Chemical Amendment of Dairy Cattle Slurry for Control of Phosphorus in Runoff From Grasslands.

It is estimated that agriculture accounts for 38% of all pollution in Ireland’s waterways. The EU Water Framework Directive (WFD; 2000/60/EC) recommends research and development of new pollution mitigation measures to achieve the 2015 target of surface and groundwaters of ‘good status’.

In intensive dairy farming systems, phosphorus (P) inputs can exceed P outputs. Over time, soil test P (STP) levels build up after repeated manure applications, and this can increase the risk of P loss to a waterbody. There is also the risk of incidental losses when landspreading slurry.

Amendments could potentially mitigate P losses in strategic areas, while allowing farmers to dispose of slurry cheaply, and utilise nitrogen (N) and other nutrients in the slurry.

The aim of this study was to identify amendments with the potential to reduce P and suspended sediment (SS) loss from agricultural grassland in Ireland arising from the land application of dairy cattle slurry.

First, an ‘agitator test’ was used to identify the optimal rates of amendment addition to slurry to reduce soluble P, and to estimate associated costs. Amendments examined included alum, poly aluminium chloride, ferric chloride, lime, flyash, flue gas desulphurisation by-product, and alum-based water treatment residuals. Intact grassland soil specimens were saturated for 24 hr. After saturation was complete, 500 ml of deionised water was added to the beaker. The agitator paddle was then lowered to the mid-depth of the beaker in the overlying water and rotated at 20 rpm for 24 hr. This rotational speed corresponded with a tangential velocity of 0.034 m/s at the perimeter of the paddle.

The most effective amendments (alum, poly aluminium chloride, ferric chloride, lime) were identified based percentage reduction in dissolved reactive P (DRP) and on cost effectiveness.

Next, flumes 200-cm-long by 22.5-cm-wide by 5-cm-deep with side walls 2.5 cm higher than the soil surface, containing intact soil from a dairy farm treated with dairy slurry and amended slurry were subjected to simulated rainfall with an intensity of 11.5 mm hr^-1. All experiments were conduced in triplicate. All runoff samples were tested for SS, DRP, total dissolved phosphorus (TDP) and total phosphorus (TP).

Results showed that alum had the best potential to be a cost-effective amendment at low application rates (1.11 Al: TP). Compared to a slurry-only treatment (the study control), alum was best at reducing DRP in surface runoff. It reduced mean flow-weighted concentration (MFWC) of DRP by an average of 83% over 3 successive rainfall events, compared to 69% for lime, 86% for poly-aluminium chloride, and 67% for ferric chloride. Alum reduced the MFWC of TP loss by an average of 93% and particulate P (PP) by over 95%, compared to the study control.

Future work will examine these treatments at field-scale, and will examine the effect of amendments on metal availability, and on gaseous emissions form slurry.