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, 368 (1621), 20130116

Consequences of Human Modification of the Global Nitrogen Cycle

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Consequences of Human Modification of the Global Nitrogen Cycle

Jan Willem Erisman et al. Philos Trans R Soc Lond B Biol Sci.

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.

Figures

Figure 1.
Figure 1.
Distribution of Nr 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 Nr deposition exceeds 5 kg N ha−1 yr−1, but is increasing over the period 2000–2030.
Figure 2.
Figure 2.
The exceedance (red bar) of the effects levels of Nr for ecosystems or human population, and the contribution of Nr (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 Nr 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 formula image in air, water or food; contribution of Nr, 80% (20% of the exposure to above-recommended Nr is due to natural sources). Air pollution (human health): exceedance, 60% of global population exposed to air quality above recommended safe levels; contribution of Nr, 20% of the formation of fine particles is due to human-caused Nr. Air pollution (crop loss): exceedance, 4% of global crop loss owing to air pollution; contribution of Nr, 50% of crop loss is due to human-caused Nr, primarily through tropospheric ozone enrichment. Freshwater pollution: exceedance, 10% of freshwater ‘systems’ area where NO3-N exceeds 1 mg l−1; contribution of Nr, 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 Nr, 15% of global biodiversity loss estimated to be due to Nr. Coastal zone dead zones: exceedance, 80% of large marine ecosystems (64 in total) ‘with a Nr problem’; contribution of Nr, 50% of global coastal zone pollution estimated to be due to Nr. Climate change: exceedance, 20% of the pre-industrial N2O concentration; contribution of Nr, net cooling of 15% due to all Nr impacts on drivers of radiative forcing. Stratospheric ozone: exceedance, 20% of the pre-industrial N2O concentration; contribution of Nr, 40% of all stratospheric ozone depletion is estimated due to Nr.

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