Nitrate leaching from lysimeter-grown short-rotation willow coppice in relation to N-application, irrigation and soil type

The use of willow vegetation filters for wastewater treatment is gaining interest in Sweden. A study was conducted in order to determine reasonable rates of wastewater application to such vegetation filters. NO3-N leaching loads and N-uptake by the plants were quantified in relation to nitrogen application rate, irrigation rate, shoot growth, and soil type during three years after planting of willow (Salix viminalis) in sixteen 1200-l lysimeters filled with clay or sand soil. Nitrogen was applied at rates corresponding to 110–244 kgN/hayr combined with irrigation of 3–6 mm/day during 100–110 days each growing season. NO3-N leaching loads were very high the first year after plant establishment (on average 341 kgN/ha from clay and 140 kgN/ha from sand lysimeters). However, NO3-N leaching loads decreased and were low or negligible during the second (43 from clay and 17 kgN/ha from sand lysimeters) and third year (3 kgN/ha from clay and less than 1 kgN/ha from sand lysimeters). Harvest of the plants after the second growing season did not seemingly affect NO3-N leaching loads during the third year. Soil type and N application rate strongly influenced NO3-N leaching loads, whereas irrigation rate had only a slight effect, and thus, dosing of wastewater should be based on N-amounts. Assuming an annual shoot growth of 10 tonnes (dry weight)/ha in a willow vegetation filter, wastewater could be applied at rates corresponding to 160–190 kgN/ha without substantial NO3-N leaching losses.