I’m not sure this would actually work? Is there anything wrong with it? I think it would only work somewhere that doesn’t experience cold winters. The idea that it uses things that are already highly available in a very efficient synergistic combination.
On a surface look, probably it works. Maybe things like nitrate/phosphate balance or pest/microbe control impact the long term production.
The problem, as with all CCS strategies is: how do you pay for it? Maybe you can sell those products for a rate that covers the labour and wear and upkeep of the system? It might just be more cost effective to take the carbon-free energy and use it to displace fossil fuel consumption…
Excess energy sale, biochar, animal feed, fertilizer sales. Along with carbon capture credits if somewhere that pays for that.
Oh I also forgot, technically you could probably raise fish in the pond as well, but I don’t know if they’d eat all the azolla. The point also is to keep the pond shallow, less than 10 cm, to make it easier to construct, harvest, and manage.