Abstract

Fencing and re-vegetating the banks of agricultural drains is a widely used practice thought to improve drainage water quality. Monitoring of small nested headwater catchments draining sections with fencing and vegetation (Veg), and without fencing (UF) showed that after scraping the base of the drain in the Veg catchment, the phosphorus (P) and sediment concentration was lower than the UF section. When the site was revisited 9 years later this study showed the Veg drain lost a third of the sediment load of the UF drain but the Total Phosphorus (TP) per unit area load from the Veg drain was approximately 3 times higher than the UF drain. The Veg drain also had a higher TP and Filterable Reactive Phosphorus (FRP) concentration than the UF drain. The impact on nitrogen was variable but both the Total Nitrogen (TN) and nitrate (NO₃) concentration were higher from the Veg drain than from the UF drain. This suggests that the absence of fencing and the presence of livestock in the UF section allowed streamflow to mobilise and importantly, expose sediment, which adsorbed soluble forms of P that was retained in the drain. The vegetation within the fenced area appeared to have little impact on P or sediment loss. Other techniques that favour chemical retention by P sorption rather than physical filtration would need to be employed to reduce P loads in these sandy catchments where soluble P forms dominate.

The P concentration in the Veg section dropped relative to the UF section after later treatment with a 30 mm layer of cracked lateritic gravel but it was less effective than the earlier excavation of the bed of the Veg section of the drain, exposing the clay subsoil. The increase in P retention from the gravel is likely to be short lived because the P sorption capacity of the gravel was exhausted in 12 months.

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