Agriculture Reference
In-Depth Information
3.1
Introduction
Composting depends intimately on the use of plant materials, i.e., lignocellulosic
biomass. The goal of this chapter is to review the results of recent research that
help to clarify the various roles of lignocellulosic biomass when mixtures of bio-
degradable materials are composted. Four key roles will be emphasized. First,
lignocellulosic materials typically make up a large proportion of the starting mix-
ture in a compost pile. Second, the cellulose and hemicellulose (polysaccharide)
components of biomass can be seen as a main source of energy that fuels the
composting process. Third, the lignin component of the biomass can be regarded
as a main source of what later becomes chemically transformed into humus—the
component of mature compost that holds onto moisture and minerals, enhancing
the quality of soils. Fourth, the bulky nature of lignocellulosic materials provides
bulking and access to aeration not only during composting, but also after the com-
post has been applied to soil.
Building upon an extensive review article published three years earlier (Hubbe
et al.
2010
), the present article is mainly concerned with research articles published
more recently. Indeed, based on the pace and quality of such publications, it ap-
pears that research related to composting has been accelerating. Also, some other
review articles have appeared that deal with different aspects of the topic (Singh
et al.
2010
; Kumar
2011
; Maeda et al.
2011
; Wichuk et al.
2011
; Yeoh et al.
2011
;
Franke-Whittle and Islam
2013
).
A few definitions will be provided here on behalf of readers who may not have
studied the composition of plant materials. The term “lignocellulose” denotes mate-
rial derived from the photosynthetic growth of plants, leading to the buildup of cell
walls. The cell walls of plants are comprised mainly of two kinds of chemicals—
lignin, which is a phenolic polymer material, and the polysaccharides cellulose and
hemicelluloses, which are polymers made up of sugar units. Cellulose, which has
a tendency to form somewhat crystalline, highly fibrous assemblies, is one of the
main polysaccharides in all lignocellulosic materials. Hemicellulose does not read-
ily form crystalline domains. Rather, side-groups in its linear structure inhibit crys-
tallization and allow it to act as a more compliant component in the structure of
plants. Some of the hemicellulose is covalently bound to lignin (Tunc et al.
2010
),
and therefore it helps to achieve a better integration of the inherently hydrophilic
cellulose with the much less hydrophilic lignin component. Lignocellulosic materi-
als include not only wood, which is a prime example, but also grasses, sugarcane
residues, and various other byproducts from agriculture.
Though the term “composting” already will have been defined in preceding sec-
tions of this monograph, a few points can be emphasized here. Composting can be
viewed as a process whereby a mixture of organic materials is subjected to enzy-
matic action leading to the heating up of the mixed material, some loss of dry mass,
changes in the chemical nature of the solids, and the ultimate stabilization of the
material so that it becomes more suitable as an additive for fertile soil. The enzymes
promoting the chemical changes are mainly secreted by naturally present bacteria
and fungi, though, as will be discussed, there has been recent research related to
