Agriculture Reference
In-Depth Information
in a later section (Section I.2.2.2.2), the relative humidity of the soil air remains close to
saturation until very low soil moisture potentials are attained. Carbon dioxide levels are
also higher than those above the surface and a range of other gases may be present in
substantial concentration, depending on physical conditions in the soil. The composition
of the soil atmosphere is critical to the growth of plants and to the activities of the aerobic
micro-organisms associated with them. Oxygen must be able to flow readily to both
the roots and their microbial symbionts (notably the mycorrhizal fungi) to fulfil their
respiratory requirements and carbon dioxide must also be able to diffuse away. A wide
range of pedogenetically-important soil processes are also controlled by the nature of
the soil atmosphere, including the general nature of decomposition processes, the
activities of the aerobic members of the soil community and the direction of reduction:
oxidation processes.
Gas movement within soils results largely from diffusion. However, barometric
pressure changes, wind gusts over the surface, temperature gradients, the penetration
of water during infiltration (which displaces the soil air ahead of the wetting front),
fluctuations of shallow water tables and the extraction of water by plant roots may
all induce significant mass flows into and out of surface soils (Patwardhan
et al.,
1988).
The diffusion rates of gases in soils depend on their diffusion coefficients in air,
the concentration gradients present, the pore size distributions (particularly the propor-
tion of air-filled pores), and the geometry of the pore system through which the gases
move. Gas diffusion rates through soil water are too low to be of much biological
consequence in terms of respiration. Nonetheless, the concentrations of dissolved
gases certainly influence both biological and other pedogenetic processes.
Structures created by soil invertebrates, especially the large ecosystem engineers
(notably earthworms, termites, ants and beetles, see II.2.4 and Chapter IV) may
significantly influence gas diffusion. Kretzschmar and Monestiez (1992), for
example, have demonstrated that earthworm burrows significantly increase gas diffusion
rates from soils, particularly at high water potentials
where only a small
proportion of the pore space is filled with air.
1.2.1
THE COMPOSITION OF THE SOIL ATMOSPHERE
The concentrations of the major gases in the soil atmosphere are contrasted with those
of the general atmosphere in Table I.9. In the atmosphere, water vapour may constitute
<0.01 % to 3 % by volume of dry air, in addition to the gases. Concentrations in the soil
are influenced by a range of soil physical, chemical and biological properties and by
cultural practices. The most important properties include moisture status, texture,
structure, soil organic matter concentrations and, at a smaller scale, density of and
proximity to roots. Such cultural practices as the addition of organic materials, nitrogenous
and other fertilisers may also affect the soil atmosphere.
The substantially lower diffusion rates of gases in soil water than in air mean that
when soil moisture levels approach saturation, respiration by soil organisms will
rapidly increase carbon dioxide concentrations, reduce oxygen levels and lead to some
degree of anaerobiosis. Conversely, in equivalent dry soils, a greater proportion of
the pore space will be filled with air and hence diffusion will be more rapid. Fine-textured
