The impact of humans on the nitrogen cycle, with focus on temperate arableagriculture

The 6 billion people alive today consume about 25 million tonnes of protein nitrogen each year, a requirement that could well increase to 40-45 millio n tonnes by 2050. Most of them ultimately depend on the Haber-Bosch process to fix the atmospheric N-2 needed to grow at least part of their protein a nd, over the earth as a whole, this dependency is likely to increase. Human s now fix some 160 million tonnes of nitrogen per year, of which 98 are fix ed industrially by the Haber-Bosch process (83 for use as agricultural fert ilizer, 15 for industry), 22 during combustion and the rest is fixed during the cultivation of leguminous crops and fodders. These 160 million tonnes have markedly increased the burden of combined nitrogen entering rivers, la kes and shallow seas, as well as increasing the input of NH3, N2O, NO and N O2 to the atmosphere. Nitrogen fertilizers give large economic gains in mod ern farming systems and under favourable conditions can be used very effici ently. Losses of nitrogen occur from all systems of agriculture, with organ ic manures being particularly difficult to use efficiently. Although nitrat e leaching has received much attention as an economic loss, a cause of eutr ophication and a health hazard, gaseous emissions may eventually prove to b e the most serious environmentally. Scientists working on the use and fate of nitrogen fertilizers must be careful, clear headed and vigilant in looki ng for unexpected side effects.