Effects of liquid and dewatered sewage sludge applied to a Scots pine stand ( Pinus sylvestris L.) in Central Sweden

Abstract

Ecological effects of application of sewage sludge with 4 and 20% dw, respectively, were compared at Jädraås, Central Sweden, in a 50-year old pine forest ( Pinus sylvestris L.) on a sandy soil with low initial nutrient contents. Each type of sludge was applied in a dose of approximately 20 t dw per hectare, and in three replicates. The sludge application produced significantly increased pH, ammonium-N and nitrate-N concentrations as well as decreased C/N ratios in the mor layer. The effects on pH and C/N ratios were still significant after 11 years. The ammonium-N and nitrate-N concentrations increased to significantly higher values in the mineral soil 1 year after application of the liquid (4% dw) sewage sludge, than after application of the dewatered (20% dw) sludge. Dewatered sludge gave more pronounced long-term effects. Sewage sludge increased the concentrations of many nutrients in the soil, as well as in the pine needles ( Pinus sylvestris L.) of N, Ca and Mg, and in current shoots of Vaccinium vitis-idaea of P, K and Mg. No significant differences were observed between liquid and dewatered sludge except for P, with higher concentrations observed in the mor layer after application of liquid sludge. In spite of similar concentrations of Cd and Ni in the soil, a higher uptake in pine needles was found after application of 4% dw sewage sludge. Cu and Ni increased slightly in Vaccinium vitis-idaea with both types of sludge. Basal area growth and volume production increased strongly, but no significant differences between the two types of sludge were found, except a slight tendency for a somewhat faster response on the relative basal area increment after application of the liquid sludge with 4% dw. It was concluded from the ecological viewpoints that dewatered sludge with approximately 20% dw would be most beneficial.