The most dramatic effects of wood ash and lime on enchytraeids have been reported in laboratory microcosms containing only a small tree seedling as a primary producer (Liiri et al. 2002c, 2007) or no plants (Pokarzhevskii and Persson 1995). It has long been realized that ash effects on decomposers are stronger in laboratory experiments than in the field, but the reason for this has been unknown (Huhta et al. 1986). In a recent experiment by Nieminen (2008b) the negative effects of wood ash on enchytraeid size and abundance were offset by sucrose addition without any change in pH, supporting the hypothesis that wood ash effects can be alleviated by relaxing carbon limitation to microbes.

Huhta et al. (1983) used quite large pieces (40 cm x 60 cm) of Scots pine forest soil. Nematode populations started to decline in control microcosms 3 weeks after the start of the experiments, and later collembolan and enchytraeid populations declined as well. A mixture of birch ash and superphosphate reduced the populations of enchytraeids, mites and later also collembolans as much as lime. Nematode populations were initially stimulated by the treatments, but the effect turned nega­tive towards the end of the experiment. The experiment of Liiri et al. (2007) lasted for 1 year, and enchytraeids went close to extinction in untreated control pots as well as in ash-treated systems. In the experiment of Liiri et al. (2002c) enchytraeid biomass as well as the abundance of fungal-feeding nematodes declined even in unamended microcosms despite sufficient moisture, suggesting that the experimen­tal ecosystems were severely resource limited. In the same way, the enchytraeid population declined in control microcosms in the experiment of Pokarzhevskii and Persson (1995). Nieminen and Setala (2001) demonstrated carbon limitation in experimental microcosms: fungal-feeding nematodes propagated rapidly after cel­lulose addition to pine microcosms. Thus, carbon availability to microbes seems to be limiting decomposer activity in most laboratory microcosms, and consequently, resource limitation may have emphasized the effects of disturbances such as wood ash and lime observed in laboratory microcosms.

Carbon additions not only provide resources for microbes, they can also increase soil moisture (Szili-Kovacs et al. 2007). Using a simple carbohydrate such as sucrose as the only carbon source has the advantage that it is easier to distinguish nutritional effects from other effects. The heterotrophic microcosms reported in Nieminen (2008b) were maintained relatively moist and because no carbon effect on moisture was detected at the end of the experiment, the carbon effects were probably nutritional in that study. In contrast, the topsoil in the greenhouse experi­ment (Nieminen 2009) was much drier at the end of the experiment. Since quite a large amount of solid sucrose was used (Nieminen 2009), a mulching effect was initially possible. In addition, sucrose is hygroscopic, but other mechanisms are also possible. In any case, the increased moisture in the C1000 sucrose treatment was critical for the persistence of enchytraeids (Nieminen 2009). Since enchytraeids are sensitive to drought, and consequently likely to suffer locally from the increasing frequency of extreme conditions such as drought (Lindberg et al. 2002), it can be suggested that more attention should be paid to the interaction of soil carbon and moisture.

The hierarchical nature of the soil food web is also worth noting. Even though low molecular weight carbon compounds are taken up and utilized on timescales ranging from seconds (Hill et al. 2008) to days (van Hees et al. 2002), the effects of labile carbon additions at higher trophic positions were still evident after one growing season.