Agriculture Reference
In-Depth Information
Because of ecosystem transfer processes‚ carbon is normally most concentrated near
the soil surface and‚ at a smaller scale‚ in close proximity to the roots. Figure I.33
presents the depth distributions of carbon in six contrasting soils. That in Figure I.33a is
an alfisol formed from sandstone parent materials and supporting a savanna woodland
dominated by
Eucalyptus
species (Isbell and Smith‚ 1976). The low carbon concentra-
tions reflect the semi-arid environmental conditions and the constraints of a reduced
nutrient status resulting partly from the considerable ages of these landscapes. The data
in Figure I.33b are from a spodosol formed on a beach ridge in the humid (average
annual rainfall >3000 mm) tropics of north-eastern Australia (Murtha‚ 1986). It supports
a sclerophyllous vegetation (species of
Acacia‚ Casuarina
and
Melaleuca
) and illustrates
the bleached E horizon and illuvial increase in humic materials and sesquioxides
characteristic of the B horizons of these soils. Figure I.33c illustrates the distribution
of carbon in an oxisol supporting a tall‚ species-rich‚ closed-canopy rainforest in humid
(
ca.
3000 mm) tropical northeastern Queensland (Spain
et al
.‚ 1989a). The substantial
carbon stocks (234 Mg to 120 cm) of this permeable‚ strongly-structured soil are
related to the high secondary productivity of the rainforest (fine litterfall 8.6 Mg
3 year mean) and the iron and aluminium oxide-rich soils. The depth distribution of
carbon presented in Figure 1.33d is that of a very young andisol from the footslope of
