Agriculture Reference
In-Depth Information
highly mature ecosystems whose continued function is dependent on the efficient
recycling of nutrients and the fixation of nitrogen, consistent with Odum's (1983)
hypothesis of the diminished 'leakiness' of mature ecosystems.
Stark (1978) presented a theoretical model of the biological consequences of
the evolution of soil properties over time. After an initial period where shallow
soils support only cryptogams, a vegetation develops that is largely dependent on
indirect nutrient cycling, that is the take up of inorganic nutrient elements directly from
the soil. As the soil becomes progressively impoverished in nutrient elements over time,
the biological community adapts to the diminished nutrient supply by developing
such nutrient conservation mechanisms as efficient mycorrhizal networks which cycle
nutrients directly from the biomass to the host plant. However, nutrient stocks continue
to diminish with continued weathering leading over time to a series of vegetation
communities dominated by, sequentially, small trees then shrubs then cryptogams.
Finally, a terminal exhausted state was considered to supervene wherein effectively
no nutrients are available.
Because much of the ecosystem stock of certain nutrients may be tied up in the
biomass or is associated with litter and surface-soil organic matter, these systems are
highly susceptible to degradation. Such disturbances as clearing, high-temperature
firing and severe erosion may deplete the biomass and soil organic matter stocks and
lead to further substantial reductions in the remaining nutrient stocks. This is consistent
with Odum's (1985) hypothesis that disturbance tends to return ecosystems to earlier
'leakier' developmental stages. Following disturbance of some very highly and deeply
weathered soils, the previous ecosystem trajectory may not necessarily ensue.
Successfulagricultural development of highly-weathered soils may therefore be difficult.
Clearing normally involves the destruction of most of the above ground biomass and
considerable disturbance to at least the surface soils. The above ground biomass is
usually disposed of by firing and this contributes directly to site nutrient losses through
volatilisation and leaching of the ash by subsequent rainfall (Stark, 1978). Because soil
organic matter may be the major source of negative charge in these soils, its depletion
following forest clearing can lead to greater proportional reductions in cation exchange
capacity and nutrient retention capacity than in less highly-weathered environments
(Allen, 1985). After clearing, loss of soil structure may ensue and continuing soil dis-
turbance through cultivation often leads to an even lower organic matter equilibrium
in the surface soils and an even more diminished capacity to retain nutrient elements.
Overstocking is of frequent occurrence in pastoral situations and may lead to a reduced
vegetation cover, structural degradation and accelerated erosion.
4.5
Toposequences
Topography is an important factor in soil formation at micro, local and greater scales and
erosional transport of materials downslope may substantially influence landscape
morphology and soil development processes (Millot, 1977). Soils in different parts of
the landscape do not operate independently from others around them; they contribute
water, solutes, suspended and larger materials to those downslope and are similarly
