Changed global nutrient cycles

Nitrogen

We have already seen in the previous chapter that atmospheric concentrations of N2O have risen over the last two centuries largely as a result of fossil fuel combustion, but this is not the only, or indeed the most significant disruption of the nitrogen cycle as a result of human activities. One of the most remarkable trends during the course of the twentieth century has been the relentless increase in the extent to which anthropogenic processes have begun to dominate the conversion of non-reactive nitrogen to reactive forms, i.e. those that are biologically, photochemically and radiatively active in the biosphere and atmosphere (Galloway, 2004).

Prior to the opening of the twentieth century, reactive nitrogen was produced mainly through nitrogen-fixing organisms – bacteria, both free living and symbiotic, and blue-green algae. Human activities such as the cultivation of rice and leguminous crops contributed around 5% of the total. Anthropogenic contributions since then have increased in several ways. The growing human population has generated an increasing demand for food, leading to higher levels of nitrogen fixation through the cultivation of rice and legumes. By the late twentieth century, annual nitrogen production linked to cultivation was around 33 Terragrams (1 Tg = 1012 g). More importantly, the demand for nitrogenous fertilisers quickly outstripped the supplies from guano and nitrate mining, and the development of the Haber–Bosch process led to a massive increase in the quantity of anthropogenically produced reactive nitrogen.