INTERNAL ENVIRONMENT, MICROCLIMATE AND RESOURCES - Soil Ecology - page 43

As the Eh is progressively reduced, the oxygen concentration in the soil solution
diminishes until at ca. 350mV, it disappears completely. Sequentially, other electron
acceptors in the soil start to be utilised (Patrick and Jugsujinda, 1992):

at ca . 250 mV, starts to be denitrified to N gases;

at ca. 200 mV, starts to be reduced to

at ca. 100 mV, starts to be reduced to and

at ca. -150 mV, starts to be reduced to
Anaerobic conditions have severe effects on unadapted higher plants. Small increases
(1-2 %) in carbon dioxide concentrations may stimulate root growth but higher concen-
trations (>5 %) cause increasing impairment. Root function may also be impaired by
a range of factors, including oxygen deficiency, that can lead to a shortage of the internal
substrates needed to drive cell metabolism. In addition, toxins may accumulate in
the root environment, or internal metabolites such as acetaldehyde and ethanol may
build up and these, in turn, reduce photosynthesis, respiration and mineral uptake (Tiedje
et al ., 1984). In agricultural situations, crop yields may be reduced by only short periods
of soil inundation (Patwardhan et al ., 1988). The activities of such beneficial aerobic
micro-organisms as mycorrhizal fungi and the aerobic nitrogen fixers are reduced.
The decomposition rate of organic matter is depressed under anaerobic conditions (Tate,
1979) and the amount of this depression is related to the Eh (de Laune et al ., 1981) even
below +350 mV, the level at which free oxygen is generally absent. In addition, much of
the soil meso- and macrofauna may be eliminated where anaerobiosis is prolonged
because of waterlogging. However, the activities of obligate anaerobic nitrogen-fixing
bacteria such as the Clostridium spp. may be stimulated.
Inundation may range in degree from occasional to seasonal (as by the melting of
snow or highly seasonal rainfall) to regular (as in tidal environments) and, in the extreme,
to an occasional exposure to the atmosphere. Throughout the world, sites that are regu-
larly saturated with water for varying periods of time on a diurnal, monthly or seasonal
basis occur in areas known as wetlands. The plants that occur naturally in such water-
logged situations possess morphological and physiological adaptations that permit
adequate root respiration to occur. These adaptations allow such plants to survive the
deleterious effects of their variably anaerobic soil environments (Drew, 1983).
Among the most striking morphological adaptations to existence in anaerobic soils
are the modified roots (pneumatophores) that occur in a range of mangrove species.
These structures are specialised gas exchange organs and occur in several forms; they
permit the species possessing them to succeed in the often anaerobic coastal and estuar-
ine soils. Further adaptive air-conducting structures found in wetland plants include
the internal aerenchyma (air conducting tissues) that occurs within the stems and roots
of such plants as rice ( Oryza sativa ) and water lilies ( Nymphaea spp.) and supplies
oxygen to their roots and, in some species to the rhizosphere. Other species may respond
to anaerobiosis by facultatively producing aerenchymatous adventitious roots, or increas-
ing the proportion of gas-filled porosity in roots (van Noordwijk and Brouwer, 1993).
A number of physiological adaptations also exist including the ability to avoid toxic
concentrations of such metabolites as ethanol by promoting their leakage into the root
environment, or the transpiration stream (Drew, 1983).