Long-term effects of sewage sludge application in a conifer plantation on a sandy soil

In 1973-74, the Danish Land Development Service (Hedeselskabet) initiated an experiment with the surface application of 45 tds/ha of anaerobic digested sewage sludge to a sandy soil in a 75 year old spruce plantation. After Cu fertilisation in 1976, the application of sludge increased tree growth for approximately four and a half years to levels comparable to full forest fertilisation. Three and a half years after sludge application, approximately 65% of the added N remained in the sludge layer and 20% had been immobilised in the raw humus layer. Approximately 50% of the P content was removed from the sludge layer and was found almost entirely in the top soil. Heavy metals, with the exception of Zn and Pb, were found almost entirely in the sludge and raw humus layers. After one and a half to two years, leaching of NO3-N increased concentrations in the ground water to an average level of 10 mg NCyi, decreasing to initial levels after 7 years. There was no significant leaching of P to the ground water. Full-scale sludge application of 25 tds/ha on neighbouring areas showed similar trends. Before reforestation in 1988, the experimental areas were logged, leaving clearcut and shelterwood plots. Clearcutting caused an increase in NO3-N in the ground water samples taken from the sludge-applied plots. The concentrations are still increasing. The increase of NO3-N on shelterwood plots occurred approximately 6 months later and was less significant. With the exception of Zn, mobility of heavy metals has not been affected significantly. Investigations of the plots are to continue until 1993.