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Narrow-spectrum pesticide - An “ideal” pesticide that only kills the organism for which it is intended and does not harm any other species. Compare wide-spectrum pesticide.

National emission limitations - The maximum permissible amount (by law) of a particular pollutant that can be discharged into the nation’s rivers, lakes, and oceans from point sources.

Natural forest management system - Controlled and regulated harvesting of forest trees, combined with silvicultural and protective measures, to sustain and increase the commercial value of subsequent stands; relies on natural regeneration of native species.

Natural resources -Goods and services – coal, fresh water, clean air, arable land, and wildlife, for example – that are supplied by the environment.

Nitrification - The conversion of ammonia (NH3) to nitrate (NO3-) by certain bacteria (nitrifying bacteria); part of the nitrogen cycle.

Nitrogen cycle - The worldwide circulation of nitrogen from the abiotic environment into living things and back into the abiotic environment.

Nitrogent fixation - The conversion of atmospheric nitrogen gas to ammonia, nitrates, and other nitrogen – containing compounds, by algae. The nitrogen-fixing bacteria include clostridium and azobacter (which are free-living and are believed to contribute minimally to soil nitrogen) and rhezobium (which lives simbyotically in root modules). Atmospheric nitrogen fixation can be caused by lightning.

Nondegradable - A chemical pollutant (such as the toxic elements mercury and lead) that cannot be decomposed (broken down) by living organisms or other natural processes. Compare biodegradable.

Nonmunicipal solid waste - Wastes generated by industry, agriculture, and mining.

Non-point-source pollution - Pollutants that enter bodies of water over large areas rather than being concentrated at a single point of entry. Examples include agricultural fertilizer runoff and sediments from construction. Compare point-source pollution.

Nonrenewable resources - Natural resources that are present in limited supplies and are depleted by use; include minerals such as copper and tin and fossil fuels such as oil and natural gas. Compare renewable resources.

Non-renewable water - Water in aquifers and other natural reservoirs that are not recharged, or are recharged so slowly that significant withdrawals will cause depletion.

No-tillage - A method of conservation tillage that leaves both the surface and subsurface soil undisturbed. Special machines punch holes in the soil for seeds. See conservation tillage.

Nutrient cycling - The process of retaining and efficiently recycling essential nutrients and micronutrients within the ecosystem.

Nutrient depletion - The detrimental removal of nutritional elements from the soil.