Day 5: Nitrogen & Phosphorus Cycles 

Nitrogen Cycle: Bacteria in Action 

Nitrogen is a critical nutrient for all forms of life. nitrogen gas (N2) makes up 78% of the volue of the atmosphere, but N2 cannot be used as anutrient by plants. It becoes a plant nutrient only as a component of nitrogen-containing ammonia (NH3), ammonium ions (NH4+) and nitrate ions (NO3 -). These chemical forms of nitrogen are created in the nitrogen cycle by lightning, which converts N2 to NH3, and by specialized bacteria in topsoil. Other bacteria in topsoil and in the bottom sediments of aquatic systems convert NH3 to NH4+ and nitrate ions NO3- that are taken up by the roots of plants. The plants then use these forms of nitrogen to produce various proteins, nucleic acids, and vitamins. Animals that eat plants consume these nitrogen-containing compounds as do detritus feeders and decomposers. Bacteria in waterlogged soil and bottom sediments of lakes, oceans swamps and bogs convert nitrogen compounds into nitrogen gas (N2). The gas is released to the atmosphere to begin the nitrogen cycle again. 

Humans intervene in the nitrogen cycle in several ways. When we burn gasoline and other fuels, the resulting high temperatures convert some of the N2 and 02 in air to Nitric oxide (NO). In the atmosphere, NO can be converted to nitrogen dioxide gas (NO2) and nitric acid vapor (HNO3), which can return to the earth's surface as damaging acid deposition, commonly called acid rain. Acid rain damages buildings, statues, and forests. 

We remove large amounts of N2 form the atmosphere and combine it with H2 to make ammonia (N2 + 3H2--->2NH3) and ammonium ions (NH4 +) used in fertilizers. We add nitrous oxide (N2O) to the atmosphere through the action of anaerobic bacteria on nitrogen-containing fertilizer or organic animal manure applied to the soil. This greenhouse gas can warm the atmosphere and take part in reactions that deplete stratospheric ozone, which keeps most of the sun's harmful ultraviolet radiation from reaching the earth's surface 

We alter the nitrogen cycle in aquatic ecosystems by adding excess nitrates (NO3-) to these systems. The nitrates contaminate bodies of water through agricultural runoff of fertilizers, animal manure, and discharges from municipal sewage treatment systems. This plant nutriet can cause excessive growth of algae that disrupt aquatic systems. Human nitrogen inputs into the environment have risen sharply and are projected to continue rising.

Phosphorus Cycles through Water, Rock, and Food Webs 

Phosporus is another nutrient that supports life, The cyclic movement of phosphorus (P) through water, the earth's crust, and living organisms is called the phosphorus cycle. The major reservoir for phosphorus in this cycle is phosphate rocks that contain phosphate ions, which are important plant nutrients. Phosphorus does not cycle through the atmosphere because few of its compounds exist as a gas. Phosphorus cycles more slowly than water, carbon, and nitrogen. 

As water runs over exposed rocks. It slowly erodes away inorganic compounds that contain phosphate ions. Water carries these ions into the soil, where they are absorbed by the roots of plants and by other producers. Phosphate compounds are then transferred by food webs from producers to consumers and eventually to detritus feeders and decomposers. 

When phosphate and other phosphorus compounds wash into the ocean, they are deposited as marine sediment and can remain trapped for millions of years over time, geological proesses can uplift and expose these seafloor deposits from which phosphate can be eroded and freed up to reenter the phosphorus cycle. 

Most soils contain little phosphate, which often limits plant growth on land unless phosphorus (as phosphate salts mined from the earth) is applied to the soil as a fertilizer. Lack of phosphorus also limits the growth of producer populations in many freshwater streams and lakes. This is because phosphate salts are only slightly soluble in water and do not release many phosphate ions to producers in aquatic systems. 

Human activities, including the removal of large amounts of phosphate from the earth to make fertilizer, disrupt the phosphorus cycle. By clearing tropical forests, we redue phosphate levels in tropical soils. Eroded topsoil and fertilizer washed from fertilized crop fields, lawns, and golf courses carry large quantities of phosphate ions into streams, lakes, and oceans. There they stimulate the growth of producers such as algae and various aquatic plants, which can upset chemical cyclingand other processes in bodies of water. According to a number of scientific studies we are disrupting the phosphorus cycle because our inputs of phosphorus into the environment (primarily for use as fertilizer) have exceeded the planet's environmental limit for phosphorus.