Phosphorus, oxygen and water cycles

In Year 7 you studied the water cycle, and in Year 9 the carbon and nitrogen cycles. Phosphorus behaves very differently from both, and it may be the cycle that matters most for feeding the world.

The phosphorus cycle

Every living cell needs phosphorus. It is part of DNA and RNA, of the energy-carrying molecule ATP, and of the phospholipids that make cell membranes. Your bones and teeth are mostly calcium phosphate. Unlike carbon and nitrogen, phosphorus has no common gas form, so it cannot travel through the atmosphere. It moves only through rock, soil, water and living things, and most of its cycle is extremely slow.

1. Weathering of rocks slowly releases phosphate ions into soil and water.
2. Plants absorb phosphate through their roots; animals get it by eating plants or other animals.
3. Wastes and dead organisms are broken down by decomposers, returning phosphate to the soil.
4. Some phosphate washes into rivers and the sea, where it eventually settles into sediments. Only when tectonic forces lift those sediments into new land, over millions of years, does it return.
5. Seabirds provide a shortcut: they eat fish at sea and drop phosphorus-rich guano on islands and coasts.

Australia: the phosphorus-poor continent

Australia's soils are among the oldest and most weathered in the world. Most of the continent has not been scraped by glaciers or covered by fresh volcanic ash for millions of years, so rain has slowly washed the phosphorus out. When Europeans first tried growing wheat on these soils, crops often failed, until farmers began spreading superphosphate, made by treating phosphate rock with sulfuric acid. Much of that rock came from Nauru and Banaba (Ocean Island), mined by a commission run by Australia, New Zealand and Britain. On Banaba, mining damaged the island so badly that most Banaban people were moved to Rabi Island in Fiji in 1945.

Native plants evolved to cope. Banksias, hakeas, grevilleas and waratahs (the Proteaceae family) grow cluster roots: dense bottlebrushes of short rootlets that release organic acids such as citrate. The acids dissolve phosphate locked onto iron and aluminium minerals in the soil so the plant can absorb it. Many of these plants are so well adapted to low phosphorus that ordinary garden fertiliser poisons them, which is why nurseries sell special "native" fertiliser.

Peak phosphorus

Modern farming depends on mined phosphate rock: without it, the world could feed only a fraction of today's population. But phosphate rock is a non-renewable resource, and about 70% of the known reserves are in Morocco and Western Sahara. In 2008 the price of phosphate rock rose about eightfold in a year, causing problems for farmers worldwide. Some scientists warned of "peak phosphorus", when production would start to fall. Newer estimates of the reserves are much larger, and most experts now expect them to last a few hundred years, but the supply is fragile and much of what we mine is wasted.

• Most fertiliser phosphorus is never taken up by crops. Some binds tightly to soil; some washes into rivers, where it helps cause algal blooms (Year 8).
• Almost all the phosphorus in our food ends up in sewage, manure and food waste. Treatment plants can recover it as a mineral called struvite, which makes good slow-release fertiliser.

The oxygen cycle

Oxygen cycles alongside carbon. Photosynthesis releases oxygen, and respiration, decay and burning use it (Year 9). About half of all photosynthesis on Earth is done by microscopic phytoplankton in the ocean, so the sea supplies about half the oxygen production of the planet.

• Rocks are by far the largest store of oxygen, locked in silicates, carbonates and iron oxides. Weathering of iron minerals (rusting) removes oxygen from the air.
• In the stratosphere, ultraviolet light splits oxygen molecules into single atoms (photolysis). These atoms join other O₂ molecules to form ozone (O₃), which absorbs UV. You studied how CFCs destroyed ozone, and how the Montreal Protocol is fixing it, in 4E.
• Oxygen dissolves in water, which is how fish breathe. Cold water holds more dissolved oxygen than warm water, which is one reason warmer rivers suffer fish kills.

Plants in the water cycle

Plants move a huge amount of water from the soil to the atmosphere by transpiration: water evaporates from leaves through tiny pores called stomata. Over land, transpiration returns more water to the air than evaporation from soil, lakes and rivers combined. Clearing forests therefore changes rainfall, river flow and even the salinity of soils: in parts of WA and Victoria, replacing deep-rooted native trees with shallow-rooted crops let the water table rise, bringing salt to the surface and creating dryland salinity.