Consequences of human modification of the global nitrogen cycle

TLDR

Increasing food demand and energy use drive fertilizer application and fossil fuel combustion, amplifying reactive nitrogen losses that exceed thresholds for drinking water, air quality, eutrophication, biodiversity, ozone, climate, and coastal ecosystems, with a single nitrogen atom capable of triggering a cascade of negative impacts. The authors aim to review reactive nitrogen’s environmental and human health impacts, assessing the magnitude of various problems and nitrogen’s relative contribution to each. They synthesize data on reactive nitrogen losses and their cascading effects across ecosystems and human systems. Reactive nitrogen is the primary driver of terrestrial and coastal eutrophication and nitrous oxide emissions, while in other contexts it exacerbates broader issues such as freshwater pollution and biodiversity loss, often obscuring its central role in trans‑boundary pollution.

Abstract

The demand for more food is increasing fertilizer and land use, and the demand for more energy is increasing fossil fuel combustion, leading to enhanced losses of reactive nitrogen (N r ) to the environment. Many thresholds for human and ecosystem health have been exceeded owing to N r pollution, including those for drinking water (nitrates), air quality (smog, particulate matter, ground-level ozone), freshwater eutrophication, biodiversity loss, stratospheric ozone depletion, climate change and coastal ecosystems (dead zones). Each of these environmental effects can be magnified by the ‘nitrogen cascade’: a single atom of N r can trigger a cascade of negative environmental impacts in sequence. Here, we provide an overview of the impact of N r on the environment and human health, including an assessment of the magnitude of different environmental problems, and the relative importance of N r as a contributor to each problem. In some cases, N r loss to the environment is the key driver of effects (e.g. terrestrial and coastal eutrophication, nitrous oxide emissions), whereas in some other situations nitrogen represents a key contributor exacerbating a wider problem (e.g. freshwater pollution, biodiversity loss). In this way, the central role of nitrogen can remain hidden, even though it actually underpins many trans-boundary pollution problems.