The oxygen cycle is not considered a key nutrient cycle in ecosystems because oxygen is not a limiting nutrient for most organisms.

Nitrogen fixation is the process by which nitrogen gas from the atmosphere is converted into a usable form for plants by certain bacteria and archaea.

Fossil fuels are not a major reservoir of carbon in the carbon cycle because they are not actively involved in the exchange of carbon between living organisms and the environment.

Decomposition is the process by which organic matter is broken down into simpler compounds by microorganisms, releasing nutrients back into the environment.

The atmosphere is not a major reservoir of phosphorus in the phosphorus cycle because phosphorus is not present in significant quantities in the atmosphere.

Denitrification is the process by which nitrates and nitrites are converted back into nitrogen gas by certain bacteria and archaea.

Rocks and minerals are not a major reservoir of nitrogen in the nitrogen cycle because nitrogen is not present in significant quantities in rocks and minerals.

Nitrification is the process by which ammonia is converted into nitrates and nitrites by certain bacteria and archaea.

Fossil fuels are not a major reservoir of carbon in the carbon cycle because they are not actively involved in the exchange of carbon between living organisms and the environment.

Decomposition is the process by which organic matter is broken down into simpler compounds by microorganisms, releasing nutrients back into the environment.

The atmosphere is not a major reservoir of phosphorus in the phosphorus cycle because phosphorus is not present in significant quantities in the atmosphere.

Denitrification is the process by which nitrates and nitrites are converted back into nitrogen gas by certain bacteria and archaea.

Rocks and minerals are not a major reservoir of nitrogen in the nitrogen cycle because nitrogen is not present in significant quantities in rocks and minerals.

Nitrification is the process by which ammonia is converted into nitrates and nitrites by certain bacteria and archaea.

Fossil fuels are not a major reservoir of carbon in the carbon cycle because they are not actively involved in the exchange of carbon between living organisms and the environment.