Phosphorus and Sulfur Cycles

Facts about the Phosphorus cycle

Cycle most affected by humans! Phosphorus is mostly found in the form a

phosphateIn living things it is in phospholipids, DNA

and RNA

More facts

Very SLOW cycle. Phosphorus can be trapped for millions of year.

The only cycle that does not have a gas phase and does not enter the atmosphere.

The main stores of phosphorus in the biosphere are in the Earth’s crust.

Bacteria not a major player

Washes from the land into streams, then the sea.

Can be deposited as sediment and remain for millions of years.

Often a limiting factor for plant growth on land.

Also limits growth in lakes and streams because phosphate salts are only slightly soluble in water.

Weathering

With weathering rocks release phosphate . The phosphate enters the soil where plants take it up by the roots.

Plants, Animals and Decomposers

When animals eat plants the phosphorus becomes incorporated into their systems. When plants or animals release wastes or die the decomposers break down the phosphorus and release it to the soil

Runoff

Phosphorus enters rivers and oceans by runoff. It then can enter the marine food chains. Phosphorus can cycle through animals for 100,00o years before settling to the bottom to become sediments which become rocks.

How Humans affect the Phosphorus Cycle

Humans have altered the cycle by mining and fertilizing

Humans reduce the available phosphate in tropical soils by clearing tropical forests.

Humans disrupt aquatic systems with phosphates from runoff of animal wastes and fertilizers.

Sewage treatment also adds phosphorus to lakes, rivers and oceans

Extra phosphorus can put some organisms, such as algae, at an advantage which can disrupt ecosystems.

Facts about the Sulfur Cycle

Sulfur is an essential component of proteins (primarily two amino acids: cysteine and methionine), therefore essential for life.

An “average” cell may have approximately 1% S by dry weight.

The largest physical reservoir is the Earth's crust where sulfur is found in gypsum (CaSO4) and pyrite (FeS2).

The largest reservoir of biologically useful sulfur is found in the ocean as sulfate anions (2.6 g/L), dissolved hydrogen sulfide gas, and elemental sulfur.

The sulfur cycle contains both atmospheric and terrestrial processes. Within the terrestrial portion, the cycle begins with the weathering of rocks, releasing the stored sulfur.The sulfur then comes into contact with air where it is converted into sulfate (SO4).

The sulfate is taken up by plants and microorganisms and is converted into organic forms; animals then consume these organic forms through foods they eat, thereby moving the sulfur through the food chain. As organisms die and decompose, some of the sulfur is again released as a sulfate and some enters the tissues of microorganisms. 

There are also a variety of natural sources that emit sulfur directly into the atmosphere, including volcanic eruptions, the breakdown of organic matter in swamps and tidal flats, and the evaporation of water.

Human Impacts on the Sulfur Cycle

Human impact on the sulfur cycle is primarily in the production of sulfur dioxide (SO2) from industry (e.g. burning coal) and the internal combustion engine.

Sulfur dioxide can precipitate onto surfaces where the following can happen: it can be oxidized to sulfate in the soil (it is also

toxic to some plants) oxidized to sulfate in the atmosphere as sulfuric

acid, a principal component of acid rain. Lower the pH in lakes and rivers

At pHs lower than 5 most fish eggs will not hatch and lower pHs can kill adult fish.

Effect of acid rain on a forest, Jizera Mountains, Czech Republic