Estimating N₂ O processes during grassland renewal and grassland conversion to maize cropping using N₂ O isotopocules

Enhanced nitrous oxide (N₂ O) emissions can occur following grassland break-up for renewal or conversion to maize cropping, but knowledge about N₂ O production pathways and N₂ O reduction to N₂ is very limited. A promising tool to address this is the combination of mass spectrometric analysis of N₂ O isotopocules and an enhanced approach for data interpretation.

The isotopocule mapping approach was applied to field data using a δ¹⁵ N^sp_N2O and δ¹⁸ O_N2O map to simultaneously determine N₂ O production pathways contribution and N₂ O reduction for the first time. Based on the isotopic composition of N₂ O produced and literature values for specific N₂ 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 N₂ O reduction.

The isotopic composition of soil-emitted N₂ O largely resembled the known end-member values for bacterial denitrification. The isotopocule mapping approach indicated different effects of N₂ O reduction on the isotopic composition of soil-emitted N₂ O for the two soils under study. Differing N₂ O production pathways in different seasons were not observed, but management events and soil conditions had a significant impact on pathway contribution and N₂ O reduction. N₂ O reduction data were compared with a parallel ¹⁵ N-labelling experiment.

The field application of the isotopocule mapping approach opens up new prospects for studying N₂ O production and consumption of N₂ O in soil simultaneously based on mass spectrometric analysis of natural abundance N₂ O. However, further studies are needed in order to properly validate the isotopocule mapping approach.