Agriculture Reference
In-Depth Information
of manganese and iron to oxidised and the oxidation of hydrogen and methane to other
organic substances. Most of these processes are beneficial to plant growth. In contrast,
where soils are flooded or waterlogged, partial or complete anaerobiosis soon super-
venes because of continued microbial activity and the low diffusion rates of oxygen and
carbon dixoide in water. The degree of anaerobiosis in the soil atmosphere is reflected in
the concentrations of both gases and, as concentrations of both vary approximately
inversely, either or both may be used as indicators (Stolzy
et al
., 1981). It may also be
measured as the redox potential (Eh) and this ranges from -300 mV (highly reducing
conditions) to 900 mV (highly oxidising conditions).
Anaerobiosis may occur within aggregates even in well drained and structured soils
where the inter-aggregate pore space is aerobic. Sextone
et al.
(1985) have shown
the existence of strong oxygen gradients within aggregates from such soils; the oxygen
concentrations at the centres of these aggregates were effectively zero (Figure I.15).
As a consequence of microbial activity, a number of materials accumulate in anaerobic
soils and in the anaerobic volumes of otherwise well-aerated soils. These include carbon
dioxide, hydrogen sulphide, methane, ethane, ethylene, a range of higher hydrocarbons
(van Cleemput and El-Sebaay, 1985) and reduced forms of such elements as manganese
and iron. Denitrification gases such as NO and may also be present although the
actual gases present will depend on both the Eh and the nature of the microbiota
present (Munch and Ottow, 1986).
