8.7 Nutrient cycles

Water cycle (ESGBD)

Over two thirds of the Earth's surface is covered by water. It forms an important component of most life forms, with up to \(\text{70}\%\) of plants and animals being composed of water. Vast quantities of water cycle through Earth's atmosphere, oceans, land and biosphere. This cycling of water is called the water or hydrological cycle. The cycling of water is important in determining our weather and climate, supports plant growth and makes life possible.

• Evaporation: Most water evaporates from the oceans, where water is found in highest abundance. However some evaporation also occurs from lakes, rivers, streams and following rain.

• Transpiration: Is the water loss from the surface area (particularly the stomata) of plants. Transpiration accounts for a massive \(\text{50}\%\) of land-based evaporation, and \(\text{10}\%\) of total evaporation.

• Evapotranspiration: The processes of evaporation and transpiration are often collectively referred to as evapotranspiration.

• Condensation: The process by which water vapour is converted back into liquid is called condensation. You may have observed a similar process occurring when dew drops form on a blade of grass or on cold glass. Water in the atmosphere condenses to form clouds.

• Precipitation: Water returns to Earth through precipitation in the form of rain, sleet, snow or ice (hail). When rain occurs due to precipitation, most of it runs off into lakes and rivers while a significant portion of it sinks into the ground.

• Infiltration: The process through which water sinks into the ground is known as infiltration and is determined by the soil or rock type through which water moves. During the process of sinking into the Earth's surface, water is filtered and purified. Depending on the soil type and the depth to which the water has sunk, the ground water becomes increasingly purified: the deeper the water, the cleaner it becomes.

• Melting and freezing: Some water freezes and is 'locked up' in ice, such as in glaciers and ice sheets. Similarly, water sometimes melts and is returned to oceans and seas.

The processes involved in the water cycle are shown in Figure 8.21.

Oxygen cycle (ESGBF)

Oxygen is one of the main gases found in the air, along with nitrogen. Oxygen is re-cycled between the air and living organisms in the following ways:

• Breathing and respiration: organisms such as animals and plants take in oxygen from the air during breathing and gaseous exchange processes. The oxygen is used for cellular respiration to release energy from organic nutrients such as glucose.
• Photosynthesis: during photosynthesis, plants absorb carbon dioxide from the air to synthesise sugars, and release oxygen.
• There is a complementary relationship between photosynthesis and cellular respiration in that the former produces oxygen and the latter consumes oxygen.

The oxygen cycle is shown in Figure 8.22.

Carbon cycle (ESGBG)

Carbon is the basic building block of all organic materials, and therefore, of living organisms. Most of the carbon on earth can be found in the crust. Other reservoirs of carbon include the oceans and atmosphere.

Carbon moves from one reservoir to another by these processes:

• Combustion: Burning of wood and fossil fuels by factory and auto emissions transfers carbon to the atmosphere as carbon dioxide.
• Photosynthesis: Carbon dioxide is taken up by plants during photosynthesis and is converted into energy rich organic molecules, such as glucose, which contains carbon.
• Metabolism: Autotrophs convert carbon into organic molecules like fats, carbohydrates and proteins, which animals can eat.
• Cellular respiration: Animals eat plants for food, taking up the organic carbon (carbohydrates). Plants and animals break down these organic molecules during the process of cellular respiration and release energy, water and carbon dioxide. Carbon dioxide is returned to the atmosphere during gaseous exchange.
• Precipitate: Carbon dioxide in the atmosphere can also precipitate as carbonate in ocean sediments.
• Decay: Carbon dioxide gas is also released into the atmosphere during the decay of all organisms.

Photosynthesis and gaseous exchange are the main carbon cycling processes involving living organisms. Figure 8.23 depicts the carbon cycle.

Nitrogen cycle (ESGBH)

Nitrogen (N$_{2}$ ) makes up most of the gas in the atmosphere (about \(\text{78}\%\)). Nitrogen is important to living organisms and is used in the production of amino acids, proteins and nucleic acids (DNA, RNA).

• Nitrogen gas present in the air is not available to organisms and thus has to be made available in a form absorbable by plants and animals.
• Only a few single-cell organisms, like bacteria can use nitrogen from the atmosphere directly.
• For plants, nitrogen has to be changed into other forms, eg. nitrates or ammonia. This process is known as nitrogen fixation.

The nitrogen cycle involves the following steps:

• Lightning: Nitrogen can be changed to nitrates directly by lightning. The rapid growth of algae after thunderstorms is because of this process, which increases the amount of nitrates that fall onto the earth in rain water, acting as fertiliser.
• Absorption: Ammonia and nitrates are absorbed by plants through their roots.
• Ingestion: Humans and animals get their nitrogen supplies by eating plants or plant-eating animals.
• Decomposition: During decomposition, bacteria and fungi break down proteins and amino acids from plants and animals.
• Ammonification: The nitrogenous breakdown products of amino acids are converted into ammonia (NH$_{3}$ ) by these decomposing bacteria.
• Nitrification: Is the conversion of the ammonia to nitrates (NO$_{3}$ $^{-}$ ) by nitrifying bacteria.
• Denitrification: In a process called denitrification, bacteria convert ammonia and nitrate into nitrogen and nitrous oxide (N$_{2}$ O). Nitrogen is returned to the atmosphere to start the cycle over again.

The nitrogen cycle is shown in Figure 8.24.