The flow of energy through the biosphere is unidirectional: sunlight energy is incorporated into ecosystems through photosynthesis, and is lost through respiration. The elements essential for life are transferred from the environment. But the substances from which living organisms, are made consist of chemical elements , which are both constant in from (i.e., they cannot be degraded into unusable form) and finite (i.e., they are not continuously supplied from an external source). Therefore, the flow of these elements through the ecosystems must be cyclical. Moreover, the fluxes of synthesized and decomposed (oxidized) organic matter must be strictly balanced. These cycles are termed biogeochemical cycles because their flow patterns involve both living organisms and non-living environment.
As indicated in Table 8.1, plans generally require sixteen elements to grow and reproduce. Oxygen, carbon, and hydrogen are building blocks for all organic compounds (carbohydrates, lipids, proteins, and nucleic acids). Together, these three elements account for ninety-six percent of the plant’s dry weight.
Thirteen of the essential elements are minerals. These naturally occurring, inorganic substances become available to plants in ionized form. Six of these minerals are called macronutrients; each makes up the plant. The rest are micronutrients, or trace elements, which represent only a few parts per million of the plant’s dry weight.
Essential trace elements are required by man in amounts ranging from 50 micrograms to 18 milligrams per day. Acting as catalytic or structural components of larger molecules, they have specific functions and are indispensable for life.
In general, the physical environment serves as a large reservoir through which elements move rather slowly, compared to haw rapidly are exchanged between organisms and the environment within the ecosystem. Broadly speaking, oxygen and hydrogen move in the form of water cycle, whereas carbon and elements move through atmospheric and sedimentary cycles.
In atmospheric cycles, the elements occurs in a gaseous phase and a large portion of it exists in the atmosphere. In sedimentary cycles, the elements does not have a gaseous phase; it moves from land to sediments in the oceans, then back to the land through geological uplifting over immense spans of “geological” time. Time of biological turnover of the elements essential for life (biogens) is estimated as 10 to 30 years for land ecosystems and about 10 days for oceanic ecosystems, whereas the geologic turnover time is 10.000-100.000 years.
Nitrogen is one of the macronutrients. It is the component of all proteins and nucleic acids. The atmosphere is the largest nitrogen reservoir: nearly eighty percent of it is composed of gaseous nitrogen N2. The N2 molecules are held together by stable, triple covalent bonds (NN), and few organisms have the metabolic equipment for breaking them.
Nitrogen fixation is a process in which a few kinds of bacteria (nitrogen fixes) convert gaseous nitrogen N2 to ammonium NH3 which dissolves rapidly in water to produce ammonium NH4+. The fixed nitrogen is used by bacteria in the synthesis of amino acids, then of proteins and nucleic acids. Today, nearly all the nitrogen in soils has been put there by a variety of nitrogen-fixing organisms.
Nitrification:
Fixed nitrogen is lost to the soil because ammonia, nitrite, and nitrate are soluble and readily washed out of the soil.
Also, fixed nitrogen is lost to the soil through a bacterial process called denitrification: Under anaerobic conditions, denitrifying bacteria change nitrate to gas N2 , which gradually escapes into the atmosphere.
Denitrification:
Since the beginning of life, nitrogen has been abundant in the atmosphere and oceans but scarce in the Earth’s crust. Of all nutrients influencing the grows of land plants, nitrogen is often the one in shortest supply. Unlike natural ecosystems, in which nutrients such as nitrogen are cycled, modern agriculture exists only due to constant, massive infusions fertilizer.
