Steve Turner wrote in message . ..
Archimedes Plutonium wrote:

I am trying to nail-down the inverse or reverse relationship. As to why
plants need animals to reform nitrogen.

Plants lack the biochemical pathways for large scale protein
degradation. That is the specialty of saprophytes (e.g. fungi and
bacteria). Legumes are able to convert atmospheric nitrogen to usable
form by virtue of rhizobia bacteria in root nodules (it is the
bacteria which do the conversion). Plants can also use inorganic
(mineral) nitrate as a source of nitrogen. Higher animals are a
relatively minor source of nitrogen.

True enough. There are some plants that eat meat (like Venus Flytrap)
but they are generally located in nitrogen depleted areas.

There is an important symbiosis between plants and animals, in that
plants use carbon dioxide and emit oxygen, and animals do the
opposite.
Um, not quite. Plants respire just fine. It's just that they make
their own oxygen so that they can use respiration in an oxygen
atmosphere to reduce the sugars they've made to use energy.
Photosynthesis is the storage of energy. Respiration is the use or
release of that energy. Animals do the latter. Plants do both.

Some symbiosis not because of the nitrogen or carbon cycles but for
breeding purposes. Some higher plants have learned to use animals to
pollinate them and spread their seeds. Wind pollination is inefficient
and random. Animal pollination is efficient relative to wind
pollination. Animals carrying seeds away from the mother plant helps
spread them in different directions instead of just downwind. In
return, animals get valuable nutrition. Most lower plants don't take
advantage of this situation as they developed before animals were
important to the ecosystem.

In conclusion, the hypothesis that the nitrogen cycle must include a
direct animal to plant transfer does not seem to have any proof and
the symbiosis exists for other reasons, in indirect transfer (animal
to ground to plant) and in other cycles.

Todd O.