Agriculture Reference
In-Depth Information
apparent between the grassy and the shrub savanna, substantially-increased surface soil
carbon concentration result from fire protection. As shown by changes in the stable
carbon isotope compositions (as values, see section I.3.2.4.2 below) of the sites
studied, the effects on organic matter are most apparent in the surface soils. Increased
values also occur at depth in the fire-protected soils although only in the surface soils
do these values approach those of the gallery forest.
In addition to large reductions in organic matter concentrations, substantial changes in
its quality follow clearing and cultivation, as shown in Sections I.1.1.3.2. and I.1.1.3.3.
Such changes appear to be largely confined to the light fraction (Skjemstad
et al.,
1997)
although fertiliser nitrogen applied to the cultivated soil may also be stabilised into
a number of heterocyclic forms (Gregorich
et al.,
1996).
3.2.4.2
Isotope natural abundance
The ratio of the two stable isotopes of carbon and in biological and other
materials may be expressed as values which relate this ratio to an international
standard. values vary considerably between plants depending whether they use
the C3 or the C4 photosynthetic pathways and, to a lesser extent on a number of other
environmental factors. C3 plants incorporate the carbon of atmospheric into C3
compounds and have values of -22 to -38‰ (mean
ca.
-27 ‰) while C4 plants
incorporate their carbon into C4 compounds and have values of -9 to -21‰
(mean
ca.
-13 ‰); CAM plants are intermediate with average values of
ca. -
17‰
(Tieszen, 1991).
The values of soil organic matter reflect the history of the organic materials that
have been input to the soil system, with only small changes. There is no evidence that
differences in isotopic composition affect the nutritive value of soil organic matter for
soil organisms and this property of soil organic matter is a useful label, especially when
rapid changes of vegetation or organic inputs of a contrasting nature have taken place
(Smith and Epstein, 1971; Cerri
et al.,
1985; Balesdent
et al.,
1987; Martin
et al
., 1990).
