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Discussion Starter #1
Are there any plants simliar to legumes, which produce nitrogen and help to keep soil fertile, in aquaitc plants? I was thinking it might be interesting to attmept something of a self sustaining ecosystem if such a plant existed. Any thoughts?

The only thing I have found so far is an "aquatic legume" by the species name of "Neptunia natans". But i havent been able to determine if it grows sumbersed or not.
 

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I don't think legums actually proguce N, they mearly take it from the air and transport it to nodules in the roots for storage and later use. In other cases, there are bacteria(actually a form of blue green algae) that will fix N from the air and make it avaliable to the host plant. I know that the floating aquatic fern, Azolla caroliniana, hosts this bacteria in its leaves. There must be others. I think if you are looking for something to transport N to the substrate, you will need a plant with its roots in the substrate but the top portion growing emmersed. Then of course, conditions would have to be right for the plant to actually take N from the air and store it in the roots. That would be a very expensive transfer for the plant.

Seems that a Walstad type tank with a well balanced fish load is what you wre looking for.
 

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Discussion Starter #3
I was more wondering if there were any plants that would fix excess nitrogen via bacterial symbiosis similar to that of the legumes. That way fertilizers wouldnt be wasted, and could be stored for use by other plants.
 

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I htink that in the aquatic substrate, the bacteria that will work with roots to fix nitrogen and keep it avaliable need very anerobic conditions to work. That is somethign we usually try to avoide for obvoius reasons. Icould be wrong about that bacteria about the anerobic part but Iknow I have seens lots of little white nodules on crypt roos that I assumed were nitrogen based. That would make sense, since it was long ago in my beginning when my tank's conditions and substrate were horrible because I did not know better.

I don't see how N dosed is wasted. If its not in the water collumn then it is in plant/algae/bactria tissue. I don't belive that N is like Fe in that it can bond wiht other ions to form compounds that are not bio avaliable to the plants. As always, I have reached the end of my knowledge. Hopefully someone smarter can answer the rest.
 

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Plants can not utilize nitrogen gas, N2. That's too bad, because 79% of the atomosphere is N2. They must get soluble forms of nitrogen, such as ammonia (NH3), nitrite (NO2), nitrate, (NO3) or urea (CO(NH2)2. Legumes and other plants that fix nitrogen live in areas where soil nitrogen is low. Their symbiotic bacteria "fix" atmospheric nitrogen (N2) by converting it into soluble, available forms, such as NH3. The process takes a lot of energy that the plant partner must provide. Legumes can use as much as half the energy they capture in the process of photosynthesis to feed their symbiotic bacteria and keep conditions just right for them. The payback is that they get nitrogen fertilizer where other plants can't. They can grow in low nitrogen habitats where other plants only struggle.

As Dennis said, the only known N-fixing aquatic plant is Azolla, which partners with the cyanobacterium, Anabena . Azolla is known to provide valuable nitrogen fertilizer for rice fields. I think what Adam wants is a plant that stores excess nitrogen fertilizer so that it isn't wasted. That is different from nitrogen fixing. Actually all aquarium plants can store excess nitrogen. They can store in their tissues over twice the nitrogen they need to grow at their best rate. If the supply of nitrogen is cut off, they can more than double their size before their supply of stored nitrogen runs out.
 

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Discussion Starter #6
Thanks for the replies, just thought it wouldnt hurt to ask. I am fascinated with the idea of creating an ecosystem that requires little or not intrusion(other then feeding fish and trimming plants) by me. I guess thats probably not entirely realistic. Thanks again.
 

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Adam wrote:
I am fascinated with the idea of creating an ecosystem that requires little or not intrusion(other then feeding fish and trimming plants) by me
.

That is easy to do. You want low to moderate light, an easy fish, like guppies, and relatively easy to grow aquarium plants. Avoid the difficult ones that require high light and/or high CO2. I would put in an initial dose of trace elements, potassium and magnesium. If the water is very soft, I would put in a source of calcium, such as ground limestone. Let the fish food provide nitrogen and phosphorus. It will be a low maintenance setup in which you can keep a wide variety of aquatic plants. You have to watch and occasionally trim to make sure that some plants are not shading out others. With a light fish load, you should only have to change the water about once a year.
 

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So Azolla does infact provide a source of N for the aquarium? Is it invasive like the rest of the floating plants?
 

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Azolla does not provide a source of N for hte aquarium. The syimbiotic bacteria in its leaves provides the Azzola with N, by fixing N2 in the air and converting it to a form useable by the plant. Its use as a N source comes from its use in rice fields where the field gets draind and the Azzola either allower to die in the soil, or get tilled in. As the plant decays, the N it has stored gets released in to the soil. HTH:) And yes, it is invasive although I am not sure how much, when compared to others like duckweed
 

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Neptunia species also fix nitrogen from the atmosphere and grow as floating plants - roots in the water but leaves on stems above the water. "Water mimosa" is:

Neptunia oleracea
Syn: N. aquatica

Leonard here posted a pic in 2011:
http://www.aquaticplantcentral.com/...lanted-aquariums/81665-neptunia-oleracea.html

As would make sense for an aquatic legume, it fixes nitrogen from the atmosphere.

http://aob.oxfordjournals.org/content/69/2/181

The Physiology and Nitrogen-fixing Capability of Aquatically and Terrestrially Grown Neptunia plena: The Importance of Nodule Oxygen Supply
E. K. JAMES*, F. R. MINCHIN† and J. I. SPRENT

Accepted October 10, 1991
 
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