Phosphorous dynamics in poplar silvopastoral systems fertilised with sewage sludge

• Lime and sewage sludge affect P cycling in a different temporal scale.
• Long term studies are needed to evaluate organic fertilization effects.
• P leaching risk is diminished by the presence of the tree.

The use of sewage sludge as fertiliser is a common practice in Europe, where sewage sludge is usually applied on a plant-available N or total metal concentration basis; as a result, phosphorus (P) concentrations can be well above crop needs. The objective of the study was to evaluate the effect of three doses of sewage sludge (0, 200, and 400 kg total N ha−1 year−1) combined with lime (2.5 Mg CaCO3 ha−1) or without lime on the P in soil and pasture, tree growth and pasture production in a silvopastoral system established with Populus × canadensis Moench over seven consecutive years. The P availability varied with the soil pH (a result of lime and sewage sludge applications) throughout the study. In general, an increase of the soil acidity (pH 5.39) increased the amounts of Fe and Al in the soil solution, which form very strong bonds with P, thereby reducing the P availability (M3-P: 20.99 g kg−1). The effect of lime on the P availability gradually declined in the years following its application. However, in the latter part of the study (2007–2008), due to the higher residual effect of the sewage sludge compared with lime, the doses of sewage sludge (200 and 400 kg N total ha−1) increased the concentrations of P (200N: 37.31 g kg−1 and 400N: 48.71 g kg−1) and Fe (200N: 321.51 mg kg−1 and 400N: 341.56 mg kg−1) in a Mehlich 3 solution, the concentration of P in the pasture (200N: 2.53 g kg−1 and 400N: 2.81 g kg−1) and the amount of P extracted by the pasture (200N: 8.78 kg ha−1 and 400N: 10.34 kg ha−1), resulting in possible reduction in environmental problems. Further, due to its agronomic benefits as a source of plant nutrients and organic matter, fertilisation with high quality sewage sludge could be recommended for silvopastoral systems under edaphoclimatic conditions similar to those in this study.