An assessment of the nitrate leaching risk for different buffer strip establishments

Nitrates have been shown to have detrimental effects on both the aquatic environment and on human health, and with the implementation of the European Water Framework Directive beckoning, assessments as to the most appropriate agricultural techniques to minimize the leaching of solutes into surrounding water courses need to be made. Riparian buffer strips are considered an effective means of attenuating concentrated nitrates in agricultural runoff. Whilst a large number of scientific investigations have focused on nitrate removal mechanisms, such as plant uptake and denitrification maximization in buffer zones, few have studied the effects that different soil management treatments have on drainage rates. The application of chloride tracer to soils under six different vegetative treatments from two differing soil textures produced breakthrough curves highlighting the dominant flow mechanisms for each management practice. Of the 18 breakthrough tracers determined, 9 exhibited a rapid macropore hydrology, 6 intermediate mesopore through-flow and three slow micropore flow. Solute breakthrough flow rate was significant between all vegetation and soil treatments (P< 0.05), indicating that both play an important role in the determination of drainage flow pathways and transport speed. Vegetation such as scrub, with a large root network, led to increased soil porosity, significantly increasing the saturated hydraulic conductivity and the maximum peak drainage rates of the soil. On the other hand, maize and grass treatments, with smaller and less dense root networks, exhibited lower soil porosity and hydraulic conductivity, increasing solute residence time within the soil matrix and hence a longer lag time to peak effluent concentration. Although many of the treatments on each soil texture were different, and so could not be directly compared, results suggested that optimal treatments were soil specific, but that grassed and maize buffer strips, on a soil with low saturated hydraulic conductivity, would provide the best compromise at mitigating solute drainage whilst maintaining management practicality.