It looks like the subject of waterlogging introduces soil chemistry that is
a sort of mirror image of what conventional agriculture and horticulture
knows for certain. By keeping the soil's moisture content below saturation,
the little gaps and air spaces allow for at least minimum level of oxygen
near the roots. Three things that does are 1) to keep almost all iron in
the oxidized (insoluble) form, 2) allow fixed nitrogen (especially nitrate)
to remain in that form, thus making it available for root absorption, and
3) suppressing the growth of anaerobic bacteria.
When soil becomes saturated, the little spaces are filled with water,
severely limiting the inmigration of oxygen from the air. Within a short
period of time, natural decay of organic matter (even if there is only a
little) depletes the available oxygen in the root zone. When the oxygen is
gone it is called anaerobic or anoxic. For most vascular plants, that
produces a life-threatening situation where root death quickly follows. For
the minority of plants adapted to living in wet soils (including a host of
aroids), a root adaptation called aerenchyma tissue keeps the roots alive
by conducting oxygen down to the root zone from other parts of the plant.
Immediately upon the disappearance of the last oxygen, special bacteria
called anaerobes begin to feed in the new environment. Chemically, the new
environment switches from an oxidizing one to a reducing one. Among the
interesting chemical reactions that happen in a reducing environment are,
1) the conversion of insoluble, oxidized iron to the reduced, soluble form,
2) the production of toxic sulfides (black, precipitated iron sulfide, or
hydrogen sulfide gas, the "rotten eggs" smell) from sulfate, if present,
and 3) the "denitrification" of nitrates first to ammonia, and eventually
to nitrogen gas. So, over time, the waterlogged soil will lose its
nitrogen, will have soluble iron available, and will have a foul smell when
disturbed. Another reaction with less significance to plants is the
conversion of carbon materials to methane gas (swamp gas).
Waterlogged soils also tend to gather and preserve organic matter since
rotting seems to be less efficient under anaerobic conditions than in a
typical terrestrial woodland or garden.
Then, as mentioned by Craig, there is the issue of soil structure. I know
this has been discussed on this list in reference to the permanence of
organic substrates. Even though waterlogged soils retard the degradation of
organic materials, they do eventually break down. The resulting stuff is
called (no kidding) "muck". I know from my own culture of Cryptocoryne that
the organic substrate known as fibrous peat works well for 12 to 24 months.
After that time (and there are some variations in these times) the peat
breaks down into about half muck and the plants do less well. I am guessing
that in the wild, new detritus is laid down fairly regularly. The plants
then adjust and grow into the new material on a continuing basis. In our
artificial cultures we need to transplant when the breakdown occurs.
One thing that interests me is how aquatic plants find nitrogen. It seems
pretty certain from the available resources that nitrogen is just not
present in anaerobic soils. Do these plants rely on atmospheric outfall,
through foliar absorption? Do they associate with symbiotic bacteria or
fungi that fix nitrogen? Maybe somebody knows this.
Another thing is the idea that soil iron is readily available in anaerobic
soils and essentially unavailable in aerobic ones (iron is actually pretty
common as an element in most soils with an inorganic basis. The iron is
tied up as the insoluble, trivalent form.) It is stated that the aerenchyma
tissue (oxygen conducting) of wetland plants essentially produce a thin
aerobic region immediately surrounding the root. Since soluble iron has
some finite lifetime, it can be easily imagined that normal ion transport
across the boundary between aerobic and anaerobic allows for ready
absorbance of iron by the root. Is there possibly an analogous system in
place around deep roots in terrestrial plants? What I mean is, are there
enough isolated regions of anaerobic bacteria activity in mostly aerobic
soils, where soluble iron is produced, which then migrates into the aerobic
zone?
Well, this is a lot of stuff and I will end. Please offer corrections and
other comments as you see fit.
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