Agriculture Reference
In-Depth Information
special attention being paid to the transfer of organic matter into the mineral soil profile‚
and the relative importance of climatic‚ edaphic‚ resource-quality‚ microbial and zoo-
logical determinants.
These ideas have been developed and refined to include a wide range of environments
and litter types and to define the processes involved in organic matter transformations.
Bal (1982)‚ for example‚ defined 'humon' as the continuum of organic materials typical
of the different stages of decomposition of organic residues. This concept emphasises
the point that the decomposition of leaves is a continuous process that starts before
abscission‚ continues at the soil surface where litter is deposited and may be completed
in different microsites or soil layers when transfers occur (for example‚ the passive transfer
of decomposing leaves when seasonal litter fall adds new strata‚ Figure I.42). Since these
pioneering studies‚ several classification systems have been proposed (see
e.g.‚
Delecour‚ 1980; Green
et al.‚
1993; Brethes
et al.‚
1995).
As stated by Satchell (1974)‚ 'The concept of the humus type as a biological system
is implicit in Müller's descriptions of mull and mor' and the concept of humus forms
may be used to describe litter systems. However‚ unlike humus forms‚ the litter system
as defined in this chapter comprises only the holorganic layers - the O horizon (Chapter
II.1.1) - that overlie the predominantly mineral soil horizons. Whenever litter is exported
to different sites and controlled by other biological systems of regulation‚ it is no longer
considered to form part of the litter system. This is especially true of the litter and faecal
pellets that initially accumulate in the holorganic layers but are later transferred to other
systems within the mineral horizons by anecic and endogeic earthworms (drilospheres)
or by termites (termitospheres).
2.2.2
MULLS
In mull litter systems‚ decomposing leaves and other litter materials do not accumulate
at the soil surface‚ either because they are completely decomposed in less than one year‚
or because they are exported to different systems of decomposition‚ such as the drilo-
spheres or the termitosphere. In mull systems‚ phenol-protein complexes are efficiently
broken down and organic acids are neutralised through the intense mixing of organic
compounds and clay minerals. In consequence‚ the holorganic layer usually only
consists of an L layer with an abrupt transition to the A1 mineral horizon. In most cases‚
decomposition is initiated in the litter system and is completed following well defined
pathways within the drilo- or termitospheres (see Chapter IV‚ Sections 4 and 5).
Mulls generally develop in sites where climatic conditions favour biological activity
for at least a significant part of the year and edaphic conditions must also be suitable for
anecic earthworms or termites. Typical soil properties include a pH close to neutrality
and a frequently loamy texture which facilitates the ingestion of small soil particles by
decomposers. Under such conditions‚ the litter is generally of high quality‚ easily
decomposed and palatable to large decomposers.
In temperate-climate areas‚ mull litter systems are largely created through the activities
of anecic earthworms. Litter is transferred into drilosphere structures including the large
casts of the anecic earthworms comprising the macro-aggregates of the A1 layer and
the walls of burrows and chambers. In some situations‚ edaphic conditions allow a rapid
