17.3 How matter is cycled through Ecosystems

Nature cycles materials through ecosystems, unlike humans who often discard unwanted items.
Elements like carbon, nitrogen, sulfur, and calcium cycle through ecosystems.
The cycle involves producers being eaten by herbivores, herbivores by carnivores, and carnivores by top carnivores.
When top carnivores die and decay, their carbon atoms become part of the soil, restarting the cycle.
A biogeochemical cycle is a closed cycle where water, nitrogen, phosphorus, or carbon pass from the nonliving environment to the living environment and back.
Ecologists refer to this as cycling between a living and non-living reservoir within an ecosystem.
The amount of a substance is typically lower in the living reservoir compared to the non-living reservoir.

Water is a crucial nonliving component of an ecosystem.
The water cycle starts in the nonliving reservoir, with water vapor in the atmosphere condensing and falling as rain or snow.
Some water becomes groundwater after being absorbed by the soil, while most evaporates back into the atmosphere.
Water enters the living part of the cycle through absorption by plant roots and then evaporates via transpiration.
Transpiration is a sun-driven process where heat draws moisture out of tiny openings in plant leaves.
The water cycle is essential for maintaining healthy ecosystems. It is a sequence where water:
- Passes into the atmosphere as water vapor.
- Precipitates back to Earth as liquid or solid forms.
- Returns to the atmosphere through evaporation.

The carbon cycle is a vital cycle balancing the non-living and living components of an ecosystem.
It begins in the air or water, where carbon is absorbed by photosynthesizing organisms (bacteria, plants, or algae) as raw materials to create organic molecules.
Carbon atoms return to the atmosphere or water through:
- Respiration: Carbon dioxide is a byproduct of cellular respiration, where organisms use oxygen to oxidize organic molecules.
- Combustion: Carbon is released into the atmosphere during burning (combustion).
- Decomposition and Fossilization: Decomposing organisms in sediments can be transformed into fossil fuels (oil, coal, and natural gas) via pressure and heat, which release carbon when burned.
- Erosion: Carbonate shells of marine organisms decompose to form limestone, releasing carbon when eroded.

The phosphorus cycle starts in soil and rock, where phosphorus exists as calcium phosphate.
When dissolved in water, it forms phosphate ions $(PO_4^{3-})$ , which are absorbed by plant roots to build organic molecules.
Animals obtain organic phosphorus by consuming plants.

Nitrogen fixation is essential for plants to utilize nitrogen.
Certain bacteria have enzymes that can break down nitrogen and bind it to hydrogen to form ammonia $(NH_3)$ .
These bacteria are found in soil and in the nodules of alder trees, beans, and other plants.
The nitrogen cycle involves four stages:
- Assimilation: Absorption and incorporation of nitrogen into organic compounds by plants.
- Ammonification: Production of ammonia by bacteria during the decay of organic matter.
- Nitrification: Production of nitrate from ammonia.
- Denitrification: Conversion of nitrate to nitrogen gas.
Genetic engineers are trying to incorporate nitrogen-fixing genes into plant chromosomes to enable plants to fix nitrogen themselves, reducing the need for nitrogen fertilizers.