Agriculture Reference
In-Depth Information
Soil Respiration and Aeration
3.4
Soil fauna and microorganisms live in the soil's pore space. Plant roots grow
through the pore space and also create pores as they grow. Living organisms and
roots respire. Normally, respiration is aerobic —that is, O 2 is consumed and CO 2
and water are released as complex organic substrates are oxidized, as in the fol-
lowing example:
C 6 H 12 O 6 (glucose) 6O 2 6CO 2 6H 2 O (3.7)
If the concentration of O 2 in the soil air falls to a low value ( 1/60 in the
air above the soil), aerobic respiration will cease. The roots of plants other than
those adapted to waterlogged conditions, such as some sedges and rushes, go brown
and die. The strictly aerobic soil organisms are replaced by facultative and obli-
gate anaerobes (section 2.3.2.1), a change that is generally undesirable for soil
health. Thus, the exchange of O 2 and CO 2 between the soil air and the atmo-
sphere is most important to maintain a balanced gas composition in the soil. This
process of gas exchange or aeration depends on the soil structure, and particularly
on the ratio of macropores to micropores.
The proportions in which the major gases N 2 , O 2 , and CO 2 occur in the
atmosphere are equivalent to partial pressures of 79.0, 21.3, and 0.0367 kPa, re-
spectively. A dynamic equilibrium is established between the atmosphere and the
soil air that depends on the soil's respiration rate and the resistance to gas move-
ment through the soil. A soil in which the gases can move rapidly through the
macropores has an O 2 partial pressure of 20 kPa and a CO 2 partial pressure in
the range of 0.1-1 kPa. However, if the macropores fill with water and the soil
becomes waterlogged, the soil becomes increasingly O 2 deficient or anoxic . In this
case, the O 2 partial pressure may drop to zero and that of CO 2 may rise well above
1 kPa. If soil becomes completely anoxic and remains so for some time as a re-
sult of anaerobic respiration, the gas phase may consist almost entirely of CO 2
and CH 4 in roughly equal proportions. The chemical and biological changes that
occur as a soil becomes progressively more anaerobic, some of which are undesir-
able, are described in section 5.6.
The Mechanism of Gas Exchange
Profiles of O 2 partial pressure in a poorly drained silty clay and a freely drained sandy
loam, in both winter (wet soil) and summer (dry soil), are shown in figure 3.6. These
contrasting profiles reflect the fact that the main mechanism of gas exchange between
the soil and the atmosphere is gas diffusion. The rate of diffusion depends on the in-
trinsic diffusion coefficient of the gas molecules, the nature of the pathway for dif-
fusion, and the gradient in gas concentration. These factors are discussed in more de-
tail in box 3.7. An adequate rate of gas diffusion is also important for the effective
fumigation of vineyard soils to control nematodes (chapter 7).
3.4.1
Soil Management and Aeration
Management practices that change the proportion of large, continuous pores in a
soil affect aeration. Degradation of soil structure through continuous cultivation,
for example, may not necessarily result in much change in total porosity, but it is
3.4.2
 
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