How is phosphorus recycled?
legumes (also called pulses) are food plants such as beans, lentils and peas. Legumes are often used in crop rotation systems by organic farmers. Thy can be used in one of two ways. Thy can be grown as a crop (for example, peas or beans) and the peas or beans harvested; then the remains of the plants are ploughed into the soil. Alternatively a non-crop legume such as clover can be grown and ploughed in at the end of the year. In this second method, all of the nitrogen fixed is added to the soil; none is lost in a crop.
The core phosphorus cycle is much the same as the core nitrogen cycle. Phosphorus is present in organisms in the form of phosphates.
• phosphate is absorbed from the soil (or water) by plants
• these are passed along food chains to various herbivores and carnivores
• on death, their bodies are decomposed and phosphate ions are released from compounds like phospholipids, ATP, DNA and RNA and are returned to the soil or water
• phosphates also enter the soil (or water) as a result of the weathering of rocks and in the form of fertilizers, which, themselves, contain phosphates that have been obtained from rocks
• over millions of years, phosphate ions can leach into the seas and become part of newly forming sedimentary rock.
How is sulfur recycled?
As with the other cycles, the core cycle is between the soil, plants, animals and special decomposers. There are also components that relate to long-term rock formation and weathering as well as the formation of sulfur dioxide when fossil fuels are burned.
• Sulfate ions in the soil are taken up by plants and incorporated in plant tissue (many proteins include some sulfur-containing amino acids, such as methionine and cysteine)
• These are passed to animals by feeding and digestion
• On death of the plants and animals, sulfate-reducing bacteria release the sulfur in the proteins in the form of hydrogen sulfide (with the smell of ‘bad eggs’); the most important genus of bacteria involved in this process is Desulphovibrio; this process requires anaerobic conditions
• In some aquatic environments the hydrogen sulfide is oxidized to sulfur by photosynthetic sulfur bacteria; this reaction is the equivalent of the photolysis of water in the photosynthesis of higher plants
• Sulfur bacteria, mainly of the genus Thiobacillus, then oxidize the hydrogen sulfide (or sulfur) to sulfate (SO42–), with sulfite (SO32–) as an intermediate step; this is an oxygen requiring process that needs aerobic conditions and makes sulfate ions available once again to be taken up by plant roots from the soil
• Sulfur can also become incorporated in rocks, including those that yield fossil fuels
• Combustion of fossil fuels oxidizes the sulfur to sulfur dioxide (SO 2); this is a serious pollutant of the atmosphere and a major contributor to the formation of acid rain
• In the atmosphere, the sulfur dioxide becomes further oxidized to sulfite and sulfate which dissolve in rainwater to form a mixture of sulfurous and sulfuric acid: acid rain
What about the water cycle?
Water is essential to all living organisms in all kinds of ways:
• it makes up 70% of all cells
• it is an essential requirement of photosynthesis
• it is the basis of all transport systems in organisms
• it provides a means of removing excretory products
In addition, we use water in many ways in our daily lives:
• to wash our clothes, our dirty dishes and our dirty selves
• to flesh away waste
• to make products such as paper, steel and beer
• to generate electricity using a range of devices that convert the motion of water into electrical energy
• in a system, called ‘hydroponics’, to grow plants in a soil-free medium