By adding industrial clout to the efforts of the microbes that used to do the job single-handed, humans have increased the annual amount of nitrogen fixed on land by more than 150%. Some of this is accidental. Burning fossil fuels tends to oxidise nitrogen at the same time. The majority is done on purpose, mostly to make fertilisers. This has a variety of unwholesome consequences, most importantly the increasing number of coastal “dead zones” caused by algal blooms feeding on fertiliser-rich run-off waters.
Industrial nitrogen’s greatest environmental impact, though, is to increase the number of people. Although nitrogen fixation is not just a gift of life—it has been estimated that 100m people were killed by explosives made with industrially fixed nitrogen in the 20th century’s wars—its net effect has been to allow a huge growth in population. About 40% of the nitrogen in the protein that humans eat today got into that food by way of artificial fertiliser. There would be nowhere near as many people doing all sorts of other things to the planet if humans had not sped the nitrogen cycle up.
It is also worth noting that unlike many of humanity’s other effects on the planet, the remaking of the nitrogen cycle was deliberate. In the late 19th century scientists diagnosed a shortage of nitrogen as a planet-wide problem. Knowing that natural processes would not improve the supply, they invented an artificial one, the Haber process, that could make up the difference. It was, says Mark Sutton of the Centre for Ecology and Hydrology in Edinburgh, the first serious human attempt at geoengineering the planet to bring about a desired goal. The scale of its success outstripped the imaginings of its instigators. So did the scale of its unintended consequences.