Geochemical Cycles

Every atom of carbon or nitrogen on Earth is on loan. Picture the planet as a set of connected buckets: atmosphere, ocean, living tissue, soil, and rock. Arrows between buckets are processes, not accidents. Carbon leaves the atmosphere when a plant photosynthesizes it into sugar, moves into an animal that eats the plant, returns to air through respiration or decay, or gets buried as shell and sediment and locked into limestone for ages. That carbon atom in your breath today can be exhaled, absorbed by the ocean, pulled into a shell, buried, and compressed into rock over a million years, then released again when that rock weathers or gets pulled into a volcano. Nothing is created or destroyed, it just changes address.

Nitrogen takes a trickier path because the N2 gas filling the air is almost chemically locked. Its two atoms share a triple bond that plants and animals cannot break. Fixation is the step that cracks it open: lightning or nitrogen-fixing bacteria in soil and roots convert N2 into ammonia or nitrate, forms a plant can actually absorb and build into protein. From there nitrogen moves up the food chain, returns to soil through decay and waste, and other bacteria eventually convert it back to N2 through denitrification, closing the loop back to the atmosphere.

These cycles run at very different speeds. Carbon moving through air, ocean surface and living things cycles in years to decades, a fast pool. Carbon buried in rock or fossil fuel sits in a slow pool for millions of years. Burning fossil fuel yanks carbon out of that slow pool and dumps it into the fast one almost instantly, faster than slow burial processes can pull it back out, which is why that transfer changes the balance of the whole cycle.