In accordance with earlier studies, wood ash increased nematode abundance (Hyvonen and Huhta 1989) in the autotrophic experiment (Nieminen 2009) but not in the heterotrophic experiment (Nieminen 2008b). Although nematodes were not identified, it is probable that their diversity was less in the simpler heterotrophic experiment, and this could partly explain their different response. Another major difference between the two experiments was that in the experiment without plants, soil moisture was kept optimal throughout the experiment, whereas in the pot experiment with plants, the organic soil dried considerably after grass harvest.

The increase of nematode numbers after loose wood ash application in field lysimeters with pine seedlings was mostly due to increased abundance of bacterial feeders, in other words, the ratio of bacterial feeders to fungal feeders increased (Liiri et al. 2002b). Liiri et al. (2007) found that loose wood ash reduced the abundance of fungal-feeding nematodes, but no effect on nematodes was evident in another microcosm study by Liiri et al. (2002c). The results are in broad accordance with an increased ratio of bacteria to fungi after ash application reported elsewhere (Baath et al. 1995; Perkiomaki and Fritze 2002), although Liiri et al. (2002b) did not detect ash effects on the ratio of bacteria to fungi. This illustrates that given sufficient response time, microbe-feeding nematodes can reliably indi­cate ash effects on soil microbes. It remains unclear why nematodes are not always responsive to wood ash. One possible reason for this is the initial community composition. For example, in Liiri et al. (2002c), the abundance of fungal-feeding nematodes was low and highly variable at the beginning of the experiment, which obviously made it difficult to observe changes in the ratio of bacterial feeders to fungal feeders.

4.2 Conclusion

In summary, laboratory experiments have shown that wood ash effects on dominant enchytraeids depend on labile carbon availability to the decomposer food web. The fact that increased carbon availability alleviated wood ash effects on enchytraeids without changing the pH supports a view that wood ash effects on soil animals are partly indirect consequences of altered food resources. In other words, food limita­tion magnifies negative wood ash effects. Although the effects of loose wood ash are well known, knowledge of the effects of granulated ash and ashes amended with organic materials on soil organisms in boreal coniferous forests is still incomplete.

Acknowledgements The study was financially supported by the Runar Backstram Foundation and the Maj and Tor Nessling Foundation. I appreciate the cooperation of the Jyvaskyla Soil Ecology Group, help with Table 4.1 from J. Haimi, V. Huhta, M. Liiri and H. Setala, and con­structive comments on the manuscript by two anonymous reviewers.