Anthropogenic changes in the nitrogen cycle


The alteration of the nitrogen cycle is not uniform over the planet. Regional variation is great, leading to the need for assessments at regional to continental scales. The variation within North America reflects the global condition, with very little change in some regions and vast changes in others. The increase in nitrogen fluxes in rivers is one measure of this change.

Regions of North America vary from having seen virtually no influence of human activity (Labrador and Hudson’s Bay) to having seen more than a 10-fold increase due to human activity.

  On average, US river nitrogen fluxes have been increasing at 1% per year for the past two decades, a trend that is expected to continue unless there is major policy intervention.

Variation in the deposition of nitrogen from the atmosphere (part of acid rain) is also great in North America, with rates from less than 50 kg N km-2 yr-1 to more than 1,000 kg N km-2 yr-1.


Important drivers of change include an intensification of agriculture (with greater use of synthetic nitrogen fertilizer and more concentrated animal production facilities) and the release of nitrogen pollutants during fossil fuel combustion.

 On a per capita basis, the US uses more synthetic nitrogen fertilizer and produces more nitrogen pollution from fossil fuels than any other country. Nitrogen fluxes from the landscape are also influenced by climate and climate change and by alteration of the landscape, such as drainage and loss of wetlands.

The consequences of accelerated nitrogen cycling are well known qualitatively and include a wide range of both ecological and human health effects.


For example, nitrogen oxides in the atmosphere create ground-level ozone pollution, which adversely affects both human health and the productivity of agricultural systems and forests.Nitrogen pollution is a major component of acid rain.

Nitrogen loads to coastal marine waters lead to eutrophication, which has degraded two thirds of the coastal waters of the US. Nitrous oxide produced by bacteria in nitrogen-rich soils and sediments is the most important biogenic greenhouse gas after carbon dioxide and methane.


Nitrogen can pose carcinogenic risks both from nitrate in drinking water and from fine particles in the atmosphere, and nitrogen pollution increases pollen production, aggravating asthma and allergies.

 Synthetic nitrogen fertilizer has been an essential component of the green revolution and has contributed to increased global food production and decreased hunger and malnutrition over the past several decades, so some increase in human fixation and use of nitrogen has clearly been good.

However, further acceleration of the nitrogen cycle can have large net negative effects on public health. While there have been many attempts to quantify discrete portions of the region’s nitrogen cycle in the past 30 years, a comprehensive assessment of sources and consequences of nitrogen pollution – a major goal of the North American Nitrogen Center – has not been previously attempted.



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 Last Updated 09 Jan 2007 by DPS