Lysimeter-based full N balance as a tool to test field N2 flux measurements

Measuring soil dinitrogen (N₂) emissions is notoriously challenging under field conditions. Hence, N₂ emissions represent a significant uncertainty in the nitrogen mass balance of terrestrial ecosystems. The ¹⁵N gas flux (¹⁵NGF) method is the only method currently available for directly quantifying N₂ emissions in situ. However, this method has rarely undergone independent validation under field conditions. In this study, our objectives were to: (1) Quantify N₂ emissions and their role in the fertilizer N mass balance of a wheat rotation using the ¹⁵NGF method (2) Verify the obtained quantities of N₂ emissions using a mass balance approach and (3) Verify the temporal N₂ emission dynamics at the soil-atmosphere interface using vertical soil profiles of ¹⁵N₂ enrichment.

To achieve these objectives, we grew winter wheat in lysimeters and applied ¹⁵N enriched mineral fertilizers via fertigation in three doses (sum 170 kg N ha^-1). We then analyzed gaseous (NH₃, N₂O, N₂) and hydrological N losses, as well as fertilizer N fates in plant and soil, and ¹⁵N₂ enrichment in soil air.

Our results showed that N₂ emissions directly measured using the ¹⁵NGF method amounted to 30 ± 4 kg N ha^-1, which was equivalent to 18 ± 3 % of the applied fertilizer N. These measurements agreed with unrecovered fertilizer N obtained from the ¹⁵N fertilizer mass balance, although the latter had large inherent uncertainty (21 ± 21 kg N ha^-1). N₂O emissions, however, were negligible (0.14 ± 0.02 kg N ha^-1). The temporal variability of measured N₂ emissions after fertilizer additions was generally well explained by ¹⁵N₂ enrichment in soil gas.

Overall, we provide independent validation of the ¹⁵NGF method in measuring N₂ emissions in the field and highlight the significant role of these emissions in the nitrogen balance of crop systems. Our data also suggest that soil gas measurements in combination with diffusion modeling could serve as an alternative method for quantifying N₂ emissions. These results should encourage a wider application of the ¹⁵NGF method in order to improve our understanding of N₂ emissions and reduce the current uncertainties in estimates of these emissions.