Agriculture Reference
In-Depth Information
3.4.1
Biodegradation Processes
Recent studies have continued to increase our understanding of what happens to
cellulose, hemicellulose, and lignin as a consequence of biological activity dur-
ing composting (Caricasole et al.
2011
; Zhao et al.
2011a
,
b
; Bikovens et al.
2012
;
Hachicha et al.
2012
; Luz Cayuela et al.
2012
; Wang et al.
2012a
; Bernabé et al.
2013
; He et al.
2013
; Iwai et al.
2013
; Paradelo et al.
2013
). In general, the cited
studies confirm the extensive loss of cellulose and hemicellulose, while confirm-
ing the increasing proportion of humus. He et al. (
2013
) found that in addition to a
build-up of fulvic- and humic-acid components in the maturing compost, there was
also production of water-extractable aromatic compounds. Such compounds might
be subject to leaching in some situations. Iwai et al. (
2013
) showed that some such
compounds can be extracted from compost by seawater. On the other hand, work by
Zhao et al. (
2011a
) revealed that the water-soluble fractions of compost tend to be
susceptible to biodegradation, at least in some cases. It follows that the water-sol-
uble components are likely to be chemically transformed or consumed by the time
the compost has been judged to be sufficiently matured, depending on the intended
usage. Bikovens et al. (
2012
) showed the presence of lignin-protein complexes in
compost derived from meat processing grease wastes.
3.4.2
The Recalcitrance of Lignin and Its Decomposition
The importance of lignin biodegradation in composting is highlighted by studies
showing increased rates of compost maturation following inoculation with fungal
species that produce ligninolytic enzymes (Hachicha et al.
2012
). Likewise, Feng
et al. (
2011
) demonstrated an increased rate of degradation of lignin and other ma-
terials in compost following direct treatment with ligninolytic enzymes. Also, as
shown by Zhao et al. (
2011a
), biomass that has higher lignin content typically is
more resistant to change during composting. Another important piece of evidence is
that the main changes to the lignin component within a compost pile generally occur
only towards the end of a composting process (Huang et al.
2010
; He et al.
2013
).
Nakhshiniev et al. (
2012
) found evidence that following hydrothermal treatment of
palm biomass, a layer of lignin tended to cover and protect some of the more easily
degradable components, thus slowing the rate of biodegradation of those materials.
Though Wang et al. (
2012a
) suggested a different mechanism, their results from
fluorescent labeling studies of composted mixtures are consistent with a mechanism
in which lignin blocks access of enzymes to some of the hemicellulose, thus slow-
ing its degradation. It is possible that naturally occurring covalent bonds between
lignin and hemicellulose (Tunc et al.
2010
) would tend to promote co-location of
lignin and hemicellulose within micro-zones within the decomposed biomass.
Humic acids, such as those produced during the course of composting, are noted
for their relatively high resistance to further biodegradation (Hubbe et al.
2010
).
