Applying slurry with different techniques in spring – which pathway does the nitrogen take?

<p>One of the most important measures to reduce ammonia (NH<sub>3</sub>) and nitrous oxide (N<sub>2</sub>O) fluxes from crop production is the adoption of low-emission application techniques for slurry. Application techniques may also impact dinitrogen (N<sub>2</sub>) emission, as they can influence denitrification activity by changing slurry and soil aeration (e.g. by injection techniques), nitrate formation (e.g. by adding nitrification inhibitors) and the pH value (e.g. by slurry acidification). However, measuring N<sub>2</sub> emissions and following pathways of slurry nitrogen (N) transformation under field conditions is still challenging.</p><p>Thus, we set up a field experiment using undisturbed soil cores with growing winter wheat as small lysimeters. Cattle slurry treatments include the following application techniques: trailing hose with and without acidification (H<sub>2</sub>SO<sub>4</sub>), slot injection with and without nitrification inhibitor (DMPP). Soil cores without slurry application were used as control. In a first step, soil nitrate was <sup>15</sup>N labelled by homogeneous injection of a K<sup>15</sup>NO<sub>3</sub> solution (98 at% <sup>15</sup>N, equal to 4 kg N ha<sup>-1</sup>). One week later, we applied 72 kg N ha<sup>-1</sup><sup>15</sup>N-labelled slurry (NH<sub>4</sub><sup>+</sup> labelled at 65 at% <sup>15</sup>N). NH<sub>3</sub> emissions were measured by Dräger-Tube method (Pacholski, 2016). N<sub>2</sub>O and N<sub>2</sub> emission were measured using the <sup>15</sup>N gas flux method with N<sub>2</sub>-depleted atmosphere (Well et al., 2018). To close the N balance and follow the different N transformation pathways, <sup>15</sup>N losses by leaching, <sup>15</sup>N uptake by plant and residual <sup>15</sup>N in roots, plant residues, microbial biomass and soil were analysed by IRMS.</p><p>N<sub>2</sub>O emission were very low (up to 0.1 kg N<sub>2</sub>O-N ha<sup>-1</sup>) and not significantly different between treatments during the experimental period of 60 days. Since the N<sub>2</sub>O/(N<sub>2</sub>+N<sub>2</sub>O) ratio of denitrification (N<sub>2</sub>Oi) was also very low, most labelled N was lost via N<sub>2</sub> (up to 3 kg N ha<sup>-1</sup>). Nevertheless, the major gaseous loss pathway was NH<sub>3</sub> with up to 8 kg N ha<sup>-1</sup> in the trailing hose treatment. Slot injection significantly reduced NH<sub>3</sub> emission, while N leaching losses were up 5 kg N ha<sup>-1</sup>. Recovery of <sup>15</sup>N was higher in the soil N pool (32-48 %) than in plants (19-37 %) and roots (5-7 %). Total <sup>15</sup>N recovery was almost complete, indicating that the experiment was able to catch the relevant N pathways.</p><p><strong>References:</strong></p><p>Pacholski, A., 2016. Calibrated passive sampling-multi-plot field measurements of NH<sub>3</sub> emissions with a combination of dynamic tube method and passive samplers. Journal of visualized experiments: JoVE 109, e53273.</p><p>Well, R., Burkart, S., Giesemann, A., Grosz, B., Köster, J., Lewicka-Szczebak, D., 2018. Improvement of the <sup>15</sup>N gas flux method for in situ measurement of soil denitrification and its product stoichiometry. Rapid Communications in Mass Spectrometry 33, 437–448.</p>