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

One of the most important measures to reduce ammonia (NH₃) and nitrous oxide (N₂O) fluxes from crop production is the adoption of low-emission application techniques for slurry. Application techniques may also impact dinitrogen (N₂) 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 N₂ emissions and following pathways of slurry nitrogen (N) transformation under field conditions is still challenging.

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 (H₂SO₄), slot injection with and without nitrification inhibitor (DMPP). Soil cores without slurry application were used as control. In a first step, soil nitrate was ¹⁵N labelled by homogeneous injection of a K¹⁵NO₃ solution (98 at% ¹⁵N, equal to 4 kg N ha^-1). One week later, we applied 72 kg N ha^-115N-labelled slurry (NH₄⁺ labelled at 65 at% ¹⁵N). NH₃ emissions were measured by Dräger-Tube method (Pacholski, 2016). N₂O and N₂ emission were measured using the ¹⁵N gas flux method with N₂-depleted atmosphere (Well et al., 2018). To close the N balance and follow the different N transformation pathways, ¹⁵N losses by leaching, ¹⁵N uptake by plant and residual ¹⁵N in roots, plant residues, microbial biomass and soil were analysed by IRMS.

N₂O emission were very low (up to 0.1 kg N₂O-N ha^-1) and not significantly different between treatments during the experimental period of 60 days. Since the N₂O/(N₂+N₂O) ratio of denitrification (N₂Oi) was also very low, most labelled N was lost via N₂ (up to 3 kg N ha^-1). Nevertheless, the major gaseous loss pathway was NH₃ with up to 8 kg N ha^-1 in the trailing hose treatment. Slot injection significantly reduced NH₃ emission, while N leaching losses were up 5 kg N ha^-1. Recovery of ¹⁵N was higher in the soil N pool (32-48 %) than in plants (19-37 %) and roots (5-7 %). Total ¹⁵N recovery was almost complete, indicating that the experiment was able to catch the relevant N pathways.

References:

Pacholski, A., 2016. Calibrated passive sampling-multi-plot field measurements of NH₃ emissions with a combination of dynamic tube method and passive samplers. Journal of visualized experiments: JoVE 109, e53273.

Well, R., Burkart, S., Giesemann, A., Grosz, B., Köster, J., Lewicka-Szczebak, D., 2018. Improvement of the ¹⁵N gas flux method for in situ measurement of soil denitrification and its product stoichiometry. Rapid Communications in Mass Spectrometry 33, 437–448.