Cadmium in willow plantations - municipal waste products as fertilizers

Plantations of willows (i.e. Salix spp) seem to absorb cadmium (Cd) from arable land to a greater extent than other plant cultivations. Thus, energy forestry, based upon cultivation of Salix, could possibly be used for biofuel production in combination with biological purification of arable land contaminated with Cd by atmospheric deposition and P fertilizers during the passed 100 years. Application of sewage sludge (5-7 ppm Cd of dry solids) up to 550 g Cd per hectare, or 140 g Cd hectare and year, resulted in clearly increased concentrations of Cd in the upper 10 cm layer of the soil. In deeper soil layers the differences compared with control plots were small. Lower rates of sludge application, corresponding to less than 70 g added per hectare and year, did not materially affect the Cd-content in the soil irrespective of layer. A general pattern was that the content of Cd in plant tissues decreased with increased stem diameter, increased age of stems and stands as well as increased biomass growth. Thus, concentrations of Cd in Salix material, in clones from some of the first generation of energy forest plants (S. viminalis, S. dasyclados and S. schwerinii). were fairly high in young stands. In general, 1-year-old shoots contained 5-6 ppm Cd of dry solids in stems and 10-15 ppm in leaves. In older shoots, up to three years, and older stands, up to six years, the Cd contents decreased to 1-2 ppm Cd and 2-4 ppm Cd of dry solids for stems and leaves, respectively. Abundant supplies of water and nutrients seem to stimulate the Cd uptake in Salix in terms of higher concentrations in plant tissues as well as higher plant growth levels compared with conventional energy forestry. Stands with older stems, not irrigated and not fully fertilized resulted in uptake rates in the range of 10-30 g Cd per hectare and year. 14 refs, 9 figs, 1 tab