Biogeochemical cycles are the natural processes that move elements and compounds (like carbon, water, nitrogen, and phosphorus) between living organisms, the atmosphere, water, soil, and rock through sources, sinks, and reservoirs.
A biogeochemical cycle is just the path an element takes as it loops between living things (the "bio"), rock and soil (the "geo"), and chemical reactions (the "chemical"). Carbon, water, nitrogen, and phosphorus don't disappear when they get used. They get passed around, stored for a while, and released again. Think of each element as a passenger that keeps boarding and exiting different reservoirs: the atmosphere, the ocean, living tissue, fossil fuels, rock.
Each cycle has sources (where the element gets released), sinks (where it gets stored), and reservoirs (the place it sits, sometimes for seconds, sometimes for millions of years). In the carbon cycle, for example, carbon moves between photosynthesis and cellular respiration in living things over short timeframes, but dead plant and animal matter can lock carbon away as fossil fuels for millions of years. Burning those fossil fuels yanks that stored carbon back into the atmosphere fast (EK ERT-1.D.4). That mismatch between slow storage and fast release is the whole story behind a lot of human-caused environmental change.
Biogeochemical cycles run through several units, which is exactly why this term is worth knowing cold. In Unit 1, the carbon cycle (Topic 1.4) is your model for sources, sinks, and reservoirs, supporting learning objective AP Enviro 1.4.A. In Unit 2, these cycles are supporting ecosystem services, the behind-the-scenes processes (like nutrient cycling) that make provisioning, regulating, and cultural services possible (AP Enviro 2.2.A). When humans disrupt a cycle, ecosystem services break down with economic and ecological costs (AP Enviro 2.2.B). In Unit 5, urbanization throws cycles off balance: burning fossil fuels and landfills add CO2 to the carbon cycle, and impervious surfaces and overuse mess with the hydrologic cycle (AP Enviro 5.10.A). The big exam theme here is that natural cycles are stable until people speed them up or break the links.
Keep studying AP Environmental Science Unit 1
The Carbon Cycle (Unit 1)
This is the cycle you'll be tested on most directly. It's the clearest example of sources and sinks, and it ties straight to climate change when fossil fuel burning moves ancient carbon into the atmosphere faster than sinks can absorb it.
Ecosystem Services (Unit 2)
Nutrient cycling is itself a supporting ecosystem service. When a biogeochemical cycle gets disrupted, the services that depend on it (clean water, fertile soil, climate regulation) start to fail, which is exactly what AP Enviro 2.2.B asks you to describe.
Impacts of Urbanization (Unit 5)
Cities are cycle-disruption machines. Impervious surfaces block water from reaching soil and cause flooding (hydrologic cycle), while fossil fuel burning and landfills pump CO2 into the atmosphere (carbon cycle). Same term, applied to a real human land-use problem.
Ecological Footprints (Unit 5)
An ecological footprint measures how much resource demand and waste a person or society generates. Big footprints overload biogeochemical cycles by pulling resources out and dumping waste back in faster than the cycles can rebalance.
Expect biogeochemical cycles in multiple-choice stems that show a diagram and ask you to identify a source, sink, or reservoir, or to trace where an element goes next. The carbon cycle is the most likely target. On FRQs, you'll usually be asked to explain a specific cycle's steps, then connect a human activity (burning fossil fuels, deforestation, urbanization) to how it changes that cycle and what the consequences are. The move the exam rewards is going past "name the cycle" to "explain how humans alter it and why that matters." No released FRQ has used the phrase "biogeochemical cycles" verbatim, but questions on the carbon cycle, water cycle, and ecosystem service disruption all test this exact thinking.
The carbon cycle is ONE biogeochemical cycle, not a synonym for all of them. "Biogeochemical cycles" is the umbrella term covering carbon, water, nitrogen, and phosphorus cycles. So every carbon cycle question is a biogeochemical cycle question, but not the reverse. If a problem asks about water or phosphorus, you're still in biogeochemical-cycle territory.
Biogeochemical cycles move elements like carbon, water, nitrogen, and phosphorus between living things, the atmosphere, water, soil, and rock.
Every cycle has sources (where the element is released), sinks (where it's stored), and reservoirs that hold it for short or very long periods.
In the carbon cycle, living things swap carbon quickly through photosynthesis and respiration, but fossil fuels store it for millions of years until burning releases it fast.
Nutrient cycling is a supporting ecosystem service, so disrupting a cycle damages the services that depend on it (AP Enviro 2.2.B).
Urbanization disrupts cycles by adding CO2 to the carbon cycle and by using impervious surfaces that throw off the hydrologic cycle (AP Enviro 5.10.A).
The exam wants you to connect a human activity to a cycle change and then to its ecological or economic consequences.
They're the natural processes that cycle elements and compounds (carbon, water, nitrogen, phosphorus) between living organisms and their environment through sources, sinks, and reservoirs. The carbon cycle in Topic 1.4 is the main example you'll be tested on.
No. The carbon cycle is just one biogeochemical cycle. "Biogeochemical cycles" is the umbrella term that also includes the water (hydrologic), nitrogen, and phosphorus cycles, so the carbon cycle is one example within that bigger category.
Burning fossil fuels moves stored carbon into the atmosphere fast, deforestation removes carbon sinks, and urbanization with impervious surfaces blocks water from reaching soil. Each of these speeds up or breaks a cycle faster than it can rebalance, causing economic and ecological consequences (AP Enviro 2.2.B).
A source releases an element (like burning fossil fuels releasing carbon), a sink stores it (like the ocean absorbing CO2), and a reservoir is simply where the element sits, sometimes for seconds and sometimes for millions of years. Knowing these three terms is the key to answering carbon cycle questions.
Nutrient cycling falls under supporting ecosystem services, the behind-the-scenes processes that make provisioning, regulating, and cultural services possible (AP Enviro 2.2.A). Without functioning cycles, you don't get fertile soil, clean water, or a stable climate.