0 to break up nitrates (= chemicals containing nitrogen and oxygen), for example in soil, and so release nitrogen into the air:
1 to remove nitrogen (= one of the gases that surrounds the earth) or chemicals containing nitrogen, for example, from the soil (= material on the surface of the ground), and so release it into the air
Water can be denitrified by the use of bacteria, but that cannot be done on a large scale.
Hobbyist aquaria often do not have sufficient bacteria populations to adequately denitrify waste.
Some organisms appear to be able to denitrify and remove phosphorus.
