Agriculture Reference
In-Depth Information
the litter layers and with increasing depth in the mineral soil, probably due to the
cumulative effects of a number of small fractionations (Nadelhoffer and Fry, 1994).
Addition of nitrogenous fertilisers may lead to substantial changes in the
values of
the organic matter breakdown products.
Sulphur isotopes fractionate very little between trophic levels and are therefore
efficient tracers (Peterson and Fry, 1987). measurements have recently been used
to plot the change in plant sulphur source that occurs during the growth of the North
American desert xerophyte
Tamarix aphylla
(Yang
et al.,
1996). values of the rings
of older trees were shown to change systematically from the centre of the stem (
ca.
13‰)
to the exterior (
ca.
20‰). Yang
et al.
(1996) used a three stage model and a knowledge
of the values of the groundwater (17-20‰) and the atmospherically-derived sulphur
of the near-surface unsaturated zone (
ca.
7‰) to explain the distribution of sulphur
stable isotopes. During the first stage of early growth, values of
ca.
13‰ indicated
that plant sulphur originated from atmospherically-derived sources in the upper soil
horizons but, with ongoing root development (stage 2), these values progressively rise
reflecting the increasing utilisation of phreatic water sources. In the third stage, the
tissues have stable
values of
ca.
17‰, similar to those of the dominant phreatic
sulphur source.
3.2.4.3
Spatial distribution patterns
Vertical distribution
Soil organic matter concentrations and overall stocks generally decline from the surface
to deeper horizons (see, for example, Tables I.19, I.30, Figure I.36) reflecting the local-
isation of inputs,
i.e.,
litter deposited at the surface and roots largely concentrated in
the upper horizons. Further, because of their deeper root systems, higher organic matter
concentrations may occur at depth in forest than in grassland soils.
This is illustrated in Table I.19 which presents the distributions of C and N in the upper
120 cm of the profile of the undisturbed rainforested oxisol described in Figure I.33c.
Both C and N masses decline with depth and more than 57 % and 65 %, respectively, of
the masses of these elements occur in the top 30 cm of the soil. As shown, substantial
masses of carbon still occur at depth in this and in similar soils elsewhere in the region.
At this site, from 4.7 to more than
of C were present at depths of 150-180 cm
in the profiles.
Some grasslands may have carbon stocks equivalent to those illustrated above. Fisher
et al.
(1994) reported the large carbon mass of in the top 100 cm of a soil
supporting improved pastures in humid tropical Colombia; more than 40 % of this
amount was present below a depth of 40 cm.
As considered in Section 1.1.3.2, the quality of the organic matter resource changes
notably with increasing depth. Only the most recalcitrant materials remain in the subsoils.
Horizontal distribution
Soil organic matter concentrations vary greatly in the horizontal dimension. This is
a consequence of small to medium-scale variation in the quantity and quality of inputs
and of such environmental conditions as moisture, temperature and soil depth.
