The sulfur cycle is the way sulfur moves between rocks, waterways and living systems.

Sulfur (S) is an element important to ecosystems and the climate. The majority of sulfur is stored underground, for example under the ocean or in rocks. It also occurs naturally in places like swamps (where organisms have decomposed) and volcanoes. Humans add excess sulfur into this cycle, usually through atmospheric pollution caused by fossil fuel combustion.

• As rocks are worn down by erosion they release sulfur that was once stored, becoming SO4 once it touches the air
• Plants absorb this sulfur through photosynthesis and make this sulfur organic
• Animals, specifically herbivores and omnivores consume the sulfur when they eat the plants
• Sulfur moves through the food chain as secondary consumers and tertiary consumers eat the primary consumers
• When animals and plants die, the sulfur dissipates into the atmosphere as sulfate and also through the body of the decomposers
• The sulfur in the atmosphere is returned to the soil and water cycle when it rains

Sulfur can also be naturally released when volcanoes explode. The hydrological cycle plays a critical role in moving sulfur around.

Humans are responsible for 1/3 of the sulfur in the atmosphere. Sulfur is released by the combustion of fossil fuels, as mentioned above, and is most prevalent as sulfur dioxide. Sulfur is one of the root causes of acid rain along with nitrogen and carbon which negatively affects the environment. Acid rain is created when the H₂O falls and combines with the Sulfur trioxide to create Sulfuric acid. Sulfuric acid is also used in most fertilizers and so can be introduced into the water cycle from runoff off of nearby farms into rivers and lakes. This causes eutrophication and algal blooms, and so reduced the Dissolved oxygen and biodiversity in that water system. Sulfur in the atmosphere as sulfur dioxide also absorbs radiation, however which keeps the earth's surface cooler and combats some of the effects of global warming.