The global problem Too much or too little nitrogen

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Nitrogen, contained in amino acids, proteins and DNA, is necessary for life. While there is an abundance of nitrogen in nature, almost all is in an unreactive form (gaseous nitrogen, N2) that is not usable by most organisms. N compounds fall into two groups - non-reactive and reactive. Non-reactive N is N2. Reactive N (Nr) includes all biologically active, photochemically reactive, and radiatively active N compounds in the atmosphere and biosphere of the earth. Thus Nr includes inorganic reduced forms of N (for example, NH3, NH4+), inorganic oxidized forms (for example, NOx, HNO3, N2O, NO3-), and organic N compounds (for example, urea, amines, proteins, nucleic acids) that can undergo biochemical transformations. In the absence of human intervention, the supply of reactive nitrogen in the environment is not sufficient to sustain the current abundance of human life. Thus humans learned in the early 20th century how to convert gaseous N2 into forms that could sustain food production. Over 40 per cent of the world's population is here today because of that capability (Galloway et al, 2004, 2008).

Reactive N is introduced to the natural terrestrial environment primarily by biological nitrogen fixation in forests and grasslands, particularly in the tropics. Human activity introduces reactive N inadvertently by fossil fuel combustion and purposefully through biological nitrogen fixation associated with agricultural crops and through the Haber-Bosch process that allows the manufacture of synthetic fertilizer. Human introduction of Nr has changed with time relative to natural sources (Figure 4.2). In 1860, natural terrestrial nitrogen fixation introduced between 100 and 200Tg per year of Nr (Galloway et al, 2004). Within the last few decades, human activities have roughly doubled this supply. Nr creation continues to increase every year. It is dominated by agricultural activities, but fossil fuel energy plays an important role, and the increasing use of biofuels is adding a new and rapidly changing dimension. From 1860 to 1995, energy and food production increased steadily on both an absolute and per capita basis; Nr creation also increased from around 15Tg N yr-1 in 1860 to 156Tg N yr-1 in 1995, and increased further to 187Tg N yr-1 in 2005, in large part because cereal production increased by about 20 per cent and meat production by 26 per cent over about 10 years. These rising agricultural demands were sustained by a matching rise in Nr creation by the Haber-Bosch process from 100 to 121Tg N yr-1 (FAO, 2006).

Global Nitrogen Pool Population

Figure 4.2 Global anthropogenic creation rates of reactive nitrogen by the Haber-Bosch process, biological nitrogen fixation associated with agricultural crops, and combustion of fossil fuels

Figure 4.2 Global anthropogenic creation rates of reactive nitrogen by the Haber-Bosch process, biological nitrogen fixation associated with agricultural crops, and combustion of fossil fuels

Source: Adapted from Galloway et al (2003)

Cultivation-induced biological nitrogen fixation (C-BNF) occurs in several agricultural systems, with crop, pasture and fodder legumes being the most important. The C-BNF estimate for 1995 was 31.5Tg N and, because of the increase in soya bean and meat production over the past decade, Galloway et al (2008) estimated that in 2005 C-BNF was 40Tg N. In parallel, primary commercial energy production by coal, natural gas and petroleum combustion increased from 8543 million tons of oil equivalents (Mtoe) to 10,600 Mtoe (24 per cent) (BP Global, 2007).

The proportion of Nr created by the Haber-Bosch process that is not used for fertilizer-N production is used as a raw material in various industries, to make products such as nylon and other plastics, resins, glues, melamine, animal/fish/shrimp feed supplements, and explosives (Galloway et al, 2008). In 2005, around 23Tg N, accounting for 20 per cent of Haber-Bosch Nr, was used in this way (Prud'homme, 2007), but little is known about the ultimate fate of the Nr.

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