Impact of pig slurry amendments on phosphorus, suspended sediment and metal losses in laboratory runoff boxes under simulated rainfall

Losses of phosphorus (P) when pig slurry applications to land are followed by a rainfall event or losses from soils with high P contents can contribute to eutrophication of receiving waters. The addition of amendments to pig slurry spread on high P Index soils may reduce P and suspended sediment (SS) losses. This hypothesis was tested at laboratory-scale using runoff boxes under simulated rainfall conditions. Intact grassed soil samples, 100 cm-long, 22.5 cm-wide and 5 cm-deep, were placed in runoff boxes and pig slurry or amended pig slurry was applied to the soil surface. The amendments examined were: (1) commercial grade liquid alum (8% Al(2)O(3)) applied at a rate of 0.88:1 [Al:total phosphorus (TP)] (2) commercial-grade liquid ferric chloride (38% FeCl(3)) applied at a rate of 0.89:1 [Fe:TP] and (3) commercial-grade liquid poly-aluminium chloride (PAC) (10% Al(2)O(3)) applied at a rate of 0.72:1 [Al:TP]. The grassed soil was then subjected to three rainfall events (10.3 ± 0.15 mm h(-1)) at time intervals of 48, 72, and 96 h following slurry application. Each sod received rainfall on 3 occasions. Results across three rainfall events showed that for the control treatment, the average flow weighted mean concentration (FWMC) of TP was 0.61 mg L(-1), of which 31% was particulate phosphorus (PP), and the average FWMC of SS was 38.1 mg L(-1). For the slurry treatment, there was an average FWMC of 2.2 mg TP L(-1), 47% of which was PP, and the average FWMC of SS was 71.5 mg L(-1). Ranked in order of effectiveness from best to worst, PAC reduced the average FWMC of TP to 0.64 mg L(-1) (42% PP), FeCl(3) reduced TP to 0.91 mg L(-1) (52% PP) and alum reduced TP to 1.08 mg L(-1) (56% PP). The amendments were in the same order when ranked for effectiveness at reducing SS: PAC (74%), FeCl(3) (66%) and alum (39%). Total phosphorus levels in runoff plots receiving amended slurry remained above those from soil only, indicating that, although incidental losses could be mitigated by chemical amendment, chronic losses from the high P index soil in the current study could not be reduced.