References

Interactions Between Microorganisms and Soil Micro- and Mesofauna 259
2.3
The Bacterial vs. Fungal Food Chain
The chemical composition of soil organic matter has significant effects on
long-termpatternsofdecompositionanddetritusfoodchainsinsoils.The
abundance of structural carbon compounds, such as cellulose and lignin,
and the C/N ratio of organic material are important determinants (Melillo
et al. 1982; Taylor et al. 1989; Verhoef and Brussaard 1990; Agren and Bosatta
1996). Pathways with a strong bacterial component mainly occur in moist
soils that are rich in nitrogen and contain readily decomposable substrate
(Fig. 4). These conditions promote a rapid turnover of carbon and a fast
nutrient cycling to plants and, therefore, bacterially based energy channels
are best described as “fast cycles”. In these bacterial food chains, the major
grazers of microbes are protozoa and nematodes (Moore et al. 1988; de
Ruiter et al. 2002).
The fungal food chain is favored by a high C/N ratio in the soil, especially
if recalcitrant materials predominate (Rosenbrock et al. 1995; Frankland
1998; Fig. 4). Fungi have higher substrate assimilation efficiency than
bacteria and are able to break down complex polyaromatic compounds
such as lignin and humic or phenolic acids. In contrast to bacteria, fungi
have a relatively long generation time and react slowly to consumption by
grazers. Fungal energy channels are therefore considered “slow cycle” systems
(Moore et al. 1988; de Ruiter et al. 2002). Major grazers in the fungal
food chain are fungus-feeding microarthropods, which form an abundant
and species-rich community (Beare et al. 1997; Scheu and Setälä 2002). In
Bacterial
DECOMPOSITION PATHWAYS
Fungal
• moisture, N-rich, labile detritus • high C:N, cellulose, lignin,
resistant detritus
• fast cycle • slow cylce
• major predators are protozoa • major predators are
and nematodes
microarthropods
resource controlled
BOTTOM UP
top predators important
TOP DOWN
Fig. 4. Litter decomposition pathways based on either bacteria or fungi and their characteristics