No difference in nitrate leaching between digestates and untreated cattle slurry over eight years of organic arable farming

Organic fertilizers often contribute to nitrate (NO 3 − ) leaching and impair drinking water quality, because the mineralization of applied organic nitrogen (N) is often asynchronous with crop N uptake. Crop yield, crop N uptake, seepage water and NO 3 − leaching were investigated in a lysimeter facility (Zurich, Switzerland) under temperate climate conditions during two consecutive four-year organically managed crop rotations. The study compared untreated cattle slurry, anaerobically digested slurry, liquid digestate from a non-agricultural biogas plant and a zero-N fertilization control. The ammonium N (NH 4 -N) fraction of the organic fertilizers was labeled with 15 N tracer to investigate the fate of NH 4 -N in the plant-soil–water system. Organic fertilization significantly increased crop yields and N uptake, with no differences among organic fertilizers, despite the higher NH 4 -N proportion of the digestates. Mean annual NO 3 − leaching was low (12–18 kg N ha −1 ) and was not increased by organic fertilization, indicating that N inputs were balanced by crop N uptake. Digestates did not differ from untreated cattle slurry in NO 3 − leaching. Higher modeled ammonia (NH 3 ) emissions of digestates suggest that N losses may occur primarily via NH 3 volatilization rather than NO 3 − leaching. The 15 N recovery showed ~1% of the applied NH 4 -N was lost through NO 3 − leaching, whereas 23–31% was recovered in crops, and 18–23% remained in the topsoil. These results highlight the importance of crop N uptake and the role of the soil as a long-term reservoir for fertilizer-derived N, with direct NO 3 − leaching from the NH 4 -N fraction of organic fertilizers playing only a minor role.