Lysimeter-based full fertilizer 15N balances corroborate direct dinitrogen emission measurements using the 15N gas flow method

Abstract

The ¹⁵N gas flux (¹⁵NGF) method allows for direct in situ quantification of dinitrogen (N₂) emissions from soils, but a successful cross-comparison with another method is missing. The objectives of this study were to quantify N₂ emissions of a wheat rotation using the ¹⁵NGF method, to compare these N₂ emissions with those obtained from a lysimeter-based ¹⁵N fertilizer mass balance approach, and to contextualize N₂ emissions with ¹⁵N enrichment of N₂ in soil air. For four sampling periods, fertilizer-derived N₂ losses (¹⁵NGF method) were similar to unaccounted fertilizer N fates as obtained from the ¹⁵N mass balance approach. Total N₂ emissions (¹⁵NGF method) amounted to 21 ± 3 kg N ha^− 1, with 13 ± 2 kg N ha^− 1 (7.5% of applied fertilizer N) originating from fertilizer. In comparison, the ¹⁵N mass balance approach overall indicated fertilizer-derived N₂ emissions of 11%, equivalent to 18 ± 13 kg N ha^− 1. Nitrous oxide (N₂O) emissions were small (0.15 ± 0.01 kg N ha^− 1 or 0.1% of fertilizer N), resulting in a large mean N₂:(N₂O + N₂) ratio of 0.94 ± 0.06. Due to the applied drip fertigation, ammonia emissions accounted for < 1% of fertilizer-N, while N leaching was negligible. The temporal variability of N₂ emissions was well explained by the δ¹⁵N₂ in soil air down to 50 cm depth. We conclude the ¹⁵NGF method provides realistic estimates of field N₂ emissions and should be more widely used to better understand soil N₂ losses. Moreover, combining soil air δ¹⁵N₂ measurements with diffusion modeling might be an alternative approach for constraining soil N₂ emissions.