Strong potential of slurry application timing and method to reduce N losses in a permanent grassland

Livestock manure is a major source of nitrogen (N) for plants, but also of ammonia (NH3) and nitrous oxide (N2O) emissions to the atmosphere, and nitrate (NO3−) leaching to groundwater. Slurry application practices targeted at reducing each of these N losses individually are relatively well known. Yet, our understanding of the potential relationships between N loss pathways is far from being complete. In this field study on grassland, we tested the effect of slurry application timing (autumn vs. winter vs. spring) and method (injection vs. broadcasting) and their interactions on N2O emissions, NH3 volatilization and NO3− leaching, as well as on plant N uptake. We found that autumn application increased NO3− leaching by 65 % compared with winter and spring due to 63 % higher rainfall following application. On average, autumn application reduced plant N uptake by 26 %. N2O emissions after winter slurry application were 43 % higher that after either spring or autumn applications. Slurry application method had no effect on NO3− leaching. Slurry injection led to 32 % higher N2O emissions compared with broadcast application. Slurry injection decreased NH3 volatilization only in autumn when the soil was relatively dry before application, but not in winter or spring when the soil was wet. Changes in slurry application timing led to variations in total N losses of up to 146 %, and application method of up to 19 %, highlighting the great potential of slurry application practices to steer an efficient N management. Overall, autumn application should be avoided to promote more sustainable grassland production; however, if slurry must be applied in autumn, injection should be recommended to reduce NH3 volatilization.