2.3.2 [HL] Nitrogen Cycles

Human activities change the nitrogen cycle

Agriculture

•Nitrogen fixation via industrial techniques, such as the Haber process

•increased the amount of global nitrogen fixation, leading to increased amounts of usable nitrogen in the form of fertilizers

•Nitrate fertilizer, used to increase crop yield, runs off or leaches into bodies of waters, such as rivers and lakes, causing eutrophication and disruption to

ecosystems

•Eutrophication leads to low O2 levels in aquatic ecosystems, changing food-web structure and resulting in habitat degradation. The addition of nitrogen can lead to changes in biodiversity and species composition that may lead to changes in overall ecosystem function.

Other Human Factors

Aquaculture
- fish release NH 3 as an excretory product.
Deforestation
- Forest trees store nitrogen in the form of amino acids and protein
Urbanization
- Traffic in cities has historically used petrol derived from fossil fuels. Fossil fuels contain nitrogen, so when they are burned, nitrogen oxides are released into the atmosphere.
This contributes to the formation of smog
- Increases in the human population have led to increased food needs and production.
- Increase in food needed, fertilizers (many of which contain nitrogen in the form of NO3- ions) are used to increase crop yield
- Increased sewage output leads to increased quantities of NH4+ and NO3- ions in rivers, lakes and the sea.

Harber Process

The Haber process is an industrial process that produces NH3 from N2 gas and hydrogen gas (H2) for use as fertilizer. The process takes place under high temperatures and pressures and the reaction between N2 and H2 is made possible by using an iron catalyst

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The benefit of the Haber process is that it provides NH3 for fertilizers. Nitrogen-based fertilizers provide nitrogen, which plants need for growth. Using nitrogenous fertilizers can lead to increased crop yields. This has enabled sufficient food to be produced for the growing global human population. The NH3 produced can also be used in other processes

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There are disadvantages of the Haber process. Soil fertilizers are easily soluble in water and are therefore easily transported from soil to bodies of water by land run-off and soil leaching. This leads to increased nitrogen in lakes, rivers and other water resources, which can lead to eutrophication. Eutrophication leads to the reduction of biodiversity in aquatic systems. The Haber process also requires high levels of energy, derived from fossil fuels, which results in high greenhouse gas emissions (e.g. CO2)

Applications of the Haber Process

The lithosphere

The lithosphere is the solid, rocky outer layer of the Earth, consisting of the crust, the outermost layer of the mantle and soil. The lithosphere contains carbon stores in fossil

Fuels and in rocks such as limestone that contain calcium carbonate. Fossil fuels arer formed by the decomposition and decay of dead organisms over millions of years
Limestone is produced from shells and reef-building coral, which contain calcium carbonate

The residence time of carbon is the average length of time it remains in any carbon store. This ranges from long-term (millions/thousands of years) to short-term (tens/hundreds of years). For example, the residence time for carbon in the atmosphere and terrestrial biomass is short term. The residence time for carbon in limestone and fossil fuel stores is long term and can be hundreds of millions of years

Reef-building corals and molluscs can become fossilized in limestone

Corals are colonies of small animals (polyps) embedded in a skeleton that they secrete. They form underwater structures, known as coral reefs, in warm, shallow water where sunlight penetrates. The hard parts of reef-building corals (the coral skeleton) and molluscs contain calcium carbonate that can become fossilized in limestone. Limestone is the largest store of carbon in Earth systems. Not all limestone is formed by fossilization of animal remains. It can also be formed by both biological and non-biological processes.

Formation of coal, oil and gas

In past geological eras, organic matter from partially decomposed plants became fossilized in coal. Organic matter from partially decomposed marine organisms became fossilized in oil and natural gas held in porous rocks. Formation of fossil fuels – coal, oil and gas – was greatest in specific geological eras when conditions were most suitable for the preservation of organic matter. The accumulation of significant stores took tens of millions of years.

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