1.5 The Nitrogen Cycle

Overview

• Movement of nitrogen (N) molecules between sources and sinks.

• Sources release N into the atmosphere; sinks take N out of the atmosphere in relatively short time compared to the carbon cycle.

• Major reservoirs: plants, soil, atmosphere.

• Atmosphere is the main N reservoir; most N exists as N2 gas, which is not usable by plants or animals.

• N is a critical nutrient for DNA and amino acids (required to form proteins).

Nitrogen Fixation

• Process of converting N2 gas into biologically available NH3 (ammonia) or NO3- (nitrate).

  Ammonia (NH3) is a colorless gas with a pungent odor, produced in the nitrogen cycle through nitrogen fixation and organic matter decomposition, and is highly soluble in water.

• Bacterial fixation: Specific bacteria in soil or symbiotic relationships with plant root nodules convert N2 into ammonia (NH3).

• Synthetic fixation: Humans combust fossil fuels to convert N2 gas into nitrates (NO3-), which are added to synthetic fertilizers used in agriculture.

Other N Cycle Steps

These processes occur in the following order within the nitrogen cycle:

1. Assimilation: Plants and animals take in nitrogen (N) and incorporate it into their bodies; plants absorb nitrate (NO3-) or ammonia (NH3), and animals assimilate nitrogen by consuming plants or other animals.

2. Ammonification: Soil bacteria and decomposers convert waste and dead biomass into ammonia (NH3), returning it to the soil.

3. Nitrification: Soil bacteria convert ammonia (NH4) into nitrite (NO2-) and then into nitrate (NO3-).

4. Denitrification: Conversion of soil nitrate (NO3) into nitrous oxide (N2O) gas, which returns nitrogen to the atmosphere.

• Ammonification: Soil bacteria and decomposers convert waste and dead biomass into NH3 and return it to the soil.

• Nitrification: Conversion of NH4 into nitrite (NO2-) and then nitrate (NO3-) by soil bacteria.

• Denitrification: Conversion of soil N (NO3) into nitrous oxide (N2O) gas, returning it to the atmosphere.

Human Impacts

• Climate: N2O is a greenhouse gas that warms the climate, produced mainly by denitrification of nitrate in overwatered agricultural soils.

• Ammonia volatilization: Excess fertilizer use can release NH3 gas into the atmosphere, causing acid precipitation and respiratory irritation. It results in less N remaining in soil for crops.

• Leaching & Eutrophication: Synthetic fertilizers lead to nitrate leaching into local waters, causing algal blooms that block sunlight and kill aquatic plants.

The nitrogen-related compounds discussed in this section of the nitrogen cycle include:

1. N2 (Nitrogen gas): The most abundant form of nitrogen in the atmosphere, which is not directly usable by most organisms.

2. NH3 (Ammonia): A compound produced through nitrogen fixation and organic matter decomposition, highly soluble in water and used by plants.

3. NO3- (Nitrate): Formed through the nitrification process, it is a key form of nitrogen that plants assimilate for growth.

4. NH4+ (Ammonium): Intermediate form of nitrogen, also utilized by plants, and involved in the nitrification process.

5. N2O (Nitrous oxide): A greenhouse gas produced by denitrification, which can impact climate change.