Effects of different slurry application techniques on Nitrogen Use Efficiency (NUE) in an extensive grassland

Nitrogen (N) fertilization is essential to increase grassland productivity, but losses of excess N to the environment are causing environmental and health issues such as nitrate contamination and eutrophication of water bodies, greenhouse gas emissions, and impaired soil and air quality.  Especially organic fertilization with cattle slurry is known for high environmental N losses. In that respect, different refined cattle slurry management strategies targeted to increase nitrogen use efficiency (NUE) are legally prescribed. However, a holistic assessment of the nitrogen-related agronomic and environmental impacts is still missing. This study aims to test different slurry application techniques for their effects on NUE, hydrological and gaseous N losses, productivity and fodder quality, soil organic nitrogen formation and total N balances. In a small-scale experiment ¹⁵N enriched slurry was applied on 1 m² grassland plots using the following application methods: (1) traditional slurry broadcast spreading under dry weather; (2) application like (1) followed by a heavy rainfall event to increase slurry infiltration into the soil; (3) broadcast spreading of slurry diluted with water; (4) injection of slurry into the soil via shallow slits; and (5) injection of slurry into the soil via deep slits. Variants (4) and (5) represent modern trailing shoe injections requiring extensive machinery. Fates of fertilizer N such as plant uptake, stabilization in soil through microbial immobilization as well as NO₃ leaching were investigated by ¹⁵N tracing approaches in order to create full fertilizer N balances. Sampling and harvest began 3 months after fertilization with ¹⁵N-labeled slurry and first results indicate that both injection treatments lead towards a markedly higher slurry-N retention in the soil compared to broadcast spreading, which was not achieved by slurry dilution and traditional application plus strong irrigation. Based on further isotopic analyses, full fertilizer N balances for the different cattle slurry application techniques will be provided.