The Nitrogen Cycle

The level of soil nitrate in agricultural ecosystems can be increased by adding fertilisers. 

Plants & Animals die > decomposition begins > microorganisms replenish nitrate levels in soil.

Four main stages of the Nitrogen Cycle: Ammonification, Nitrification, Nitrogen Fixation & Denitrification, which all involve saprobiotic microrganisms.

Ammonification (production of ammonia from organic ammonia-containing compounds)

> E.g. Urea, proteins, nucleic acids & vitamins.

> Saprobiotic microorganisms (mainly fungi & bacteria) feed on these materials releasing ammonia, which forms ammonia ions in the soil. This is where Nitrogen returns to the non-living part of the ecosystem.

Nitrification

> Some bacteria obtain their energy from reaction involving inorganic ions. E.g. Ammonia ions > Nitrate ions = OXIDATION reaction, so releases energy. Carried out by Nitrifying Bacteria. Occurs in two stages:

  1. Oxidation of ammonia ions to Nitrite ions (NO2ֿ)

  2. Oxidation of Nitrite ions to nitrate ions (NO3 ֿ)

Nitrifying bacteria require oxygen for this process so they need soil with many air spaces. Farmers try and keep soil light and well aerated by ploughing as well as being well drained so air spaces do not become filled with water.

Nitrogen (g) > nitrogen-containing compounds. Can be carried out industrially or naturally when lightning passes through the atmosphere / by microorganisms:

  1. Nitrogen-fixing bacteria release gaseous nitrogen to ammonia, which they then use to manufacture amino acids. Nitrogen-rich compounds are released from them when they die & decay.

  2. Mutualistic nitrogen-fixing bacteria (live in nodules on roots) obtain carbohydrates from plant and plant acquires amino acids from bacteria.

Denitrification

In waterlogged soil with little oxygen, less aerobic nitrifying and nitrogen-fixing bacteria found and more anaerobic denitrifying bacteria. Convert soil nitrates to gaseous nitrogen. Reduces amount of nitrogen-containing compounds for plants. Soil must be well aerated to prevent build-up of denitrifying bacteria.

Intensive food production makes large demands on the soil because mineral ions are continually being taken up by the crops growing on them. Mineral ions not replaced because crops either used as food or fodder for animals which get eaten.

Fertilisers become necessary otherwise levels of mineral ions becomes main limiting factor > reduced productivity. Two types:

> Natural (organic) fertilisers = consist of the dead and decaying matter of plants / animals as well as animal waste (manure, bone meal)

> Artificial (inorganic) fertilisers = mined from rocks/deposits then converted into different forms & blended to appropriate balance of minerals for particular crop. Compounds containing Nitrogen, Phosphorus & Potassium usually present.

Research suggests combination of natural & artificial fertilisers gives greatest increase in long-term productivity. Remember there is a point at which further addition of fertilisers no longer increases productivity.

E.g. Nitrogen fertilisers. Nitrogen needed for production of proteins & DNA > if adding Nitrogen then plants grow quicker, taller & with greater leaf area > increased rate of photosynthesis / productivity = cheaper food production.

Use of Nitrogen-containing fertilisers has had detrimental effects:

> Reduced species diversity: nitrogen-rich soils favour growth of fast-growing crops such as grasses & nettles which out-compete other species > die as a result.

> Leaching: can lead to pollution of watercourses.

> Eutrophication: caused by leaching of fertiliser into watercourses.

Leaching

Process by which nutrients leave the soil. Rain water will dissolve any soluble nutrients (nitrates) and carry them deep into soil beyond reach of plant roots. Nitrates end up in streams and rivers which could lead to freshwater lakes. Harmful effect if lake used for drinking water. Can also cause eutrophication.

1. Nitrate levels usually low in lakes and rivers = limiting factor to plant / algal growth.

2. Nitrate concentration increases as a result of leaching > algae / plants gow exponentially.

3. Upper layers of water become densely populated with algae = 'algal bloom'

4. Dense layer of algae absors light, preventing it penetrating lower depths.

5. Light = limiting factor for algal / plant growth at lower depths > eventually die.

6. Saprobiotic algae grow exponentially using dead plants as food.

7. Saprobiotic algae require oxygen > increased demand for oxygen

8. Oxygen concentration in water reduced and nitrates are released from decaying organisms.

9. Oxygen = limiting factor for aerobic organisms (fish) > eventually die

10. Less competition for anaerobic organisms > grow exponentially

11. Anaerobic organisms further decompose dead matter, releasing nitrates / toxic materials e.g. hydrogen sulphide which make the water putrid.