Biogeochemical Cycle
Biogeochemical cycles: overview
Biogeochemical cycles: integration of biological, geological, and chemical aspects of nutrient cycles.
Recycling of inorganic matter between living organisms and their nonliving environment.
Each cycle has a reservoir (nutrient pool) and an exchange (cycling) pool; elements move between abiotic and biotic components.
Six elements commonly in organic molecules: C, N, H, O, P, S.
Cycles can be classified as:
Gaseous cycles (reservoirs in air/ocean): nitrogen, oxygen, carbon, and water.
Sedimentary cycles (reservoirs in Earth's crust): phosphorus, sulfur, iron, calcium, etc.
Energy flow vs. matter flow:
Energy enters as solar energy and exits as heat; it does not recycle.
Matter (biogeochemical components) cycles within the biosphere.
Energy flow in the biosphere
Energy enters the system as solar energy and flows directionally through the ecosystem.
Producers (plants) convert solar energy into chemical energy via photosynthesis.
Consumers (herbivores, carnivores) obtain energy by consuming other organisms.
Decomposers (bacteria, fungi) recycle nutrients back to the abiotic environment.
At each step, energy is eventually lost as heat; matter is recycled within the system.
Water cycle (Hydrologic cycle)
Precipitation: water falls from atmosphere to Earth's surface.
Infiltration: water enters the soil from the ground surface.
Percolation: water moves through soil to groundwater reserves.
Transpiration: water loss from plants via stomata.
Evaporation: liquid water turns into water vapor.
Condensation: water vapor turns into liquid.
Surface runoff: water moves over the land surface.
Carbon cycle
Major reservoirs: atmosphere (CO₂), oceans, biomass, soils, and fossil fuels.
Processes:
Photosynthesis:
6CO2 + 6H2O \rightarrow C6H{12}O6 + 6O2Respiration:
C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O + ATPFossil fuel combustion releases CO₂; decomposition releases CO₂; oceans dissolve CO₂ forming carbonates.
Carbon cycle dynamics:
Plants fix CO₂ into organic matter; animals respire releasing CO₂ back to the atmosphere.
Ocean-atmosphere exchange and carbonate rock formation (limestone) store carbon long-term.
Human impact: burning fossil fuels increases atmospheric CO₂, influencing climate; oceans and soils act as buffers but may become saturated.
Nitrogen cycle
Atmosphere: ~78% N₂; nitrogen is essential for amino acids and nucleic acids.
Key processes and players:
Nitrogen fixation: N₂ → NH₃ (ammonia) by nitrogen-fixing bacteria (root nodules in legumes; cyanobacteria in aquatic systems).
Nitrification: NH₃ → NO₂⁻ → NO₃⁻ (bacteria such as Nitrosomonas convert ammonia to nitrite, then to nitrate).
Ammonification: organic N → NH₃ (decomposition releases ammonia).
Denitrification: NO₃⁻ → N₂ (returning nitrogen to atmosphere).
Plant uptake: plants absorb NH₄⁺ or NO₃⁻ from the soil.
Humans indirectly affect the cycle via fertilizer use and fossil fuel combustion.
Quick reference: key terms and reactions
Nitrogen fixation: N₂ → NH₃ (bacteria, legumes, cyanobacteria).
Nitrification: NH₃ → NO₂⁻ → NO₃⁻ (Nitrosomonas → Nitrobacter).
Ammonification: organic N → NH₃.
Denitrification: NO₃⁻ → N₂.
Carbon: a foundational element in organic molecules; central to energy flow and storage.
Photosynthesis:
6CO2 + 6H2O \rightarrow C6H{12}O6 + 6O2Respiration:
C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O + ATPReservoir vs. exchange pool (biogeochemical cycles): long-term storage vs. ongoing cycling.
Practice prompts (brief prompts for quick recall)
Why are biogeochemical cycles important for living systems?
They balance nutrient availability and support energy flow without depleting essential elements.
How do human activities affect biogeochemical cycles?
They can increase the rate or magnitude of fluxes (e.g., CO₂ from fossil fuel burning; nutrient runoff causing eutrophication; nitrogen deposition affecting fixation balance).
Are you part of these cycles?
Yes: you participate in cycles via respiration, consumption, and nutrient exchange with the environment.