Very good article sluggo, that's the kind of information I was looking for.
So, Steve - the ebb and flow of pressure in the plants' roots is enough to slowly turn the substrate? I would still think that worms / snails would help bring more nutrients / oxygen to the depleted areas around the roots regardless of the guttation or O2 transport.
More supplemental O2 to roots = Better?
Guttation allows new water from the main body of the tank that has oxygen to replace water lower in oxygen in addition to what the roots deliver. Though it doesn't turn over the substrate like critters would. I've only seen snails [ I have Malaysian live bearing snails] move substrate and as for worms it would be difficult to see how such a week critter could move a pebble or some other type of not dirt substrate. Has it been clearly demonstrated worms and snails actually turn over a substrate [ like ants or earthworms] or do they in the case of snails just move the very top layer around ?
Snails and worms along with a plethora of other substrate critters work well in a natural environment, but maybe not so well in a closed system. The important thing is to have a very slow flow of new water from the main body of the tank through a substrate that does not compact.