Eutrophication and the disrupted nitrogen cycle

After the Second World War, there was a global need for increased food production to feed a growing population, and new techniques to increase agricultural production were developed once the war was over. In the 1950s and 1960s this led to "the third agricultural revolution" (or "the green revolution" as it was later called), and in 1970 one of the early innovators, the American scientist Norman Borlaug, received the Nobel peace prize "for saving billions of people from starvation". Key elements in this development were introduction of high-yield crop varieties, increased irrigation, and the use of pesticides and synthetic fertilizers, which significantly increased the availability of e.g. organic nitrogen in agricultural systems. A rough estimate suggests that the yearly global human creation of reactive nitrogen (basically all forms of nitrogen besides N₂) has more than doubled since 1950, with more than a 70% increase just since 1990 (Galloway and Cowling 2021).

However, while the green revolution rapidly increased food production and contributed strongly to alleviate global malnutrition, the tremendous global increase in synthetic fertilizer use in agriculture, with subsequent leaking into water-ways and seeping into ground-water reservoirs also led to human health issues, eutrophicated watersheds and coastal waters, with anoxic deep water and subsequently diminished abundance and biodiversity of bottom fauna. These problems started to receive serious scientific attention and public awareness in the late 1970s and early 1980s, and the first warning signals came from well studied enclosed water basins like the Baltic Sea (e.g. Larsson et al. 1985; Elmgren 1989). However, it was also early realized that the environmental problems following increased nutrient loads from agrochemicals and other antrophogenic activities where of a global nature, since these chemically and biologically reactive nutrients are distributed around the globe via wind transport, land run-off and ocean currents (e.g. Caraco 1995; Galloway 1998), and affect not only aquatic ecosystems, but also terrestrial (e.g. Matson and Vitousek 1987).

In this Anniversary Collection, we highlight four innovative and highly cited Ambio papers (Elmgren 1989; Caraco and Cole 1999; Galloway and Cowling 2002; Matson et al. 2002), covering a period when the global problem of increased output of reactive nitrogen became generally acknowledged and science shifted focus from problem identification to mitigative actions and providing management decision-making advice. Elmgren (1989) described how eutrophication had changed energy flows in the total Baltic Sea ecosystem and discussed how it would likely change in the future, if not mitigated. Caraco and Cole (1999) modelled and analysed the human impact on nitrate transport to the oceans via the world’s major rivers, and how nitrogen saturation could dramatically increase nitrogen export. Matson et al. (2002) discussed how effectively reactive nitrogen was retained in different systems and how the global atmospheric deposition of excess nutrient affects terrestrial ecosystems with different soils and vegetation characteristics. Finally, Galloway and Cowling (2002) gave an historic overview of changes in reactive nitrogen, sources and consequences, over the nineteenth and twentieth century, as well as an interesting and innovative outlook for the future.

The authors of all four original Ambio articles share with us the background of their early work and reflect on the significance of the featured papers when they were published, their impact today, and discuss where to go from here (Caraco 2021; Elmgren 2021; Galloway and Cowling 2021; Hall et al. 2021). Furthermore, in two invited perspectives, high profile researchers in this field reflect on the legacy of the original articles, how they helped us understand the global eutrophication issue and reframe policy targets regarding environmental impacts from reactive nitrogen, and also discuss coming challenges (Bonsdorff 2021; Melillo 2021).

Today we know much more about eutrophication and its consequences than 20 years ago. Successfully reduced emission of reactive nitrogen from transport and energy production systems has contributed to cleaner air and reduced the global emission from fossil fuel combustion to the environment, but the still increasing human use of antropogenic nitrogen compounds in food production and agriculture leads to cascading ecosystem effects, and poses a major environmental challenge for the twenty-first century. Some of the contributing authors point out that there is after all room for cautious optimism since mitigative actions, like improved sewage management and restoration of wetlands, have had local and regional effects. Therefore some ecosystems are in better shape with regard to eutrophication today than 20 years ago, although many problems remain to be solved.

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