Quantification of N2O emission pathways via a 15N tracing model

A 15N tracing model was developed to analyse nitrous oxide (N2O) dynamics in terrestrial ecosystems, which build on previous tracing models for the quantification of the main mineral nitrogen (N) transformations and soil nitrite (NO2−) dynamics. The N2O dynamics in the model are directly associated with three NO2− sub-pools. Four pathways for N2O production in soil were considered in the model: i) reduction of NO2− associated with nitrification (NO2−nit → N2Onit), ii) reduction of NO2− associated with denitrification (NO2−den → N2Oden), iii) reduction of NO2− associated with organic N (Norg) oxidation (NO2−org → N2Oorg), and iv) codenitrification (N2Ocod), a hybrid reaction where one N atom in N2O originates from organic N and the other from NO2−den. Soil N2O can further be reduced to N2 and/or can be emitted to the atmosphere. The reaction kinetics and emission notations are based on first-order approaches. Parameter optimization was carried out with a Markov Chain Monte Carlo (MCMC) technique that is suitable for models with large number of parameters. The 15N tracing tool was tested with a data set from a 15N tracing study on grassland soil. Tracing model results showed that on average over a 12 day period N2Onit, N2Oden, N2Oorg and N2Ocod contributed 9%, 20%, 54% and 18% to the total N2O emission, respectively. The results are in line with estimates based on analytical approaches that consider three N2O emission pathways. The strength of this new 15N tracing tool is that for the first time four N2O emission pathways, including a hybrid-reaction, can simultaneously be quantified. The analysis highlights that heterotrophic processes related to organic N turnover and neither autotrophic nitrification nor denitrification may be the prevailing pathways for N2O production in old grassland soil. The underlying NO2− and N2O reduction kinetics are in agreement with denitrification gene expressions and the calculated N2/N2O ratios are in the expected range. The tracing model provides insights on N dynamics which may occur in soil microsites. This information is important for the development of more realistic representations of soil N cycling in ecosystem models.