References

Role of Microorganisms in Carbon Cycling in Soils 147
terial microbial community [low number and abundance of phospholipid
fatty acids (PLFA)], whereas smaller fractions tended to have a highly complex
bacterial community using the heterogeneous substrates available in
organo-mineral associations (Fig. 4). The amendment of farmyard manure
with narrow C/N changed the microbial community structure mainly in the
coarse sand fraction. The drop in fungal abundance here may be due to the
lower C/N ratio of their substrates, which favored bacterial growth (Eiland
et al. 2001). In contrast, typical Gram-negative bacteria PLFA biomarkers(monounsaturatedpluscyclopropylfattyacids)weredetectableinthe
farmyard manure (FYM)-amended soil (Fig. 4). Several studies of soil
PLFA have documented an increase in monounsaturated fatty acids with
enhancedavailabilityoforganicsubstratesandmanures(BossioandScow
1998; Peacock et al. 2001).
5
Microorganisms and Enzymes Involved in C Cycling
Decomposition of plant litter is a complex ecological process involving
interactions of many taxa, spanning much of the range of biotic diversity
(Sinsabaugh et al. 2002). Fungi as well as bacteria are key decomposers of
the large amounts of substrates deposited on top of and below the ground
each year. State-of-the-art reviews have recently been published on fungal
communitiesinvolvedindecomposition(KjøllerandStruwe2002),andon
microbial dynamics of litter decomposition (Sinsabaugh et al. 2002). Biotic
processes of decomposition were investigated at three levels of resolution:
at the molecular level, the topics of interest were plant fiber structure and
enzymological characteristics of degradation. At the organismal level, the
focus was on functional gene analyses, regulation of enzyme expression and
growth kinetics, whereas at the community level, research concentrated on
metabolism, microbial successions and competition between microbial and
faunal communities. These three levels must be integrated to fully understand
microbial litter decomposition. Sinsabaugh et al. (2002) explained
the complex decomposition process as a successional loop in which the
substrate selects the microbial community, which then produces extracellular
enzymes that degrade and modify the substrate, which in turn drives
community succession (Fig. 5; Sinsabaugh et al., 2002 modified).
The close link between litter decomposition, litter quality and enzyme
activities is underlined in studies using different types of litter decomposed
in the same environment (Sinsabaugh and Linkins 1988; Luxhøi et
al. 2002). These and other studies showed the relationship between changes
in plant litter quality during and the activity of enzymes involved in carbon
cycling. The rapid mass loss within the first 10–18 days is due to leaching