Biogeochemical Cycles Explorer

The carbon cycle moves carbon through the atmosphere, oceans, biosphere, and geosphere. The atmosphere holds about 850 Gt C as CO₂, while oceans store roughly 38,000 Gt C as dissolved inorganic carbon.

Photosynthesis removes about 120 Gt C/yr from the atmosphere, while respiration and decomposition return a similar amount. Human fossil fuel burning adds roughly 9.5 Gt C/yr, creating a net atmospheric increase.

Nitrogen makes up 78% of the atmosphere as N₂ gas, but most organisms cannot use it directly. Nitrogen-fixing bacteria such as Rhizobium convert N₂ to NH₄⁺ (ammonium), making it biologically available.

Nitrification by Nitrosomonas and Nitrobacter converts NH₄⁺ to NO₃⁻ (nitrate), which plants absorb. Denitrifying bacteria complete the cycle by returning NO₃⁻ to N₂ gas. The Haber-Bosch process now fixes more nitrogen industrially than all natural processes combined.

Unlike carbon and nitrogen, phosphorus has no significant atmospheric phase. It cycles between rock, soil, living organisms, and aquatic sediments over geological timescales. Weathering of rock phosphate releases PO₄³⁻ into soil at roughly 15 Tg P/yr.

Plants absorb dissolved phosphate, and it returns to soil through decomposition. Human mining of rock phosphate for fertilizer has roughly doubled the rate of phosphorus entering soils, leading to excess runoff and aquatic eutrophication.

Residence time is the average time a molecule spends in a given reservoir before moving to another. It is calculated as the reservoir mass divided by the total outgoing flux rate.

Atmospheric CO₂ has a residence time of about 4 years, meaning individual molecules cycle quickly even though the total amount is increasing. Ocean dissolved carbon has a much longer residence time of roughly 400 years. Fossil carbon can remain locked away for millions of years.