Mitigation of phosphorus leaching losses via subsurface drains from a cracking marine clay soil

Mean total P (TotP) leaching losses after conventional autumn ploughing and inverting the soil to a depth of 23 cm were 0.79 kg ha−1 year−1, with 87% occurring as particulate P (PP), and the corresponding mean total N leaching losses were 27 kg ha−1 year−1, with 91% occurring as nitrate. The coefficient of variation in TotP leaching both in spring before the experiment started (64%) and during the six-year experiment (60%) was higher than the coefficient of variation in P-soil status (20%), or drainage (25%), illustrating the importance of local-scale subsurface transport in this cracking clay. However, TotP and PP leaching losses were significantly (pr > F < 0.002) lower from plots with structural liming than from the other treatments grouped together. Different P fertilisation strategies (band-spreading/broadcasting of mineral P and applying a balanced amount/no P fertiliser) had no significant effect on P leaching losses. Nitrogen leaching was significantly (pr > F < 0.001) lower from unfertilised fallow than from other treatments and was not significantly lower after shallow autumn tillage than after conventionally ploughing, whereas PP losses tended to be higher. Infiltration measurements with tension infiltrometers revealed a high variation in saturated hydraulic conductivity within plots. In view of the generally high PP losses, efforts to combat eutrophication of the nearby Baltic Sea should concentrate on soil structure improvements, while extensive tillage and totally omitting P fertilisation of cracking soils with moderate soil P status appears to be inefficient mitigation options.