The hypothesis that carbon amendment increased the ability of C. sphagnetorum to resist wood ash disturbance because of enhanced microbial production is supported by the large increase in the abundance of microbial-feeding nematodes (Nieminen 2008b), which is in accordance with the results of earlier field experiments. Baath et al. (1995) found that wood ash application reduced the ratio of fungi to bacteria in pine forest soil. In a field study the ratio of fungal to bacterial phospholipid fatty acids was lower in forests treated with loose ash than in forests treated with hardened ash, although the difference from the control was not significant (Perkiomaki and Fritze 2002). Further, ash treatment was shown to alter the carbon utilization pattern of bacteria in a microcosm study, indicating changes in the bacterial community structure (Fritze et al. 2000). Although wood ash increases CO2 evolution rates (Perkiomaki and Fritze 2002), Jokinen et al. (2006) concluded that not all microbial groups are equally stimulated by wood ash. In other words, by increasing the soil pH, wood ash application can favour fast-growing microbial species (Zimmermann and Frey 2002), which are not necessarily preferred food sources for dominant animals in forest soil (e. g. bacteria over fungi). An increase in microbial activity can also result from the bacterial decomposition of microbial residues. In particular, because the dominant enchytraeid species C. sphagnetorum is a litter feeder dependent on fungal activity, it is plausible that a shift towards the dominance of r-strategist bacteria has a negative influence on it. In accordance with this reasoning, Nieminen (2009) observed a tendency towards a lower proportion of bacterial-feeding nematodes in the sucrose treatment. We do not know whether sucrose increased the numbers of the same microbes that were negatively affected by wood ash, or if it increased other possible food sources for enchytraeids. On the basis of these and the present results, it seems that loose wood ash affects enchytraeids to some extent through a change in microbial community structure, and that sucrose counteracts this effect.

A moderate wood ash disturbance in the autotrophic experiment (Nieminen 2009) had little direct effect on decomposer animals. Since enchytraeids only persisted in sucrose-amended pots, the interaction of wood ash and carbon avail­ability could not be tested. Although wood ash did not affect enchytraeid biomass, it did increase the size of individual worms, indicating that there were fewer but larger enchytraeids in the ash-treated microcosms. Because C. sphagnetorum reproduces asexually by fragmentation, a large individual size may be indicative of delayed reproduction. On the other hand, a higher wood ash application rate led to significantly smaller enchytraeid body size in the heterotrophic experiment (Nieminen 2008b), suggesting that when these organisms reproduce, they produce smaller offspring. This reasoning is supported by the data obtained by Nieminen and Haimi (2010), who found that enchytraeid populations originating from an ash — treated plot had a lower propagation rate (smaller fragmentation frequency) than populations from adjacent untreated control forest even when the animals were transferred to laboratory microcosms containing untreated organic soil. Also, the body size dynamics differed between populations with different disturbance his­tories. The overall length variation of the disturbed populations was smaller than that of the control populations, and, hence, the mean enchytraeid length could be either smaller or greater than that of control, depending on the sampling date (Nieminen and Haimi 2010).