Sulfur Cycle- The movement of sulfur around the biosphere. Sulfur exists in rocks and is released into soils and water as the rocks weather over time. Producers absorb sulfur through their roots in the form of sulfate ions and the sulfur cycles through the food web. Volcanic eruptions are a natural source of atmospheric sulfur in the form of sulfur dioxide. Human activities such as the burning of fossil fuels and the mining of metals add sulfur to the atmosphere.