Typically plants will only uptake as much Nitrate as they have energy (light) and additional photosynthetic intermediates for. Otherwise the plant will just grow more slowly, and in turn use fewer nitrates. If the plant dies it will immediately start breaking down into Nitrates, Hydrocarbons, and other fish unfriendly chemicals, as well as invite fungus, bacteria, and parasites.
If your aim is only to reduce nitrates, do water changes.