An Introduction to Environmental Chemistry

microclimate too; these factors in turn affect the potential for erosion and

dissolution.

4.6.4 Vegetation (v)

The development of vegetation is clearly influenced by parent material, relief and

climate. However, the development of vegetation also feeds back positively to

modulate these factors. For example, soils will tend to form where plants help

stabilize the substrate, preventing erosion by surface water or wind. At the same

time plant roots may help break up parent material, while evapotranspiration will

influence microclimate. Vegetation is the main contribution of organic matter to

soil and although most soils contain less than 5% organic matter (by weight), it

is an extremelyimportant component. The type of vegetation developed controls

the nature of the soil organic matter (SOM), comprising a complex mixture of

various biopolymers (Box 4.10) including cellulose and lignin (Table 4.6). SOM,

often called humus, is composed of the elements carbon, hydrogen, oxygen, phos-

phorus, nitrogen and sulphur bonded together to form huge macromolecules.

The presence of SOM influences soil structure and thereby its water-holding

capacity, keeping mineral surfaces and soilwater in close contact.

4.6.5 Influence of organisms (o)

Soil dwelling macro- and micro-fauna process soil organic matter by feeding on

it, deriving energy from the oxidation of the reduced substrate (Box 4.3). Phy-

tophagous organisms consume living plant material, while sacrophagous organ-

isms consume dead plant material. Electrons removed in oxidation processes pass

down an electron transport chain inside cells. Ultimately, the energy from these

The Chemistry of Continental Solids 99

Box 4.9 Mineral reaction kinetics and solution saturation

The discussion in Section 4.6.2 suggests that
weathering rates of minerals are proportional
to the water flow rate. This is only true if the
waters are close to saturation (Box 4.12) with
respect to the weathering mineral. If water
flow is continuous and sufficiently high, a
limit is reached beyond which further
flushing is no longer a rate-controlling
factor.
With insoluble minerals (solubility
< 10 -^4 mol l-^1 (Box 4.12)), including all silicates
and carbonates, ion detachment from
mineral surfaces is very slow, such that ions
never build up in solution close to the crystal

surface. The weathering rate of these

minerals thus depends mainly on the rate of

ion detachment from the crystal surface,

rather than the efficiency of flushing (water

flow rate). Only in very soluble minerals

(solubility> 2 ¥ 10 -^3 mol l-^1 ), for example

evaporite minerals, can ions detach rapidly

from the mineral surface to form a

microenvironment close to the crystal surface

which is saturated with respect to the

dissolving mineral. The rate of dissolution is

then controlled by the efficiency of dispersal

of these ions and water flushing effects then

become important.