Quantification of N sub(2)O emission pathways via a super(15)N tracing model

A super(15)N tracing model was developed to analyse nitrous oxide (N sub(2)O) dynamics in terrestrial ecosystems, which build on previous tracing models for the quantification of the main mineral nitrogen (N) transformations and soil nitrite (NO sub(2) super(-)) dynamics. The N sub(2)O dynamics in the model are directly associated with three NO sub(2) super(-) sub-pools. Four pathways for N sub(2)O production in soil were considered in the model: i) reduction of NO sub(2) super(-) associated with nitrification (NO sub(2) super(-) sub(nit) arrow right N sub(2)O sub(nit)), ii) reduction of NO sub(2) super(-) associated with denitrification (NO sub(2) super(-) sub(den) arrow right N sub(2)O sub(den)), iii) reduction of NO sub(2) super(-) associated with organic N (N sub(org)) oxidation (NO sub(2) super(-) sub(org) arrow right N sub(2)O sub(org)), and iv) codenitrification (N sub(2)O sub(cod)), a hybrid reaction where one N atom in N sub(2)O originates from organic N and the other from NO sub(2) super(-) sub(den). Soil N sub(2)O can further be reduced to N sub(2) 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 super(15)N tracing tool was tested with a data set from a super(15)N tracing study on grassland soil. Tracing model results showed that on average over a 12 day period N sub(2)O sub(nit), N sub(2)O sub(den), N sub(2)O sub(org) and N sub(2)O sub(cod) contributed 9%, 20%, 54% and 18% to the total N sub(2)O emission, respectively. The results are in line with estimates based on analytical approaches that consider three N sub(2)O emission pathways. The strength of this new super(15)N tracing tool is that for the first time four N sub(2)O 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 N sub(2)O production in old grassland soil. The underlying NO sub(2) super(-) and N sub(2)O reduction kinetics are in agreement with denitrification gene expressions and the calculated N sub(2)/N sub(2)O ratios are in the expected range. The tracing model provides insights on N dynamics which may occur in soil microsites. This information is import