a little copy and paste:
anaerobic bacteria... whilst those that do not require air are referred to as anaerobic,
and tend to cause putrefaction of dead organic matter.

Yes, it stinks! I usually just dump the old dirt in the garden - I'm sure there is something good in it that the garden can use. I don't use it back in the pond. You probably could - spread it out in the sun and let it get rained on a few times first.
Do you use pots with lots of holes, a few holes or none for your plants? I use rubbermaid tubs and I put a few holes around the sides. I don't want the fertilizer to escape but I do want a few holes so any anaerobic gases that might build up can escape.
Some (generally nonflowering) plants get the waterlily baskets they sell with all the holes but those plants get gravel around a rootball in the middle and no fertilizer except what they take from the water. Sherry

Like frugalgardener said, anaerobic bacteria. The smell is the chemicals these bacteria produce.

Drying out the soil slows the bacteria way down and the smell goes away (depending how good your nose is). But as soon as the soil is wet again the bacteria go right back to what they were doing. Replacing with new soil will do the exact same thing. So to me I don't see a reason for drying it out or using other soil. That is the environment the plants have evolved to handle.

waterbug guy...........are you saying that this smell (anaerobic) is nothing to worry about?

Yes.

What you are smelling is indeed toxic and many people do seem to freak out about that. But it's toxic in the same basic way CO2 is "toxic". The issue, just like CO2, is the amount. Heck, even O2 is toxic. People do seem to smell these gases a lot and all seem to live to tell the tale.

A potted plant in soil creates these gases but is slow to release them. Which is why when you stir the soil a lot of trapped gas is suddenly released all at once and you smell it.

Where I've heard of this possibly being a problem is in ponds that have a large fish load, like a rearing pen. The fish were fed a lot of food which was sinking to the bottom and decaying. Because this was rich food the bacteria grew fast and because the layer wasn't deep the gas was released into the water and the fish suddenly died. That was the theory anyways. Personally I have my doubts. But at least it's plausible.

The point is you can read many pond forums and web sites that will tell you these gases are toxic and will kill fish...and there is a grain of truth in that. But getting those gases to the levels needed to kill fish isn't easy. All the lakes and ponds in the world that have existed before there even were fish have always had these gases present seems to me to point to all the scary posts and web sites on these gases being toxic are way over hyped. Normally big fish die offs are caused by a lack of O2 long before these other gases can get into the deadly range. Because conditions for low O2 and getting these gases to build is all the same issue...poor gas exchange for the fish load.

So imo a few pots with soil in a kind of standard backyard Water Garden...not a problem, not even close to a problem.

But if anyone prefers to believe the world is coming to an end there are plenty of sources willing to scare the heck out of you. The bread and butter of myth generators is to take a fact out of context and then apply it in an almost impossible context so it still sounds logical if no one looks too close. They do more to harm our hobby than anything else.

Okay...........another question..........I seem to only smell this in soil in pots of dead plants. When I dump out the plant and soil, will the anaerobic bacteria die off when exposed to the air/sunlight for awhile? I don't have any fish, so that's not a problem. I think what you are saying is even if it smells, it can't necessarily hurt the plant.
I'm running out of plain garden soil to use and I don't like to buy soil, so I'm wanting to re-use it in pots with new plants.
I'm not afraid of it.........just didn't want to throw money away planting new plants in soil that smelled pretty bad once.
I'm thinking after awhile of sitting out and aerating, it will be fine to use.
Thanks.

The more organic matter in the soil the worst the smell because the more there is for bacteria to eat. But even if you started with soil with absolutely no organic matter this smell would still occur because things grow in the soil, bacteria, virus, fungus, bugs, life. and that stuff will die and be consumed and produce the smell depending on how good your nose is. This same process happens in the garden, compost piles, every where. The smells are a little different in each case, but the same process. Even the some of the same species of bacteria.

This is all good and bad for plants. The good part is the smell is an indication of decomposition and the release of nutrients plants can use. The down side is just like compost can be too hot so can muck. Plants like ammonia, but too much can burn the roots.

So just like a regular potted plant it might be helpful to put used soil into a compost pile to allow the decomposition to proceed further. But in general that shouldn't be needed.

Exposing the bacteria to the sun might kill some bacteria on the surface, but that might only be 0.000001% of the bacteria in there. You can stick the soil in your over and cook it until sterilized...but the bacteria are going to recolonize in a very short time. These bacteria are everywhere. It isn't that these bacteria die when exposed to oxygen, they just don't need oxygen. It isn't that they die in sunlight, they just don't need sunlight. Sunlight is harmful to all life because of UV, but these creatures aren't like vampires.

The bottom line is that humans don't like the smell but some plants have evolved to live in that kind of soil. It's normal to a degree. Like I said, plants can have a hard time if there's too much, but in general, they're fine.

Thanks waterbug guy!