A summary of the effects of different ashes on forest soil animal groups detected in both field and laboratory experiments is given in Table 4.1. Loose wood ash was used in most experiments (Table 4.1). Because the dissolution rate of granulated wood ash is much lower than that of hardened or loose ash (Eriksson 1998; Nieminen et al. 2005), the ash effects on soil organisms also differ between ash types. The solubility of CaCO3 formed during stabilization is 2 orders of magnitude lower than that of CaO and Ca(OH)2, and, therefore, stabilized ash causes a much smaller pH shock in soil than loose ash (Steenari and Lindqvist 1997).

Macroarthropods react slowly to wood ash application (Huhta et al. 1986). Although very high wood ash doses (7 Mg ha—1) can cause negative effects on microarthropods (Koskenniemi and Huhta 1986), their populations and community structure tolerate moderate wood ash application (Liiri et al. 2002a, b). The numbers of some collembolans even increased after wood ash application in a field study (Vilkamaa and Huhta 1986). In contrast, 1-5 Mg loose wood ash per hectare, particularly if mixed with humus, has been shown to reduce the populations of the enchytraeid Cognettia sphagnetorum (Vejd.) both in many laboratory studies and in a field study (Huhta 1984; Haimi et al. 2000; Liiri et al. 2001, 2002c, 2007; Nieminen 2008a). A higher dose, however, was needed for a negative response in the field than in laboratory experiments (Haimi et al. 2000). In contrast to loose ash, hardened or granulated ash did not affect enchytraeid abundance, although a transient increase in the cadmium content of enchytraeid biomass was detected in one field study (Lundkvist 1998).

In contrast to its effects on enchytraeids and microarthropods, loose wood ash has been found to increase the total numbers of nematodes (Hyvonen and Huhta 1989; Liiri et al. 2002b; Nieminen 2009).

C. sphagnetorum is an omnivorous litter — and microbe-feeding oligochaetan worm dominating the mesofaunal community in boreal coniferous forests, and it is considered a keystone species in those ecosystems (Huhta et al. 1998). Because this species is adapted to acid soil (Baath et al. 1980; Standen 1982; Abrahamsen 1983; Huhta 1984), the negative wood ash effects have been explained by the increase of soil pH after wood ash application (Aronsson and Ekelund 2004). Changing pH can affect soil animals indirectly by changing the community com­position of microbes (Perkiomaki and Fritze 2002).

In addition to nutritional effects, wood ash can also modify enchytraeid popula­tions. When Nieminen and Haimi (2010) transferred enchytraeids exposed to wood ash in the forest to laboratory microcosms containing unamended Norway spruce forest humus, they found that enchytraeid populations originating from wood-ash — treated soil propagated slower and mineralized less nitrogen (N) than populations from untreated control forest.

In summary, the most conspicuous wood ash effect on soil fauna is the reduction of enchytraeid populations after loose wood ash treatment, especially in small-scale laboratory experiments including small plants or no plants and, hence, very limited carbon flow to the soil. In contrast, up to 8 Mg granulated ash per hectare had no effect on enchytraeid abundance in the forest (Lundkvist 1998).