In June 1996, tree seedlings were planted at a distance of ca. 500 meters from the main stack of the Cu-Ni smelters. Seedlings of two native species, Betula pubescens Ehrh. (1-year-old containerized downy birch seedlings) and Pinus sylvestris L. (2-year-old containerized Scots pine seedlings), were each planted on six replicate plots (5 × 5 m2) as 49 seedlings per plot. Tree of the plots were to- tally covered with a 5 cm-thick layer of mulch, and the other three were left uncovered to serve as controls. In addition, six replicate plots (5 × 5 m2) without any trans- plants were established to serve as reference sites for soil characteristics. Three of them were covered with a 5 cm-thick layer of mulch, and the other three were left uncovered. The location of the experimental plots was randomized. One of the uncovered plots was unintentionally destroyed when slag was spread over it.
The pine and birch seedlings were planted in soil pockets (2 L, depth about 20 cm) containing mulch. Planting the seedlings in the mulch pockets penetrating down into the less contaminated soil was considered to be essential for their initial survival [17].
The mulch consisted of a mixture of household bio- compost and wood-chips (1:1, volume). The bio-compost was 14 months old and had been produced in outdoor windrows at the Ammassuo Waste Handling Centre, Espoo, Finland by mixing kitchen and garden waste from the Greater Helsinki area and coarse woodchips (diameter ca. 50 mm). The mulch was prepared one week before spreading by mixing the biocompost with woodchips (diameter < 20 mm) of Scots pine and Norway spruce (Piceaabies Karst.) stemwood [26]. The pH of the mulch was 6.3 and the carbon:nitrogen ratio 16:1 [26]. The av- erage Cu and Ni concentrations in the Ammassuo bio-compost were 60 and 3 mg·kg−1 as dry weight [26]. The mulch was spread directly on the layer of undercomposed litter with a plot wise dose of bio-compost (excluding the wood chips) of 5.4 kg·m−2 as dry weight. The input of C through mulching was 2 kg·m−2 [26].