Heavy metal concentrations have to be considered when wood ash is recycled to forests; thus, the quality of the applied ash is of great concern to avoid accumulation of heavy metals in the environment (Stupak et al. 2008). In a microcosm experiment performed by Fritze et al. (2000), Cd derived from wood ash application on forest soils did not show any harmful effects on the microbial activity or fungal and bacterial community structure. In the same experiment, ash treatments were found to induce a shift in archaeal community patterns, whereas Cd alone or with ash did not have an influence (Yrjala et al. 2004). When looking at the heavy metal contents of forest berries (Rubus chamaemorus, Vaccinium vitis-idaea, Vaccinium uligino — sum) or mushrooms (Russula paludosa, Lactarius rufus, Lactarius trivialis, Suillus variegates, Paxillus involutus) on different Finish forest sites after wood ash fertilisation (4-14 t ha-1), Moilanen et al. (2006) observed no accumulation of heavy metals or even a decrease in the long term. Similar results were found in six Pinus radiata plantations repeatedly fertilised with 4.5 t wood ash per hectare in northwestern Spain, leading to a decrease of Zn, Cu and Cd levels in some mushroom species, which was attributed to an increase in soil pH. Only Mn concentrations were elevated in all mushroom species investigated (Amanita mus — caria, Russula sardonia, Tricholomapessundatum, Laccaria laccata, Micenapura, Suillus bovinus, Xerocomus badius). Heavy metals did not accumulate in tree needles or ground vegetation (Omil et al. 2007). Another aspect of recycling ashes back to the soil is the accumulation of 137Cs (Hedvall et al. 1996). However, application of wood ash at a level of 3.0 and 4.2 kg ha-1 contaminated with 30-4,800 Bq 137Cs per kilogram on different coniferous forest sites in Sweden did not significantly increase radioactivity in the biological system (soil, field vegetation, tree parts) when measured 5-8 years after ash amendment, which was partly attributed to the antagonistic effects of wood ash K on 137Cs (Hogbom and Nohrstedt 2001). This finding was confirmed in a 100-year-old Scots pine (Pinus sylvestris L.) stand on Fe podsol in central Finland, on which ash fertilisation (1,2.5 and 5 t ha-1) led to a reduction of 137Cs concentrations in lingonberries (Vaccinium vitis-idaea L.) analysed 2 and 7 years after application of ash (Levula et al. 2000).