Estimating N 2 O processes during grassland renewal and grassland conversion to maize cropping using N 2 O isotopocules

Rationale Enhanced nitrous oxide (N 2 O) emissions can occur following grassland break‐up for renewal or conversion to maize cropping, but knowledge about N 2 O production pathways and N 2 O reduction to N 2 is very limited. A promising tool to address this is the combination of mass spectrometric analysis of N 2 O isotopocules and an enhanced approach for data interpretation. Methods The isotopocule mapping approach was applied to field data using a δ 15 N sp N2O and δ 18 O N2O map to simultaneously determine N 2 O production pathways contribution and N 2 O reduction for the first time. Based on the isotopic composition of N 2 O produced and literature values for specific N 2 O 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 N 2 O reduction. Results The isotopic composition of soil‐emitted N 2 O largely resembled the known end‐member values for bacterial denitrification. The isotopocule mapping approach indicated different effects of N 2 O reduction on the isotopic composition of soil‐emitted N 2 O for the two soils under study. Differing N 2 O production pathways in different seasons were not observed, but management events and soil conditions had a significant impact on pathway contribution and N 2 O reduction. N 2 O reduction data were compared with a parallel 15 N‐labelling experiment. Conclusions The field application of the isotopocule mapping approach opens up new prospects for studying N 2 O production and consumption of N 2 O in soil simultaneously based on mass spectrometric analysis of natural abundance N 2 O. However, further studies are needed in order to properly validate the isotopocule mapping approach.