Phytoremediation
Direct plant absorption:
Populating aquifer regions with rapidly growing trees can help capture excess nitrogen as the plants absorb it for growth.
In areas where concentration is low enough, harvestable crops can even be used to extract the excess nitrogen.
Nitrogenous air pollutants:
Although nitrogenous air pollutants have not yet been discussed, it is noteworthy to mention that plants have some potential to bioremediate them. Some studies show that plants like Arabidopsis thaliana (below) are able to neutralize the pollutant NO2 by incorporating it into amino acids. Researchers are also investigating the possibility of synthesizing an enzyme to convert NO2 to N2O or N2.
Arabidopsis thaliana
Phytoremediation is also an effective strategy for decontaminating metals.
Sources:
Morikawa (2003); Kirk-Othmer Encyclopedia (2004)