Increasing grassland productivity and reducing environmental N losses – Multiple benefits of advanced cattle slurry separation

Reducing the high nitrogen (N) losses during fertilization with cattle slurry is key to reduce environmental impacts of grassland farming. We tested the hitherto unknown potential of separated versus regular unseparated slurry (control) to mitigate total N losses in a three-year experiment using 15 N-labelled slurry. Slurry separation was enhanced using starch, clay minerals, and centrifugation, yielding an organic-rich solid fraction and a liquid fraction with low dry-matter content and ca 70 % ammonium-N. The use of separated slurry significantly increased plant productivity (+12 %), plant N uptake (+21 %), and total biomass harvest N export (+20 %) compared to the control. Additionally, fertilizer N retention in topsoil organic N (SON) increased by 8 %. Due to higher plant uptake, and higher soil storage of fertilizer N, total gaseous N losses from separated slurry were lower (33.5 % of added N) than from regular slurry (57.6 %). Leaching of fertilizer N remained negligible in both treatments. However, this did not apply for N 2 O emissions, which were of low relevance for N balance considerations, but tripled after the addition of the liquid phase of separated slurry in summer. This undesired effect however might be prevented if the solid phase is applied in summer and the liquid phase in spring when soil microbial activity is still low. In summary, separated slurry reduced N losses, increased productivity, fodder quality, and fertilizer N retention, thereby mitigating N deficits and soil N mining. Thus, with appropriate application timing, use of separated slurry can enhance both ecological and economic soil functions and ecosystem services. • We compare 2-yr full N balances for use of regular vs. separated cattle slurry. • The use of separated slurry increased grassland productivity and plant N uptake. • Use of separated slurry reduced total gaseous N losses but increased N 2 O emissions. • Increased N 2 O emissions are avoided by application of solid phase slurry in summer. • Application of separated slurry closed N balance gaps and prevented soil N mining.