The God Species_ How the Planet Can Survive the Age of Humans - Mark Lynas [50]
Reactive nitrogen is extremely potent stuff and can keep on polluting over and over again as it transforms and circulates between different compounds. A single liberated nitrogen atom may, for example, join with oxygen to form nitrogen oxides (the precursors of toxic low-level ozone smog), before later raining out of the sky as nitric acid, going on to fertilize grasslands and watercourses as nitrates, and then being converted into the powerful greenhouse gas nitrous oxide. Even nitrogen that is sensibly applied to fields will eventually form part of this cycle: After being incorporated in a crop it will inevitably one day be released again—whether into the sewage system as waste or by a decomposing corpse in a grave. The only way this apparently immortal nitrogen atom can ever be properly silenced is for it to be combined again with another nitrogen atom back into the happy pair bond N2. Then it can once again enter the atmosphere as an inert and entirely trouble-free gas. But humans cannot perform this “denitrification” trick—only certain microbes can do it, and then only under very specific circumstances such as in the absence of oxygen.
It is not all bad news, however. The human production of vast quantities of nitrogen is greening the Earth, and may be helping plants to draw down increased volumes of carbon dioxide—thereby offsetting climate change. One group of scientists suggests that nitrogen deposition by humans (not directly as fertilizer, but indirectly through stray nitrogen that blows, washes, or rains into natural ecosystems) is having just this effect in forests, with 200 kilos of carbon mopped up for each additional kg of accidentally deposited nitrogen.13 In total, this free benefit may be responsible for cleaning up 0.7 billion tonnes of carbon a year via Northern Hemisphere forests and woodlands—about a tenth of the total emitted.14 If you’re wondering how this nitrogen gets into the forests, the answer is via the rain: Ten or more kilograms fall annually per hectare in industrialized countries, an order of magnitude more than would have been deposited by lightning in preindustrial times.15
And if you don’t mind the dead zones, there are benefits too to having nitrogen in the oceans. With an estimated third of oceanic nitrogen overall of human origin, the extra plankton growth thereby stimulated may be mopping up an additional 300 million tonnes of carbon per year.16 In areas where the seas are chronically short of nutrients, human-derived nitrogen washed down by rivers may even support whole new marine ecosystems and associated fisheries. Around the Nile Delta, for example, fisheries collapsed after the construction of the Aswan Dam in 1965 caused fewer nutrients to be washed downstream. Luckily, human pollution came to the rescue. Thanks to all the fertilizer runoff and sewage sludge now coursing down the Nile, fisheries rebounded: One estimate is that at least 80 percent of all fish landed in the area are now supported by this anthropogenic excess nitrogen.17
But let’s not get carried away. For the most part, nutrient pollution is bad—even when it has a fertilizing effect. Study after study18 has shown that on land enhanced nitrogen deposition tends to lead to a loss of biodiversity as weedy, fast-growing species become dominant and crowd out more specialized plants.19 In the U.K., the best restored wildflower meadows are those that have had their fertility artificially lowered, not enhanced. Where too much human nitrogen now arrives with each successive rain shower or flood inundation, ecosystems experience “slow but chronic loss of biological diversity,” in the words of one recent survey.20 The impacts in the oceans are likely to be similar—nitrogen and phosphorus runoff have been associated with the “rise of slime” in the marine realm, as algae and toxic bacteria proliferate