The Sulfur and Phosphorus Cycles

The sulfur cycle

The element sulfur plays a large role in the health of ecosystems. Sulfur has an atmospheric cycle, meaning that it cycles between the atmosphere and the ecosystems on the land and in the ocean.

Sulfur is released into the atmosphere when volcanoes erupt. Sulfur is also released from the ocean in sulfur-containing compounds in sea spray, and as the gas hydrogen sulfide (H2S), which is a metabolic by-product of some types of bacteria.

About 95 percent of the sulfur that enters the atmosphere from the world's oceans does so in the form of strong-smelling sulfur compounds (such as dimethyl sulfide) that are breakdown products of organic molecules made by phytoplankton. The odorous compounds are lofted into the air by wave action and contribute to the smell we associate with seaside air.

Ecosystems naturally release sulfur, but human activity greatly increases the sulfur emissions into the atmosphere. By burning fossil fuels such as oil and coal, we introduce sulfur dioxide (SO2) into the air. In the atmosphere, this gas combines with oxygen and water to become sulfuric acid (H2SO4).

From the atmosphere, sulfur compounds return to land in dry particles or dissolved in rain. Too much sulfuric acid in the rain is the main cause of acid rain, which lowers the pH of soils, lakes, and rivers, damaging organisms and reducing the productivity of plants.

Some of the sulfur that falls back to Earth is taken up by, and cycled through, organisms in terrestrial and aquatic communities. Sulfur is required by all organisms because it is a component of the amino acids cysteine and methionine.

Sulfur eventually falls to the bottom of the ocean in sediments. Much later, geological activity brings sulfur back to Earth's surface. Weathering releases sulfur from surface rocks, making it available to terrestrial communities. The sulfur from rocks can also be released through volcanic activity.

Like most other nutrients with atmospheric cycles, sulfur cycles through terrestrial and aquatic ecosystems relatively quickly.

Activity 1

Complete these sentences about the sulfur cycle. Drag the terms to their correct locations.

The phosphorus cycle

Phosphorus is a vital nutrient because it is a component of crucial macromolecules, such as DNA. Phosphorus is also important to ecosystems because it strongly affects net primary productivity, especially in aquatic ecosystems.

How do organisms acquire phosphorus? The main stores of phosphorus in the biosphere are found in Earth's crust. With weathering, rocks release phosphorus in the form of phosphate. The phosphate enters the soil, where plants take it in through their roots.

When animals eat plants, they incorporate the plants' phosphorus into their own bodies. When plants, animals, and other organisms die or release wastes, phosphorus becomes available to decomposers, such as fungi and bacteria. The decomposers break down the tissues and release phosphate into the soil, where plants can take it up again.

In the ocean, phosphorus enters the marine food chain. A single phosphorus atom might cycle between the water and the bodies of organisms for an average of 100,000 years before it finally falls to the ocean bottom as sediment, forming new rocks.

Phosphorus can remain trapped in sediment for 100 million years or more. Unlike elements in the carbon, sulfur, and nitrogen cycles, phosphorus does not cycle through the atmosphere. Instead, it has a sedimentary cycle. Phosphorus in the ocean sediments remains there until it is removed by weathering or geological uplift to reach Earth's surface, where the cycle can continue.

Sedimentary nutrients usually cycle very slowly, so they are not replaced easily once they are lost from an ecosystem.

Humans have changed the natural phosphorus cycle by activities such as mining and fertilizing. Fertilizer contains abundant phosphorus, which enters the environment. Sewage treatment plants add extra phosphorus to rivers, lakes and oceans. Extra phosphorus can put some organisms, such as algae, at a growth advantage, and thereby upset the balance of organisms in an ecosystem.

Activity 2

Complete these sentences about the phosphorus cycle. Drag the terms to their correct locations. Some terms are used more than once.