Effects of pig and dairy slurry application on N and P leaching from crop rotations with spring cereals and forage leys

Two crop rotations dominated by spring cereals and grass/clover leys on a clay soil were studied over 2 years with respect to nitrogen (N) and phosphorus (P) leaching associated with pig or dairy slurry application in April, June and October. Leaching losses of total N (TN), total P (TP), nitrate-N and dissolved reactive P (DRP) were determined in separately tile-drained field plots (four replicates). Mean annual DRP leaching after October application of dairy slurry (17 kg P ha(-1)) to growing grass/clover was 0.37 kg ha(-1). It was significantly higher than after October application of pig slurry (13 kg ha(-1)) following spring cereals (0.16 kg ha(-1)) and than in the unfertilised control (0.07 kg P ha(-1)). The proportion of DRP in TP in drainage water from the grass/clover crop rotation (35 %) was higher than from the spring cereal rotation (25 %) and the control (14 %). The grass/clover rotation proved to be very robust with respect to N leaching, with mean TN leaching of 10.5 kg ha(-1) year(-1) compared with 19.2 kg ha(-1) year(-1) from the cereal crop rotation. Pig slurry application after cereals in October resulted in TN leaching of 25.7 kg ha(-1) compared with 7.0 kg ha(-1) year(-1) after application to grass/clover in October and 19.1 kg ha(-1) year(-1) after application to spring cereals in April. In conclusion, these results show that crop rotations dominated by forage leys need special attention with respect to DRP leaching and that slurry application should be avoided during wet conditions or combined with methods to increase adsorption of P to soil particles.