Nitrogen Pollution in a Global Perspective

Nitrogen fixation (the conversion of inert nitrogen to ammonium, ammonia, nitrate, or nitrogen oxides) is a process which occurs naturally, but can also occur through anthropogenic processes. Natural nitrogen fixation is performed by a number of microorganisms, called diazothrops. Well-known examples of diazothrops are the Rhizobium bacteria, which can be found in the root nodules of higher plants (especially leguminous plants). A second form of natural nitrogen fixation is as a result of lightning, which converts inert nitrogen into nitrogen oxides. Human activities, however, now have by far the highest contribution to global nitrogen fixation. These include the synthesis of fertilizers, fuel combustion, and industrial nitrogen fixation.

As a result of such activities, the anthropogenic inputs of NOj, and NHX to ecosystems have increased tenfold in 100 years, and at present account for the majority (65%) of the total global nitrogen budget. Estimates for the current global anthropogenic nitrogen fixation of 140TgNyr_1 show that 15% originates from fuel combustion, 24% stems from the cultivation of rice and nitrogen fixating crops for food production, and 61% comes from the production of fertilizers via the chemical conversion of N2 to NH3 (the Haber-Bosch process). Of the different regions in the world, North America (28.4TgNyr_1), Europe and the former Soviet Union (26.5TgNyr_1), and Asia (68.9 TgNyr-1) fix the majority of global reactive nitrogen.

Since the turnover of the reactive nitrogen to inert N2 (via the process of denitrification) is lower than the production of reactive nitrogen, nitrogen may accumulate in different chemical forms and severely disrupt N cycles in the atmosphere, and in terrestrial and aquatic ecosystems.

An overview of the global conversions and transfers of nitrogen between the atmosphere, and terrestrial and aquatic ecosystems, and its effects at different levels is given by the nitrogen cascade depicted in Figure 1.

Industrial nitrogen fixation and fuel combustion are together responsible for the vast majority of NO^ in anthropogenic nitrogen fixation. NO^ is almost completely emitted to the atmosphere, where it can increase concentrations of gases such as ozone via photochemical reactions. In the lower atmosphere, the troposphere, nitrogen oxides can form a variety of nitrate containing organic compounds (e.g., peroxyacytyl nitrate, PAN) and contribute to formation of excessive levels of ozone after reactions with volatile organic compounds (VOCs); both PAN and ozone have ecotoxicological effects. In addition, nitrogen oxides can form the acidic HNO2 and HNO3 which can have ecotoxicological effects when deposited on aquatic and terrestrial ecosystems.

The production of food (which includes nitrogen fixation via fertilizer production and cultivation) and other agricultural practices account largely for the fixation of nitrogen as NH3 and NH^. Nitrogen in these forms can be released either directly into the soil and soil pore water (e.g., via application of fertilizers in agriculture), directly

Energy production

Food production

NhL NO,

People (food; fiber)

Human activities

Atmosphere

The nitrogen cascade

Increased ozone in troposphere

Terrestrial ecosystems

Formation of

nitrate and

ammonium

aerosols

Transport and deposition of N and acidity

Transport and deposition of N and acidity

Agricultural ecosystems effects

Crop

Animals

Soil

J = denitrification to N2

Aquatic ecosystems

Agricultural ecosystems effects

(Semi) natural ecosystems effects

Vegetation

*

Animals

Soil

<—

NH4+, NH3 and mainly NO-

Groundwater effects

Surface freshwater ecosystem effects

N2O, NO

Surface freshwater ecosystem effects

J

Coastal

Ocean

effects

J

effects

Figure 1 Diagram of the nitrogen cascade, illustrating the different sources and sinks of nitrogen forms in the atmosphere, terrestrial-and aquatic ecosystems. Adapted from Galloway etal. 2003. Some key processes are emitted from the original diagram because they are not relevant to this article.

into aquatic bodies (e.g., via effluent from sewage works or from runoff from agricultural sources), or into the atmosphere (e.g., from industrial and agricultural sources). Subsequently, fixed nitrogen may be deposited from the atmosphere to terrestrial and aquatic ecosystems.

Although part of the anthropogenic reactive nitrogen is removed from ecosystems in the form of food and raw materials, this is only a small proportion of the amount which is fixed to produce the food and raw materials. Anthropogenic inputs to aquatic ecosystems from ground-water and surface runoff, from wastewater treatment runoff, and from uncontrolled sewage waters account for the majority of the remaining anthropogenic input of nitrogen into water systems.

Different forms of reactive nitrogen can cause either direct ecotoxic effects, or indirect effects by altering bio-geochemical pathways or by interactions with secondary pollutants. In addition, it strongly depends on the source where, and in what nitrogen form, ecotoxic effects of nitrogen occur The ecotoxicity of reactive nitrogen is manifested differently within and between terrestrial and aquatic systems, and therefore these are now considered in turn.

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