Full nitrogen balances for different cattle slurry fertilization techniques in a temperate grassland

Modern slurry application techniques have been shown to reduce ammonia losses, yet a comprehensive evaluation of their nitrogen (N)-related agronomic and ecological impacts is missing. Therefore, we utilized ¹⁵N-labeled cattle slurry to examine traditional and modern application techniques regarding their effects on hydrological and gaseous N losses, plant N uptake, soil organic nitrogen (SON) formation, and total fertilizer N balances. Following the broadcast spreading of slurry, 43 % of fertilizer N was lost as gaseous emissions, irrespective of precipitation. In contrast to broadcast spreading, significant total N emission savings were achieved by the broadcast application of diluted slurry combined with a reduced N supply (47 % emission reduction). Open slot injection at depths of 5 cm and 2 cm led to even greater emission reductions of 60 % and 74 %, respectively. Recent fertilizer was typically leached in minimal amounts only, yet the application of diluted slurry elevated nitrate leaching due to increased infiltration. Overall, the high productivity and plant N uptake were hardly affected by the application method, because over 90 % of the plants' N uptake relied on mineralized SON rather than recent fertilizer. This promoted soil N mining, particularly for broadcast spreading and slurry dilution, resulting in distinctly negative N balances (17 – 37 kg N ha⁻¹ deficit per fertilization-harvest cycle). Utilizing slurry injection contributed to additional SON formation, effectively offsetting the N deficit and thereby supporting the long-term maintenance of N-related soil functions.