From what I understand mangrove roots absorb nitrate and phosphate, that is why they are so beneficial to our systems. If I decided to plant the mangroves into some refugium mud would the roots still absorb the nitrates and phosphates in the water column or would they just take their nutrients from the mud and therefore making mangroves useless?

I would say they will take the nutrients from both the mud and the water column. IMO

If you put the roots into the mud, they will pull first from there. Now, some of the N and P you want them to take up will be there, but most of that which matters (with respect to nuisance algae) will be in the water. This is why, when you see mangroves used as nutrient export, they are planted such that their roots are not embedded in a substrate and if they are, it is in course sand.

Of course, planting them in mud might improve their growth and overall health, but it won't do much for your nutrient issues in the near to mid-term.

I was thinking the way the root structure is formed on a mangrove you would still have some of the roots only pulling nutrients from the water. I would imagine it will take them from where they are available.

Yes, provided those areas of availability are immediately adjacent to the root surface. Like almost all aquatic higher plants (often termed macrophytes), their nutrient intake is through their roots. Not stems, not leaves, just roots. Some freshwater macrophytes that form emergent and immersed leaf morphologies will take up nutrients from other body surfaces, but I can't think of a higher marine macrophyte that does this.

This is one reason why macroalgae are often preferable for nutrient removal in our small systems. Their loose (or non-existent) cell differentiation allows any cell with sufficient light and carbon to take up inorganic nutrients.

That makes sense! Excellent explanation...

I have read that mangroves are not really a good choice for nutrient export as their uptake is very slow. Any truth to this Fosi?

Dunno, as I've never tested it. I tried mangroves before but I've never lit them well enough and they inevitably keeled over.

I have read that mangrove nutrient uptake starts slow but once a significant area of root has been produced, the uptake rate is greatly increased. This makes intuitive sense.

I've decided not to use them in my systems because I don't care to construct a special lighting rig to keep them healthy and I'm also am very concerned about their roots growing into my aquariums' seams. The last time I tried them, I grew them in a modded AC70 and one of them grew a root right through one of baffle's silicone seams in a span of 2 weeks when I wasn't paying attention. Best to rubbermaid them I think.

Thanks Fosi thats what I was thinking I just needed someone else to chime in. I'm thinking when the mangrove starts to grow an advanced root system that if there was any N and P in the mud it would be quickly used up by the mangroves. Then either two things will happen the mangroves will slowly die or somehow the roots will still pull the nutrients it needs from the water column through the mud. Any thoughts?

As long as the water above the mud is moving, you'll get transport through the sediment driven by pressure differential. Of course, the more roots (or rocks) there are, the more the water velocity is reduced.

The presence of bioturbators, like infunal worms, will enhance material transport between the sediment and water but I don't know how how common they are in aquaria. Certainly some transport will be driven by other small macroinverts but I have no idea what kind of rate we are talking about... It may be minuscule.

If you really want to use mangroves as nutrient export and not just a cool addition to your system, you should grow them with their roots suspended in the water or in a course substrate as you would in hydroponics.

I think I will just end up planting them and throw in a clump of cheato on top of the mud. It does look good to have leaves growing out of the back of your tank. It's a conversation piece.
Friend - Is that a saltwater tank?
Me - Yup
Friend - But there are plants growing in the back!
Me - Yup
Friend - Cool....You should get a Nemo
Me - (slaps friend) You idiot!

Fair enough.