How do water plants obtain nitrogen if it is insoluble in water?

Answer 1

Marine ecosystems:

Nitrogen is fixed in the photic zone by cyanobacteria who acquire it from the atmosphere (it isn't completely insoluble). The fixed Nitrogen can enter the water as Ammonia or Urea (which is soluble) and rains down to the deeper parts of the Ocean as detritus. Large Brown and Green algae, sea grasses and other plants of the ocean acquire Nitrogen from the Ammonia or Urea dissolved in the water, while organisms in the lower zones of the Ocean can filter feed Nitrogen from the detritus (these organisms are a part of the Nitrogen cycle, but are not plants obviously).

Otherwise, algae typically grow to have a significant part of their bodies reach the surface, where they can fix Nitrogen directly.

Aquatic:

Either from ammonia released by bacteria, from bacteria living in the anaerobic 'soil' beneath the water or by growing to the surface and directly fixing the Nitrogen there.

Source:

Castro, Huber, Marine Biology 7e, McGraw Hill.

Answer 2

Water plants obtain nitrogen from the surrounding water through their roots, which have the ability to take up nitrogen in the form of ammonium or nitrate. Some water plants also have symbiotic relationships with nitrogen-fixing bacteria that can convert atmospheric nitrogen into a form that the plants can use.