Towards more sustainable organic grassland fertilization – a synthesis based on full N balances

Cattle slurry is widely used as organic fertilizer in temperate grasslands, but high nitrogen (N) losses during application cause emissions of greenhouse gases and air pollutants, water quality issues, and biodiversity loss. Furthermore, N balances frequently are negative, causing soil organic nitrogen (SON) mining, often accompanied by soil organic carbon volatilization and associated losses of agronomic and ecological soil functions. Low-emission slurry fertilization such as ground-level application has become obligatory in many countries, but such legal regulations as well as the individual decisions of farmers frequently are largely based on knowledge on ammonia losses only, but not on full N balances. Here, we provide a synthesis of data from experiments with 15N labelled organic fertilizers at 36 field plots, spanning a gradient of 1000 km from the Alps to Northern Germany. This approach allowed to assess effects of management intensity, climate change and different low emission application techniques on fertilizer N fates and full ecosystem N balances.

For intensive management with broadcast spreading of cattle slurry, on average almost half of the applied fertilizer N was lost to the atmosphere with a large contribution of dinitrogen emissions, while leaching of recent fertilizer N was negligible. Surprisingly, less than 10% of fertilizer N was taken up by plants, with the residual almost half of fertilizer N being stored in soil organic nitrogen. Nonetheless, grasslands were highly productive and largely met their N demand from mineralization of SON, which resulted in negative N balances and SON mining of on average 70 kg N ha^-1 year^-1 which increased with soil organic matter content, management intensity and experimentally induced climate change. Hence, a new paradigm for organic grassland fertilization is needed: the soil, not the plant is fertilized.

Both open slot slurry injection and traditional management with farmyard manure strongly reduced N losses compared to broadcast spreading of slurry, thereby leading to more closed N balances and counteracting N mining. However, slurry injection was more effective for acid soil rather than calcareous soil, where slurry acidification could be more promising to reduce N losses. Slurry dilution with water promoted infiltration, productivity and reduced N losses but avoided N mining only when N fertilizer amounts were maintained at the same level, which increases costs for farmers and the risk for soil compaction. In this context, slurry separation into liquid and solid phases is helpful.

In sum, we recommend either intensive grassland management with targeted low emission fertilization when productivity and fodder quality is prioritized, or extensive grassland management with grazing and fertilization with farmyard manure when soil organic matter formation and biodiversity is prioritized. Coexistence of these two diverging management approaches rather than applying medium management intensities is recommended to maximize both economical and ecological soil functions and ecosystem services at landscape scales.