Consequences of human modification of the global nitrogen cycle

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 (Nr) to the environment. Many thresholds for human and ecosystem health have been exceeded owing to Nr 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 Nr can trigger a cascade of negative environmental impacts in sequence. Here, we provide an overview of the impact of Nr on the environment and human health, including an assessment of the magnitude of different environmental problems, and the relative importance of Nr as a contributor to each problem. In some cases, Nr 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.

Keywords: biodiversity; climate; environmental effects; food security; human health; reactive nitrogen.

Figure 1.

Distribution of N_r deposition classes and exceedance of deposition levels in the period 2000–2030 on Protected Areas (PAs) under the Convention on Biological Diversity [41]. Red PAs show an exceedance of 10 kg N ha⁻¹ yr⁻¹ and deposition in 2030 higher than 2000. Orange PAs show a current exceedance, but deposition in 2030 lower than 2000. Yellow PAs might be under threat in the near future since N_r deposition exceeds 5 kg N ha⁻¹ yr⁻¹, but is increasing over the period 2000–2030.

Figure 2.

The exceedance (red bar) of the effects levels of N_r for ecosystems or human population, and the contribution of N_r (blue bar) to the total effect, relative to other components or causes (e.g. natural) of the problem. The figure extends from the local scale to the global/stratospheric scale and thus represents the N_r cascade (green arrow). The exceedance and contribution can be summarized as follows. Nitrate or nitrite intake: exceedance, 70% of global population exposed to above-recommended levels of either in air, water or food; contribution of N_r, 80% (20% of the exposure to above-recommended N_r is due to natural sources). Air pollution (human health): exceedance, 60% of global population exposed to air quality above recommended safe levels; contribution of N_r, 20% of the formation of fine particles is due to human-caused N_r. Air pollution (crop loss): exceedance, 4% of global crop loss owing to air pollution; contribution of N_r, 50% of crop loss is due to human-caused N_r, primarily through tropospheric ozone enrichment. Freshwater pollution: exceedance, 10% of freshwater ‘systems’ area where NO₃-N exceeds 1 mg l⁻¹; contribution of N_r, 40% relative to other freshwater pollution and natural causes. Biodiversity loss: exceedance, 50% of the total area of biodiversity hot spots in which N deposition exceeds 5 kg N ha⁻¹ yr⁻¹; contribution of N_r, 15% of global biodiversity loss estimated to be due to N_r. Coastal zone dead zones: exceedance, 80% of large marine ecosystems (64 in total) ‘with a N_r problem’; contribution of N_r, 50% of global coastal zone pollution estimated to be due to N_r. Climate change: exceedance, 20% of the pre-industrial N₂O concentration; contribution of N_r, net cooling of 15% due to all N_r impacts on drivers of radiative forcing. Stratospheric ozone: exceedance, 20% of the pre-industrial N₂O concentration; contribution of N_r, 40% of all stratospheric ozone depletion is estimated due to N_r.