Environmental Impacts of Human Domination of the Global Nitrogen Cycle

Humans now annually add more biologically available nitrogen to the Earth’s land surfaces than do all natural processes. For 3 billion years, available nitrogen had been the major limiting currency for life on Earth. The recent human disruption of the global nitrogen cycle is causing major environmental harm, including water and air pollution, marine dead zones, 10% of global greenhouse gas emissions, and species extinctions. Three major ways to solve this nitrogen problem are (1) much more efficient global use of nitrogen fertilizers; (2) shifts to lower-meat and healthier diets; and (3) halting the conversion of food crops into biofuels, such as US corn ethanol used for transport.

Nitrogen, carbon, hydrogen and oxygen are essential elements for all forms of life on Earth. Ever since the earliest emergence of life 3 billion years ago, nitrogen – which is required to make proteins – has been the most limiting of these elements. However, new methods of agriculture, that began with the Green Revolution in the 1960’s and have been massively expanding ever since, now add more nitrogen to terrestrial ecosystems than all natural processes combined. The resulting nitrogen pollution has highly detrimental environmental impacts.

Although nitrogen is 78% of the atmosphere, N2 gas is an incredibly stable compound because of the amount of energy needed to break the triple bond that holds the two N molecules together. For about a billion years, lightening was the major way that this N – N bond was broken to create the biologically available forms of nitrogen that all plants require. The evolution of nitrogen-fixing cyanobacteria and the later evolution of nitrogen-fixing land plants, such as legumes, contributed to increased availability of nitrogen for land plants, thus allowing the emergence Earth’s forests, grasslands and savannas. For millennia, all natural processes have tended to release a total of about 150 Tg/yr of biologically available nitrogen into Earth’s land ecosystems via lightening and natural legume fixation (10 Tg/yr and 140 Tg/yr, respectively). Mainly because of the massive intensification of global agriculture since 1960, human actions now release an additional 210 Tg of available N. In particular, in 2021 global agriculture used 115 Tg of nitrogen fertilizer, added 40 Tg more from legume crops, 20 Tg more came from fossil fuel combustion and 35 Tg more from land clearing and tilling of soils.

Human domination of the global nitrogen cycle has three major environmental impacts: greenhouse gas emissions; water and air pollution; species extinctions.

• Soil microbes convert about 1% of nitrogen fertilizer into nitrous oxide, a greenhouse gas that is 300-times more potent than carbon dioxide. This agricultural emission represents about 10% of the global sum of all greenhouse gas emissions.
• Nitrate and nitrite pollution of streams, rivers, and lakes can cause blooms of often toxic cyanobacteria that have made major water supplies in numerous countries undrinkable. These algal blooms also harm freshwater fisheries and reduce the recreational value of freshwater ecosystems. Pollution of groundwaters with nitrite and nitrate from fertilized croplands can cause blue-baby syndrome and make once-healthy well waters become unsuited for human consumption.
• Runoff of available nitrogen from farms is carried to the oceans by most major rivers, and often kills all fish in huge nearshore areas, creating large marine “dead zones.”
• Recent research has shown that ammonia from both nitrogen fertilization and livestock operations dissolves into the atmosphere, creating nucleating sites that generate tiny health-harming particulate matter called PM2.5. In the US alone, these agricultural air emissions cause about 20,000 excess deaths per year from lung and heart diseases.
• Agricultural nitrogen pollution is a major cause of species extinctions. For instance, for animals that live in freshwaters, including amphibians and fish, water pollution is often their greatest extinction risk. Atmospheric deposition of biologically available nitrogen of agricultural origin onto terrestrial ecosystems a major threat to plant species. Most plant species have a series of evolved traits that make them good competitors for soil nitrogen. However, when nitrogen deposition make a different soil nutrient, such as calcium or phosphate become limiting, plants are nitrogen-efficient lose out in competition and can be driven locally extinct by the few species that are strong competitors for calcium or phosphate.