生物地球化学的循環とは？

Living organisms require biogeochemical cycles because the matter that makes up life—including water, carbon, nitrogen, and phosphorus—exists in limited quantities within ecosystems. These essential elements must be conserved and recycled between biotic and abiotic components to remain available for organisms. Without recycling, nutrients would become depleted and life could not be sustained.

Global biogeochemical cycles involve elements with gaseous forms, such as water, carbon, nitrogen, and sulfur, which cycle through terrestrial, aquatic ecosystems, and the atmosphere. Local biogeochemical cycles involve heavier elements like phosphorus that cannot form gases and circulate only through terrestrial and aquatic ecosystems. Both types are connected through geological processes like tectonic plate uplift and rock weathering.

Decomposition is a key process that breaks down organic material and recycles nutrients back into ecosystems. The rate of decomposition depends on temperature, moisture, and nutrient availability. For example, organic material decomposes much faster in rainforests with warm, moist conditions compared to temperate environments with lower temperatures and seasonal climates.

Geological processes connect local and global biogeochemical cycles by moving matter between different ecosystem compartments. Key processes include tectonic plate uplift, which brings buried materials to the surface; weathering and erosion of rock deposits, which release elements into soil and water; and water drainage, which transports nutrients through terrestrial and aquatic environments.

Human activities can significantly alter biogeochemical cycles by adding excess nutrients to ecosystems. For example, agricultural phosphorus runoff into Lake Erie caused eutrophication, promoting toxic cyanobacteria blooms. Excessive nutrient levels from human sources can overwhelm natural cycling processes, leading to harmful ecological imbalances and water quality degradation.

Four key elements cycle through the atmosphere as gases: water, carbon, nitrogen, and sulfur. Because these elements have gaseous forms, they participate in global biogeochemical cycles that span terrestrial ecosystems, aquatic environments, and the atmosphere. This atmospheric circulation allows these nutrients to be distributed widely across different ecosystems.

Phosphorus is too heavy to occur as a gas, so it cannot enter the atmosphere and remains confined to local biogeochemical cycles. Unlike gaseous elements such as nitrogen and sulfur, phosphorus circulates only through terrestrial and aquatic ecosystems. This limitation means phosphorus availability depends on local geological processes like rock weathering and water drainage rather than atmospheric transport.