triple 15N Tracing Technique to Elucidate N Transformations in a UK Grassland Soil

To identify the production and consumption pathways and temporal dynamics of N2O emitted from soil, this study uses N-15-labelled substrate-N to quantify the underlying gross N transformation rates using the Ntrace analysis tool and link them to N-emissions. In three experiments twelve soil cores each were incubated in a lab incubation system to measure gaseous emissions, while parallel incubations under the same conditions were set up for destructive soil sampling at 7 time points. Using the triple labelling technique (applying NH4NO3 with either the NH4+-N or the NO3--N or both being N-15 labelled), this study investigated the effects of 55, 70 and 85% water filled pore space (deemed to promote nitrification, both nitrification and denitrification, and denitrification, respectively) in a clay soil on gaseous N emissions and investigates the source and processes leading to N2O emissions. To assess the utilisation of applied NO3- vs. nitrified NO3- from applied NH4+, the N-15 tracing tool Ntrace was used to quantify the rates of immobilisation of NO3- and NH4+, oxidation of NH4+, mineralisation of organic N and subsequent nitrification by the analysis of the N-15 in the soil. Gross transformation rates were calculated, indicating the relative importance of added NO3- and NO3- derived from nitrified added NH4+. Results show an important contribution of heterotrophic nitrification (organic N oxidation to NO3-) which was highest at the 55% water filled pore space (WFPS), decreasing in its contribution to N-transformation processes with increasing WFPS, while nitrification (NH4+ oxidation to NO3-) was contributing the most at 70% WFPS. The contribution of denitrification increased with increasing WFPS, but only became dominant at 85% WFPS. While denitrification still showed to be most important at high and nitrification at lower WFPS, the actual % WFPS values were not as expected and highlight the fact that WFPS is a contributor, but not the sole/most important parameter determining the type of N-transformation processes taking place.