Even if current Agricultural practices are upgraded to end the addition of more Nitrogen runoff it will take upwards of 30 years to deplete the stored Nitrogen (Nitrate).
Scientists are only now discovering how big the mess is and how difficult it will be to stop polluting the Gulf of Mexico with Nitrate from Agricultural runoff in the Mississippi River Basin and feeding the "Dead Zone". The Agricultural runoff results in stored Nitrogen (which is of course Nitrate!), and it makes its way to the Gulf of Mexico off the shores of Mississippi and results in the hypoxic "Dead Zone" where all living biota die.
Last year, the hypoxic zone reached its largest size ever. This is despite a major effort to reduce the loading of the Mississippi River Basin with additional Nitrogen from Agricultural runoff.
This is the first time that it has been documented that the enormous size of the legacy stored Nitrogen in the Mississippi River Basin presents such a huge challenge to deal with.
Credit for Article: Ocean Hypoxia: Haunted by the Past, Science p. 394, April 27, 2018