5.4 Biogeochemical cycles  (Page 5/6)

Carbon dioxide is also added to the atmosphere by the animal husbandry practices of humans. The large numbers of land animals raised to feed the Earth’s growing population results in increased carbon dioxide levels in the atmosphere due to farming practices and the respiration and methane production. This is another example of how human activity indirectly affects biogeochemical cycles in a significant way. Although much of the debate about the future effects of increasing atmospheric carbon on climate change focuses on fossils fuels, scientists take natural processes, such as volcanoes and respiration, into account as they model and predict the future impact of this increase.

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, humans industrially fix nitrogen to produce artificial fertilizers.

Organic nitrogen is especially important to the study of ecosystem dynamics since many ecosystem processes, such as primary production and decomposition, are limited by the available supply of nitrogen. As shown in [link] , the nitrogen that enters living systems by nitrogen fixation is successively converted from organic nitrogen back into nitrogen gas by bacteria. This process occurs in three steps in terrestrial systems: ammonification, nitrification, and denitrification. First, the ammonification process converts nitrogenous waste from living animals or from the remains of dead animals into ammonium (NH ₄ ⁺ ) by certain bacteria and fungi. Second, the ammonium is converted to nitrites (NO ₂ ⁻ ) by nitrifying bacteria, such as Nitrosomonas , through nitrification. Subsequently, nitrites are converted to nitrates (NO ₃ ⁻ ) by similar organisms. Third, the process of denitrification occurs, whereby bacteria, such as Pseudomonas and Clostridium , convert the nitrates into nitrogen gas, allowing it to re-enter the atmosphere.