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 ~d15Nsp N2O and ~d18ON2O 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.
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