Agriculture Reference
In-Depth Information
Chemical limitations to decomposition slowly disappear and the end products of this
process (
i.e.‚
humic colloids‚ nutrient elements and organic acids) are eventually
released. Nutrients released into the superficial holorganic layers may be absorbed by the
large biomass of fine roots and the hyphae of mycorrhizal fungi. Such roots are relatively
more abundant in infertile soils with a limited ability to retain and accumulate nutrients.
In such systems of decomposition‚ soil minerals are not mixed with the decomposing
material‚ and thus aggressive organic acids are not neutralised in the early stages of
their formation. As a result‚ they infiltrate into the mineral horizons where they have
important roles in weathering minerals and promoting the translocation of iron and
aluminium into the underlying horizons (see Chapter II‚ Section 3.1).
2.3
Structure of litter systems
Litter systems have a clearly layered vertical structure because of the burial of old litter
by that more recently fallen. Lateral variation in litter systems may result from the dis-
tributions of structures such as bark and fruits that fall close to the plants that produce
them‚ the presence of particular tree species that produce litters of contrasting qualities‚
and from topographic heterogeneity. A number of important litter-system properties are
directly derived from the vertical and horizontal heterogeneity present.
Seasonal and successional cycles in climate and vegetation also affect litter systems and
lead to changes in the intensities and types of processes operating at different
temporal scales. The major of these processes are the seasonal changes that occur in
litter inputs and those influencing the long term evolution of litter morphological‚
physical and chemical characteristics across the different humus types present at a given site.
2.3.1
VERTICAL STRUCTURE
Macro-scale features
As discussed above‚ and in Chapter II. 1 in the context of horizon designation‚ litter
systems may comprise up to three distinct layers of variable thickness. These three
layers were first designated L‚ F and H by Hesselmann (1926); all were subsequently
further subdivided into two sublayers by Babel (1971) (Figure IV.15).
The L layer comprises unfragmented leaves that have fallen within the last year;
it may be further divided into an Ln layer with only a slight level of microbial attack
and an Lv layer thoroughly colonised by fungi. The thickness of this layer depends
on the amounts and seasonal patterns of litter fall.
The F layer includes fragmented leaves intensively colonised by fungi together with
the accumulated faecal pellets of invertebrate decomposers. F layers are often sub-divided
into two layers. The Fr (or F1) layer is composed mainly of large fragments of leaves
and a few faecal pellets of the invertebrates (
e.g.‚
large millipedes‚ Diptera larvae and
epigeic earthworms) that feed on freshly fallen leaves. The second layer‚ the Fm (or F2)‚
consists mainly of the smaller faecal pellets of Diptera and micro-arthropods mixed with
fine leaf fragments. In these litter systems‚ white-rot fungi generally develop most
conspicuously in the Fm layer (Gourbière‚ 1982).
