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

Abstract

<p>Measuring soil dinitrogen (N<sub>2</sub>) emissions is notoriously challenging under field conditions. Hence, N<sub>2</sub> emissions represent a significant uncertainty in the nitrogen mass balance of terrestrial ecosystems. The <sup>15</sup>N gas flux (<sup>15</sup>NGF) method is the only method currently available for directly quantifying N<sub>2</sub> emissions in situ. However, this method has rarely undergone independent validation under field conditions. In this study, our objectives were to: (1) Quantify N<sub>2</sub> emissions and their role in the fertilizer N mass balance of a wheat rotation using the <sup>15</sup>NGF method (2) Verify the obtained quantities of N<sub>2</sub> emissions using a mass balance approach and (3) Verify the temporal N<sub>2</sub> emission dynamics at the soil-atmosphere interface using vertical soil profiles of <sup>15</sup>N<sub>2</sub> enrichment.</p><p>To achieve these objectives, we grew winter wheat in lysimeters and applied <sup>15</sup>N enriched mineral fertilizers via fertigation in three doses (sum 170 kg N ha<sup>-1</sup>). We then analyzed gaseous (NH<sub>3</sub>, N<sub>2</sub>O, N<sub>2</sub>) and hydrological N losses, as well as fertilizer N fates in plant and soil, and <sup>15</sup>N<sub>2</sub> enrichment in soil air.</p><p>Our results showed that N<sub>2</sub> emissions directly measured using the <sup>15</sup>NGF method amounted to 30 ± 4 kg N ha<sup>-1</sup>, which was equivalent to 18 ± 3 % of the applied fertilizer N. These measurements agreed with unrecovered fertilizer N obtained from the <sup>15</sup>N fertilizer mass balance, although the latter had large inherent uncertainty (21 ± 21 kg N ha<sup>-1</sup>). N<sub>2</sub>O emissions, however, were negligible (0.14 ± 0.02 kg N ha<sup>-1</sup>). The temporal variability of measured N<sub>2</sub> emissions after fertilizer additions was generally well explained by <sup>15</sup>N<sub>2</sub> enrichment in soil gas.</p><p>Overall, we provide independent validation of the <sup>15</sup>NGF method in measuring N<sub>2</sub> 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 N<sub>2</sub> emissions. These results should encourage a wider application of the <sup>15</sup>NGF method in order to improve our understanding of N<sub>2</sub> emissions and reduce the current uncertainties in estimates of these emissions.</p>