Estimating N₂O processes during grassland renewal and grassland conversion to maize cropping using N₂O isotopocules

Rationale: Enhanced nitrous oxide (N₂O) emissions can occur following grassland break‐up for renewal or conversion to maize cropping, but knowledge about N₂O production pathways and N₂O reduction to N₂ is very limited. A promising tool to address this is the combination of mass spectrometric analysis of N₂O isotopocules and an enhanced approach for data interpretation. Methods: The isotopocule mapping approach was applied to field data using a ~d¹⁵N^sp_N2O and ~d¹⁸O_N2O map to simultaneously determine N₂O production pathways contribution and N₂O reduction for the first time. Based on the isotopic composition of N₂O produced and literature values for specific N₂O pathways, it was possible to distinguish: (i) heterotrophic bacterial denitrification and/or nitrifier denitrification and (ii) nitrification and/or fungal denitrification and the contribution of N₂O reduction. Results: The isotopic composition of soil‐emitted N₂O largely resembled the known end‐member values for bacterial denitrification. The isotopocule mapping approach indicated different effects of N₂O reduction on the isotopic composition of soil‐emitted N₂O for the two soils under study. Differing N₂O production pathways in different seasons were not observed, but management events and soil conditions had a significant impact on pathway contribution and N₂O reduction. N₂O reduction data were compared with a parallel ¹⁵N‐labelling experiment. Conclusions: The field application of the isotopocule mapping approach opens up new prospects for studying N₂O production and consumption of N₂O in soil simultaneously based on mass spectrometric analysis of natural abundance N₂O. However, further studies are needed in order to properly validate the isotopocule mapping approach.