Biotechnology and Waste - Environmental Biotechnology: Theory and Application - page 185

Table8.3 Illustrativecarbonto
nitrogenratios
Material

C : N

Food wastes 15 : 1

Sewage sludge (digested) 16 : 1

Grass clippings 19 : 1

Cow manure 20 : 1

Horse manure 25 : 1

Leaves and foliage 60 : 1

Bark 120 : 1

Paper 170 : 1

Wood and sawdust 500 : 1

Most plants obtain the nitrogen they require as nitrate. The mineralisation of
nitrogen is performed by two genera of bacteria, Nitrosomonas , responsible for
converting ammonia to nitrite and Nitrobacter which completes the nitrification.
Inactivated at temperatures above 40 ◦ C and with a relatively slow growth rate,
their activity is largely confined to the maturation phase. Thus, the proper min-
eralisation of nitrogen can only be achieved after the growth and thermophilic
stages have been completed, which themselves require a suitable C : N balance
in the initial feedstock in the first place.
A number of different organisms are involved in composting, including bacte-
ria, fungi, protozoa, mites, nematodes, insects and annelids, there being a natural
succession of forms allied with the four phases of the process. Thus the initial
decomposition is brought about by mesophilic bacteria in the main, until their
increased activity raises the temperature into the range favoured by thermophilic
organisms. These thermophiles then play a major role in the breakdown of carbo-
hydrates and proteins, before they themselves become inhibited by the 70-75 ◦ C
heat of the composting pile. Later, as the temperature begins to drop, actino-
mycetes become the dominant group, giving the ageing compost a characteristic
white-grey appearance. Although largely confined to the surface layers, they are
of considerable importance in the decomposition of cellulose and lignin, which
are two of the more difficult components of biowaste to break down.
The microbial component of compost is an area of particular future potential,
particularly as a measure of product quality. While simple chemical analysis has
traditionally been used to assess composts, concentrating on NPK values and
placing it on the same footing as artificial fertiliser, there has been a growing
realisation that its complex nature means that this does not tell the full story. The
potential benefits in terms of soil flora improvement and plant pathogen suppres-
sion cannot be inferred from a compost's gross mineral contribution and so, in an
attempt to produce a more comprehensive yardstick, some producers and users
have begun to investigate assessment based on microbiological profiling. Pioneer-
ing work in the US, by BBC Laboratories of Arizona, has led to the development
of the first predictive tool for the value of a compost as a soil microbial inoculant,