Which N2O production processes are relevant when converting from grassland to cropland?

During the last decades, large areas of grassland were converted to cropland across Europe, mainly due to the increasing demand of cropland following the expansion of biogas plant production (e.g. maize). However, the conversion to cropland bears a risk of nitrous oxide (N₂O) emission due to enhanced nitrogen (N) mineralization. Until now, knowledge about N₂O production pathways due to grassland conversion and in particular N₂O reduction to N₂ is very limited (Buchen et al., 2018), even though understanding of N₂O processes and identification of sources are needed in order to devise mitigation options.

N₂O samples were collected periodically from manual chambers following chemical and mechanical conversion from permanent grassland to cropland (maize) at two sites with different texture (clayey loam and sandy loam) and fertilization regime (with and without mineral N-fertilization) in north-western Germany (Helfrich et al., 2020). Samples were analysed for natural abundance stable isotope signatures of soil-emitted N₂O (δ¹⁵N^bulk_N2O, δ¹⁸O_N2O and δ¹⁵N^SP_N2O = intramolecular distribution of ¹⁵N in the N₂O molecule) by isotope ratio mass spectrometry (IRMS) and dual-isotope of N₂O isotopic signatures (plotting δ¹⁵N^SP_N2O vs. δ¹⁸O_N2O) were used for data evaluation (Lewicka-Szczebak et al., 2017). Although, isotopic signatures were very variable throughout the year at both sites, the clayey loam site exhibited a close correlation between δ¹⁵N^sp_N2O and δ¹⁸O_N2O suggesting that values were mainly controlled by N₂O reduction to N₂. At the sandy loam site this pattern was less pronounced, possibly because processes other than bacterial denitrification (e.g. fungal denitrification and nitrification) also significantly influence isotopocule values. Altogether, bacterial denitrification was found to be the most important process following grassland conversion to maize cropping.

References:

Buchen, C., Lewicka‐Szczebak, D., Flessa, H., Well, R., 2018. Estimating N₂O processes during grassland renewal and grassland conversion to maize cropping using N₂O isotopocules. Rapid Communications in Mass Spectrometry 32, 1053-1067.

Helfrich, M., Nicolay, G., Well, R., Buchen-Tschiskale, C., Dechow, R., Fuß, R., Gensior, A., Paulsen, H., Berendonk, C., Flessa, H., 2020. Effect of chemical and mechanical grassland conversion to cropland on soil mineral N dynamics and N₂O emission. Agriculture, Ecosystems & Environment 298, 106975.

Lewicka-Szczebak, D., Augustin, J., Giesemann, A., Well, R., 2017. Quantifying N₂O reduction to N₂ based on N₂O isotopocules – validation with independent methods (helium incubation and ¹⁵N gas flux method). Biogeosciences 14, 711-732.