Short-term effects of alkaline biosolids on pH and trace metals in oligotrophic forest peat and on growth of Picea sitchensis

A field experiment was conducted in a 13-year-old Sitka spruce (Picea sitch ensis (Bong.) Carr.) plantation on shallow oligotrophic peat to investigate short-term changes in soil pH and trace metal concentrations and in tree g rowth responses to applications of rural and urban alkaline biosolids. Ther e were five treatments: (1) control (no amendment); (2)-(4) rural biosolids at application rates of 20, 40 and 60 t fresh material (FM) ha(-1); and (5 ) urban biosolids applied at one rate (40 t ha(-1)) only. The biosolids wer e spread by hand on the ground surface of each plot on 12 March 1992. Feat and foliar samples were collected in February each year from 1993 to 1995. Annual tree height was also measured. The results show that alkaline biosol id-derived Cu, Zn, Ni, Cd, Pb and Cr accumulated in the litter layer. The m agnitude of surface soil metal enrichment was related to sludge type and ap plication rate. There was spatial and temporal variation in metal concentra tions, probably due to uneven distribution of the sludge biosolids and the uneven depth of the litter layer on the forest floor. Rural biosolids marke dly increased the dilute acetic acid extractable Zn while urban biosolids g reatly elevated extractable Cu in the litter layer, reflecting the differen t metal loadings in the two sources of sewage sludge. Alkaline biosolids ra ised the pH of the litter layer, and of the top 5 cm land occasionally the top 10 cm) of the underlying peat profile and elicited tree growth response s. Small amounts of heavy metals such as Cu and Zn may have leached down th e soil profile. Heavy metals that have accumulated in the litter layer may represent a potential pool for long-term leaching as litter decomposition a nd acidification proceed. Long-term monitoring would therefore be required to determine the full environmental impact following application of alkalin e biosolids to spruce forests on oligotrophic peat.