Agriculture Reference
In-Depth Information
3.2.2.2
Factors Affecting Microbial Populations
The passage of time, i.e., the “phase” of composting, is well known to play a domi-
nant role with respect to what kind of organisms are dominant in a given sample
of organic matter subjected to composting (de Gannes et al.
2013
; Karadag et al.
2013
; Li et al.
2013
). De Gannes et al. (
2013
) found that the dominant populations
of microbes tended to become more diverse as composting progressed through the
mesophilic and mature phases. In addition, there was no evidence found to support a
hypothesis that certain microbes entered a dormant phase, facilitating their return to
prominence at a later stage of composting. Karadag et al. (
2013
) also concluded that
the phase of composting had the most dominant influence on population levels. One
may hypothesize that such a succession of populations facilitates a relatively rapid
initial decomposition of the more readily biodegradable materials, such as hemicel-
lulose and cellulose, leaving the more recalcitrant lignin moieties to be degraded
during later phases of composting, when other micro-organisms are dominant.
When seeking to answer the question, “Why should time seem to play such a
dominant role relative to microbial populations?” one of the most frequent answers
has been “temperature”. Many studies have shown that during composting the tem-
perature first rises over a few days from ambient conditions to about 50 to 65 °C
(thermophilic phase), then more gradually falls back towards ambient temperature
as the compost approaches maturity (Hubbe et al.
2010
; Caricasole et al.
2011
; Hu
et al.
2011
). In other studies the expected “rise and more gradual fall” temperature
trend has been shown to be highly punctuated by temporary severe drops in tem-
perature associated with turning of compost piles (Wang et al.
2011
; Luz Cayuela
et al.
2012
). The following authors have recently reported evidence that microbial
populations are strongly affected by the prevailing temperatures within a compost-
ing mixture (Xiao et al.
2011
; Lü et al.
2012
; Karadag et al.
2013
). Lü et al. (
2012
)
showed that by imposing a certain temperature during composting it was possible
to obtain different stable communities of biota. Other researchers, however, have
reported cases in which temperature differences did not seem to play a major role
in determining microbial populations (Adams and Umapathy
2011
). In summary, it
seems that the variables of time and temperature can never be completely separated
due to complex interactions between those two variables.
Though it would make logical sense to control the relative dominance of differ-
ent microbial groups by control of the pH during composting, there is little evidence
that such an approach has been tried. Lin et al. (
2011
) showed that such an approach
can be used effectively during anaerobic digestion of lignocellulosic materials. The
pH was observed to decline during the course of anaerobic processing of waste
materials. By contrast, under the aerated conditions of typical composting, the pH
has been found to rise to a weakly alkaline range as the compost matures (Hu et al.
2011
; Karadag et al.
2013
).
An example of pH changes during composting is given in Fig.
3.3
(Lü et al.
2012
). As shown within the re-plotted figure, the observed changes are consistent
with expected chemical changes that are typical of composting. The initial down-
ward trend (see first large arrow) is consistent with the saponification of acetate
