Community Level Systems: The Nitrogen Cycle and the Water Cycle

The Nitrogen Cycle

Nitrogen and Life

Nitrogen is crucial to life on Earth in its various forms, akin to carbon. It’s a necessary component of proteins, nucleic acids, and other vital organic molecules.
Nitrogen gas (N₂), which makes up around 79% of the Earth’s atmosphere, is not directly usable by most organisms.

Nitrogen Fixation, Nitrification, and Denitrification

The nitrogen cycle involves the transformation of nitrogen from its inert form (N₂) into forms usable by organisms through several processes.
Nitrogen fixation is the conversion of nitrogen gas into ammonium (NH₄+) by bacteria like Rhizobium, which live in the root nodules of leguminous plants, and by lightning.
Nitrification involves two steps: firstly, ammonia is oxidised into nitrite (NO₂-) by nitrosomonas bacteria; secondly, nitrite is oxidised into nitrate (NO₃-) by nitrobacter bacteria.
Denitrification, carried out by anaerobic bacteria, turns nitrates into nitrogen gas, which is then released back into the atmosphere.

The Role of Animals and Plants

Animals acquire nitrogen by consuming plants (or other animals). Upon the death and decay of these organisms, the nitrogen they contain is converted back to ammonium by decomposers.
Plants absorb nitrates from the soil through their roots, converting the nitrates into proteins and other essential organic molecules.

The Water Cycle

The Hydrosphere

The hydrosphere, comprising the combined mass of water on, under, and above the surface of the Earth, is subject to a constant cycle of movement and change.
Water is essential for all known forms of life and plays a crucial role in various global cycles like the carbon cycle and the nitrogen cycle.

Precipitation, Evaporation and Condensation

Precipitation: Water falls from the atmosphere to the Earth’s surface as rain, snow, or sleet. Some of this water is retained in plants, animals, or the soil, while the rest flows into rivers and oceans.
Evaporation: The Sun’s heat causes water from the surface of seas, rivers and lakes to evaporate and become water vapour, which rises into the atmosphere. Plants also lose water to the atmosphere through transpiration.
Condensation: As water vapour rises, it cools and condenses to form tiny droplets, which clump together to form clouds.

Runoff and Infiltration

Runoff: When rain falls, it can flow over the surface of the ground. This is known as runoff and it ends up in rivers, lakes, and oceans.
Infiltration: Some of the rainfall seeps into the ground; this is called infiltration and it replenishes groundwater supplies.

Humans and the Water Cycle

Human activities such as deforestation, urbanisation, and agriculture can impact the water cycle, changing the rate of evaporation, condensation, and precipitation.
Ensuring a sustainable water cycle is a key consideration for managing water resources, reducing flood risk, and maintaining ecosystem health.