Enhancing the P trapping of pasture filter strips: successes and pitfalls in the use of water supply residue and polyacrylamide

Abstract

Summary In intensive pastoral systems the landscape at ground level is clad in dense, filtering vegetation – yet phosphorus losses in overland flow do occur, and pollution of surface waters is a serious consequence. The use of pre‐applied polyacrylamide (PAM) or chitosan to trap particulate phosphorus (PP) and P‐sorbing potable water treatment alum residue (PWTR) to enhance vegetative filtering effects is examined here using field and laboratory overland flow simulation (flows from 0.43 to 0.34 litres s −1 (m width) −1 ) and analysis. Fitted equations suggest that up to 40% of dissolved reactive P applied (0.75 mg P litre −1 ) in overland flow could be captured in a flow length of 2.1 m (1 kg PWTR m −2 ). Unfortunately, drying decreased PWTR effectiveness, though little of the P captured was readily desorbed. This effect did not appear to be the result of gibbsite formation. Compared with the other treatments, there was a strong treatment effect of pre‐applied PAM on the change in PP losses ( P < 0.001) over time, though evidence suggests the PAM effect declined during a 44 minute flow period. We showed that the investigated two‐pronged approach to the enhancement of the effectiveness of P trapping by pasture had limitations. Laboratory sheet‐flow simulations suggest that a field‐stable P sorber with sorption characteristics similar to those of the un‐dried PWTR could be an effective retention enhancer for dissolved P. Pre‐applied PAM can have an effect on particulate‐P trapping but was rapidly dissolved and removed by flow.