Agriculture Reference
In-Depth Information
Important differences also exist between vegetation types. In tropical areas, forest
soils have greater organic reserves than savannas in the upper 15 cm, but may have
equivalent amounts in the soil profile down to 100 cm. In contrast, temperate climate
grasslands have significantly more organic matter than forest soils.
3.2.4.5
Conclusions
The low quality of the soil organic matter resource is partly due to its dilution in large
volumes of inorganic soil materials but it may be due to its chemical or physical protection
against decomposer digestion through humification and soil aggregation processes, or
through associations with particular minerals (Chapter IV.1). It is a regularly-renewed
resource, with inputs concentrated at the soil surface and in the root zone. Soil organic
matter concentrations decline rapidly with depth in most soils and commensurate changes
in quality also occur. It has a relatively homogeneous lateral distribution at the scale of
decimetres to metres and this contrasts with its discontinuous spatial distribution at
microscopic scales.
3.2.5
SOLUBLE RESOURCES
A range of water-soluble resources are produced in soils by roots (exudates
sensu lato
),
earthworms (cutaneous or intestinal mucus) and other invertebrates; soluble organic
matter may also originate from the leaching of foliage, bark and litter. These assimilable
materials are pulsed resources as defined by Price (1984) since their availability to the
soil biota is highly dispersed in time and space, and largely unpredictable. They are
characterised by large fluxes but low stocks and are known as “ecological mediators” for
their important role in activating the microbiota at selected scales of time and space
(Lavelle
et al
., 1994a) (see Chapter IV.1.4).
Estimates of the rates of production of this relatively-diverse suite of soluble and
colloidal resources are still poor since they degrade rapidly and techniques are not yet
available to measure their fluxes under field conditions. Production of root exudates is
probably in the order of several Mg in most ecosystems (Chapter III.3.2.3).
Certain invertebrates also produce such readily-assimilable organic resources as mucus.
Endogeic earthworms may add from 6 to 40 % of the dry weight of the ingested soil as
intestinal mucus, a mixture of a low molecular weight glycoprotein, sugars and amino-
acids. This mucus is released in the anterior part of the gut, and may be largely
reabsorbed in the mid and posterior gut, after part of it has been used by the ingested
soil micro-organisms to increase their activity. Although similar products have not been
identified and quantified in other soil invertebrates, it is likely that they exist.
The production rates of foliar and litter leachates have rarely been quantified.
Their occurrence in ecologically-meaningful amounts is clearly demonstrated by
the changes that occur in the redox potential (Eh) of the mineral soil underlying the
litter layers. These are related to the temporary accumulation of water-soluble organic
compounds, mainly sugars with high reducing potentials (Toutain, 1974) (Figure I.51).
In one of the very few measurements of the production of this resource, Carlisle
et al.
(1966) measured an annual flux of 350
of water soluble organic matter in a
English
Quercus petraea
forest.
