Agriculture Reference
In-Depth Information
estimated that the new microbial biomass synthesis uses 0.16-0.26 M NH
3
as N
source for cell protein per 1 M glucose (C
6
H
12
O
6
), which results in a C/N ratio
range of 32-15 parts of C per each part of N. A similar calculation has been done
by Ryckeboer et al. (
2003
) assuming a microbial yield coefficient of 30 % and an
average microbial C/N content of 10, they suggested a theoretical optimum C/N
ratio of 30 for a composting starting mix substrate while other works (Larsen and
McCartney
2000
; Tuomela et al.
2000
) have confirmed the optimum C/N ratio in
the range between 25 and 35.
The C/N ratio of compost starting mix is the leading parameter when setting up
a new composting process. However, the C/N should not be used as absolute pa-
rameter as it is important to identify the nature of C in the composted materials. The
complexity of the C compounds affects the rate at which organic wastes are broken
down. The ease with which compounds degrade generally follows the order: car
bohydrates>hemicelluloses>cellulose=chitin>lignin. Naturally, the decomposition
rate of organic matter continues at very slow rate since there is a slow release of C.
In contrast, fruit and vegetable wastes are easily degraded as they contain mostly
simple carbohydrates (sugars and starches) while, leaves, stems, nutshells, bark and
trees decompose more slowly as they contain cellulose, hemicelluloses and lignin
(Epstein
1997
). Only the biodegradable fraction of a substrate, in terms of volatile
solids (VS), is available to decomposers and so it takes part to the process. The prac-
tical formula to calculate this fraction is: biodegradable fraction = 0.83 − 0.028 × lig-
nin content of VS (Chandler et al.
1980
); it means that 83 % of a substrate is the
maximum percentage of degradable fraction of VS. However, this is true in case of
lignin-free starting mix but there is no lignin-free plant residue. On the other hand,
lignin plays a very important role in composting process being a precursor of the
humic-like substances. The microbial degradation of the mass results in a precise
trend of the C/N ratio during the composting process while the ash content per unit
of dry mass under composting is a constant value for the whole process (Breiten-
beck and Schellinger
2004
), which means that the C/N decreases by the on-going
process. Microbial respiration releases CO
2
, which reduces the C content, but the
C/N will decrease only if the diminishing of C is greater than that of N, which
is possible only in case of negligible N rate in the leachate and of low ammonia
volatilization.
1.3.2
Microorganisms' Environment in Composting
As discussed earlier, the composting process includes numerous microorganisms
and it is the most important natural process of organic waste recycling. Compost
microorganisms include useful microorganisms that perform the composting pro-
cess and others that are potentially harmful for human, animal, plant and/or the
environment. One of the benefits of the composting process is the “inactivation”
of the harmful microorganisms and the development of beneficial microbial com-
munity. Thus, composters have to provide favorable conditions of oxygen, moisture
and proper C/N ratio for microbial growth (Fuchs
2010
).
