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<p>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 (N<sub>2</sub>O) emission due to enhanced nitrogen (N) mineralization. Until now, knowledge about N<sub>2</sub>O production pathways due to grassland conversion and in particular N<sub>2</sub>O reduction to N<sub>2</sub> is very limited (Buchen et al., 2018), even though understanding of N<sub>2</sub>O processes and identification of sources are needed in order to devise mitigation options.</p> <p>N<sub>2</sub>O 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 N<sub>2</sub>O (δ<sup>15</sup>N<sup>bulk</sup><sub>N2O</sub>, δ<sup>18</sup>O<sub>N2O</sub> and δ<sup>15</sup>N<sup>SP</sup><sub>N2O</sub> = intramolecular distribution of <sup>15</sup>N in the N<sub>2</sub>O molecule) by isotope ratio mass spectrometry (IRMS) and dual-isotope of N<sub>2</sub>O isotopic signatures (plotting δ<sup>15</sup>N<sup>SP</sup><sub>N2O</sub> vs. δ<sup>18</sup>O<sub>N2O</sub>) 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 δ<sup>15</sup>N<sup>sp</sup><sub>N2O</sub> and δ<sup>18</sup>O<sub>N2O</sub> suggesting that values were mainly controlled by N<sub>2</sub>O reduction to N<sub>2</sub>. 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.</p> <p><strong>References:</strong></p> <p>Buchen, C., Lewicka‐Szczebak, D., Flessa, H., Well, R., 2018. Estimating N<sub>2</sub>O processes during grassland renewal and grassland conversion to maize cropping using N<sub>2</sub>O isotopocules. Rapid Communications in Mass Spectrometry 32, 1053-1067.</p> <p>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 N<sub>2</sub>O emission. Agriculture, Ecosystems & Environment 298, 106975.</p> <p>Lewicka-Szczebak, D., Augustin, J., Giesemann, A., Well, R., 2017. Quantifying N<sub>2</sub>O reduction to N<sub>2 </sub>based on N<sub>2</sub>O isotopocules – validation with independent methods (helium incubation and <sup>15</sup>N gas flux method). Biogeosciences 14, 711-732.</p>