Process-specific analysis of nitrite dynamics in a permanent grassland soil by using a Monte Carlo sampling technique

The aim of the present study was to separate and quantify process-specific gross nitrite (NO2-) production in a permanent grassland soil and separate the observed NO2- concentration into process-specific sub-pools. We re-analysed data from a laboratory experiment where either the nitrate and/or ammonium ions were labelled with N-15. The simultaneously occurring N transformations were quantified with a N-15 tracing tool in combination with a Monte Carlo sampling technique which has been extended by a NO2- sub-model. Three pathways of NO2- production were identified, that is autotrophic nitrification, denitrification and the oxidation of organic N. The full model contained nine pools and 14 N transformations. This is the first report where such a complex N-15 tracing model has been used and where all parameters have been optimized simultaneously. Our analysis shows that on average 52.4% of the total soil NO2- concentration was directly derived from oxidation of organic N. Nitrification and denitrification contributed on average 18.1 and 29.5%, respectively, to the total NO2- pool. However, more than 99% of the gross NO2- production was related to the NH4+ oxidation, which was rapidly further oxidized to NO3-. Our re-analysis using a Monte Carlo sampling technique resulted in parameter profiles that differed from the original analysis. As the new technique is able to optimize all parameters in one optimization run we regard this technique as superior to the previous one. A process-based understanding of NO2- dynamics in soils has important implications for the understanding of the production of the greenhouse gas nitrous oxide (N2O), which is tightly linked to NO2- turnover.