Soil Phosphorus Storage In a Poplar + Pasture Silvopastoral System Fertilized with Sewage Sludge In Northwestern Spain.

Sewage sludge is commonly applied as a fertilizer source in forage production in southern Europe, but the long-term effects of the practice on soil quality is seldom studied.  The present study evaluated the effect of three sewage sludge application rates  on phosphorus (P) saturation ratio (PSR) and the soil P storage capacity (SPSC) in a silvopastoral system in Lugo, northwestern Spain. The system was established in autumn 2001 on an abandoned agricultural land; seeds of  a mixture of Dactylis glomerata var. Artabro, 25 kg ha^-1, and Trifolium repens var Huia, 4 kg ha^-1, were sown and Populus x euramericana saplings were planted at a density of 400 trees ha^-1.   Sewage sludge was applied in spring of 2002 and 2003 at doses based on nitrogen (N) concentration at the rate of 0, 160, and 320 kg ha^-1 yr^-1 of total N; the quantities of P applied amounted to  206.6 and 149.0 kg ha^-1 yr^-1 for 2002 and 2003. The experiment was set up in a split plot design with  two rates of liming (0 and 2.5 Mg ha^-1 of CaCO_3, applied in autumn 2001 before  sowing  pasture seeds) as main plots, and sewage sludge application rates as subplots.  Soil sampling to 25 cm depth and analyses were performed in March 2003, January 2004, and January 2005.  Data showed increases in SPSC from 2003 to 2004, but a reduction in SPSC in 2005. Higher sewage sludge application rate caused a reduction in SPSC over time compared to the control and the lower application rate. The soil under high sewage sludge application over the three-year period has more capacity remaining to store P.  The study emphasizes the importance of continued monitoring of long-term effects of sewage sludge application.