soil - Lublin

authors stated that O 2 gradients inside single aggregates appeared to be steeper in
artificial or disturbed aggregates.
A. Effect of Texture on Gas Transport
Studies of the effects of soil texture on gas transport and on the composition of
soil air in artificial aggregates reveal a strong correlation between the pore size distribution
and the air entry value. The air entry value is the soil water potential at
which gas diffusion to the aggregate centre increases because water menisci are
removed from continuous pores.
In prisms of sandy-loamy texture the increase in O 2 partial pressure occurred at
a soil water potential of about – 15 kPa while polyhedrons with loamy-clay became
aerated at more negative water potentials of < - 60 kPa (Zausig et al., 1990). According
to Fick’s law, gas flow depends on the diffusion coefficient and concentration
gradient. Assuming an oxygen content of the air surrounding the aggregate of
20%, O 2 transport within the aggregate would require additional time. If chemical
and microbial oxygen demand inside the aggregate exceeds oxygen supply, zones
of low oxygen partial pressure or even anoxic microsites may develop.
Stepnieweski et al. (1991) described a method where O 2 -sensitive microelectrodes
were pushed through soil aggregates at a constant speed of 0,00166 mm ⋅ s -1 . By
this procedure continuous radial profiles of oxygen partial pressure could be measured.
The method was used to compare the internal oxygen status of artificially
formed spherical aggregates (diameter of 24 mm) of six different soil samples at
soil water potentials ranging from – 1 to – 6 kPa (Zausig et al. 1993). It was found
that the intensity of anoxia and the diameter of anoxic centres would be controlled
not only by microbial and chemical oxygen demand but also by parameters such as
aggregate hydraulic conductivity and pore size distribution, i.e., by the soil texture.
B. Effect of Soil Structure on Aeration
Aggregated soils always include secondary large interaggregate pores (coarse
pores > 50 µm) and small intraaggregate pores (finer pores < 0.2 µm). This results
in a heterogenization of the pores system and of the texture within the aggregates
which strongly affects transport phenomena. The smaller the biological activity and
the smaller the degree of organization of soil particles the smaller the intraaggregate
pores which in turn restrict gas diffusion. A large reduction in O 2 partial pressure
or even anoxia within aggregates under in situ conditions results from restricted
pore space (diameter and continuity) as well as a source of reduced carbon,
provided O 2 consumption by microbes is the main factor causing anoxia. Thus,
natural soil aggregates should have an anoxic or less-aerated centre if the soil water
potential is > - 60 kPa.
Significant amounts of organic substances may induce large decreases in redox
potential soon after saturation with water. Thus, in humic A horizons redox potentials
should drop rapidly upon wetting while in subsurface horizons with low contents
of organic substances only small redox potential changes occur.
The intensity and speed of changes of redox potential may depend on more
than the content of organic matter. The chemistry of the mineral soil components
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