The nitrogen cascade in arable crop areas of the North of France

The concept of nitrogen cascade refers to the successive transfer of the reactive forms of this element, beginning with their introduction into the environment by human activity, up to their ultimate return, through denitrification, to the pool of inert atmospheric di-nitrogen. In the case of major cash crop farming areas such as those of the Parisian basin, nitrogen surpluses linked to the fertilisation of arable soils are largely predominant as sources feeding the nitrogen cascade in comparison to reactive nitrogen discharge from urban wastewater purification plants or nitrogen oxide emitted through traffic and the generation of thermal electricity. The agricultural surplus of arable soil, which represents the potential for environmental losses at the scale of a whole crop rotation cycle, is considerable in these arable cropping regions. This is true, even in the case of a strict respect of the rules of equilibrating fertilisation to the needs of the crop during its vegetative cycle, the rules that form the foundation of "good agricultural practices" or "responsible farming" according to the specifications of a French public organism like the COMIFER. Moreover, the surplus is increasing with crop yield objectives. Most of the cropland nitrogen surplus is leached to the aquifers and the river network causing their nitrate contamination, as well as the proliferation of harmful algal species in the receiving coastal marine areas. Natural mechanisms for eliminating nitrates occur all along the aquatic continuum but they may be accompanied by the emission of nitrous oxide, a potent green-house gas, which limits the possibility of their systematic use for mitigating nitrate contamination of hydrosystems. The control of the nitrogen cascade in these regions will thus necessarily require a profound reconsideration of the current agricultural model, with its never-ending quest for higher yields.