???Moss Blocks Oxygen???

[MichaelS]

Porous soil in the first couple of inches at the surface, or in bonsai pots, may have space for gas exchange from the atmosphere to some degree, but oxygen will have to dissolve in water to make it into the roots.

That is just plain wrong Brian. The differential caused by oxygen use of roots and other life in soil draws O2 in as a gas. Whether it is 2 inches or 20 feet under the surface. All air space in the soil are connected to the atmosphere. The gas enters plant roots directly (apparently into gas filled spaces between cells) and then into the cell itself. Not only ''dissolved'' in water.
http://www.funscience.in/study-zone/Biology/Respiration/RespirationInPlants.php

Roots growing deeper than a couple of inches in the ground will not receive any significant gas exchange from the atmosphere, barring burrowing animals to create channels.

Please see my point above about the connection of ALL air spaces in soil with the atmosphere

Water carries dissolved oxygen to those roots. Water carries dissolved oxygen to bonsai roots too

O2 is still a gas if it surrounded by H2O. It is not necessary for O2 to be dissolved in water before a plant can take it up.

If you don't water enough to fully flush the deoxygenated water from your pot, eventually the root zone will become anoxic and roots will die. This is one of the primary reasons beginners are advised to water from the top until water runs freely out of the bottom of the pot.

It is physically impossible for oxygen to become depleted in soil unless the air filled pores are filled with water.