8.1 Nitrogen and phosphorus cycles

As mouths multiply, food resources dwindle. Land is a limited quantity, and the land that will grow wheat is absolutely dependent on difficult and capricious natural phenomena... I have to point the way out of this colossal dilemma. It is the chemist who must come to the rescue of the threatened communities. It is through the laboratory that starvation may ultimately be turned into plenty... The fixation of atmospheric nitrogen is one of the great discoveries, awaiting the genius of chemists.

The "difficult and capricious natural phenomena" of nitrogen fixation is just one part of the biogeochemical cycle of this nutrient, but it is, as Crookes noted, an extremely important part. Chemists have succeeded in the quest to develop reactions that can fix atmospheric nitrogen into forms that can be used by living organisms, but it should be noted that humble microorganisms did this long before boastful chemists thought about it. Industrial processes for nitrogen fixation now produce as much usable nitrogen as all of the biological nitrogen-fixers on the planet. This chemical success, however, does come with some undesirable consequences, as you will learn below.

Energy flows directionally through ecosystems, entering as sunlight (or inorganic molecules for chemoautotrophs) and leaving as heat during the many transfers between trophic levels. However, the matter that makes up living organisms is conserved and recycled. The six most common elements associated with organic molecules—carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur—take a variety of chemical forms and may exist for long periods in the atmosphere, on land, in water, or beneath the Earth’s surface. Geologic processes, such as weathering, erosion, water drainage, and the subduction of the continental plates, all play a role in this recycling of materials. Because geology and chemistry have major roles in the study of this process, the recycling of inorganic matter between living organisms and their environment is called a biogeochemical cycle .

The nitrogen cycle

Nitrogen is an essential nutrient for living processes; it is a major component of proteins and nucleic acids. Proteins are important biological molecules because all cellular activities are driven by proteins. Nucleic acids are the building blocks of DNA (hereditary material). Nitrogen is often the limiting nutrient (necessary for growth) on terrestrial ecosystems [link] .

Getting nitrogen into the living world is difficult. Plants and phytoplankton are not equipped to incorporate nitrogen from the atmosphere (which exists as tightly bonded, triple covalent N ₂ ) even though this molecule comprises approximately 78 percent of the atmosphere. Nitrogen enters the living world via free-living and symbiotic bacteria, which incorporate nitrogen into their macromolecules through nitrogen fixation (conversion of N ₂ ). Cyanobacteria live in most aquatic ecosystems where sunlight is present; they play a key role in nitrogen fixation. Cyanobacteria are able to use inorganic sources of nitrogen to “fix” nitrogen. Rhizobium bacteria live symbiotically in the root nodules of legumes (such as peas, beans, and peanuts) and provide them with the organic nitrogen they need. Free-living bacteria, such as Azotobacter , are also important nitrogen fixers. In addition to natural nitrogen fixation by microbes, humans industrially fix nitrogen to produce artificial fertilizers.