Modern farming industry and other industries are causing a large amount of nitrogen to enter the soil at double the natural rate.
The nitrogen is in the form of nitrates and much elutes into streams, rivers and lakes. There is increased plant and bacterial growth in the water, a process known as eutrophication.
However some nitrate is retained by biological processes by plants and bacteria. Most importantly there are bacteria which convert the nitrate to molecular nitrogen and nitrous oxide. These bacteria persist in the sediment of streams and banks. However at higher concentrations of nitrate the efficiency of this process declines.
Seitzinger Out of reach Nature vol 452, pp162-3

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