1.5 | Nitrogen Cycle

Overview

The Nitrogen Cycle is the movement of molecules containing nitrogen between sources and sinks/reservoirs. Sources release nitrogen into the atmosphere; sinks take nitrogen out of the atmosphere in increasing amounts

  • KEY DIFFERENCE- Nitrogen reservoirs hold nitrogen for relatively short periods of time compared to the carbon cycle (e.g., plants, soil, atmosphere. They hold nitrogen as long as they are alive)

The main nitrogen reservoir is the atmosphere, where it exists mostly as N₂ gas, and is unusable by plants or animals.

Nitrogen is a critical nutrient for plants and animals. All living things need nitrogen for DNA and amino acids to make proteins.

Nitrogen Fixation

How do living things take in nitrogen if most exists as unusable N₂ gas? The first step is Nitrogen Fixation. It is the first step in the Nitrogen Cycle and is critical to all life on Earth.

Nitrogen Fixation is the process of N₂ gas being converted into biologically available NH₃ (ammonia), or NO₃ (nitrate). This can be done naturally by way of nitrogen fixing bacteria, or by lightning storms that cause nitrogen to be converted in ammonia.

  • bacterial fixation: certain bacteria that live in the soil, or in symbiotic relationship with plant root nodules convert N₂ into NH₃

  • legumes: Rhizobacteria live in root nodules of legumes(peas, beans), and fix nitrogen for them in return for amino acids from the plants.

Nitrogen can also be fixed synthetically, when humans combust fossil fuels in order to fix N₂ into NO₃

  • Very energy intensive, but

  • The nitrate is added to synthetic fertilizers and are used in agriculture.

Other Nitrogen Cycle Steps

  • assimilation: plants and animals taking nitrogen in and incorporating it into their bodies

    • plants can access nitrogen directly via their roots, but animals have to eat plants in order to get their nitrogen, or eat other animals if they are carnivorous

  • ammonification: soil bacteria, microbes and decomposers converting waste and dead biomass back into NH₃

  • nitrification: the conversion of ammonium (NH₄+) into nitrite (NO₂), and then nitrate(NO₃) by soil bacteria

  • denitrification: the conversion of soil nitrogen (NO₃) into nitrous oxide gas (N₂O), which returns into the atmosphere

    • Converted by bacteria, eventually breaks down to form nitrogen gas and complete the cycle

Human Impacts on Nitrogen Cycle

  1. Climate - Nitrous Oxide (N₂O) is a greenhouse gas that warms the earth’s climate

    • Produced by the denitrification of nitrate in agricultural soils, especially when the soil is waterlogged or over watered

  2. Ammonia Volatilization - excess fertilizer use can lead to NH₃ gas entering the atmosphere

    • NH₃ gas in the atmosphere leads to acid rain and respiratory irritation in humans and animals

  3. Leaching & Eutrophication - synthetic fertilizer use leads to nitrates (NO3) leaching, or being carried out of soil by water