Introduction
The patterns of cycling nutrients in the biosphere involves both biotic and abiotic chemical reactions. Understanding the biogeochemical cycle of any biologically important element requires the knowledge of chemical processes that operate in the biosphere, lithosphere, atmosphere, and hydrosphere.
The biogeochemical cycles of all elements used by life have both an organic and an inorganic phase. For most of these nutrients, how efficiently these elements cycle from the organic component back to the inorganic reserviors determines how much is available to organisms over the short term. This cycling involves the decomposition of organic matter back into inorganic nutrients. The major reservoirs for all metabolically important elements are found either in the atmosphere, lithosphere (mainly rock, soil and other weathered sediments) or hydrosphere. Flow from these reservoirs to the organic phase is generally slower than the cycling of nutrients through organic matter decomposition.
Nutrients Essential for Life
Living organisms require the availability of about 20 to 30 chemical elements for the various of metabolic processes that take place in their bodies. Some products of this metabolism require relatively few nutrients for their production. For example, carbohydrates are photosynthesized from just water and carbon dioxide. Some organic substances, like amino acids and proteins, are more complex in their chemical make up and therefore require a number of different nutrients.
The types of nutrient needed by life is often categorized into two groups. Elements required in relatively large amounts are generally referred to as macronutrients. Macronutrients that constitute more than 1% each of dry weight include carbon, oxygen, hydrogen, nitrogen, and phosphorus. Macronutrients that constitute 0.2 to 1% of dry organic weight include sulfur, chlorine, potassium, sodium, calcium, magnesium, iron, and copper.
Nutrients needed in trace amounts are generally called micronutrients. These elements often constitute less than 0.2% of dry organic matter. Some common micronutrients required by living organisms include aluminum, boron, bromine, chromium, cobalt, fluorine, gallium, iodine, manganese, molybdenum, selenium, silicon, strontium, tin, titanium, vanadium, and zinc.
Nutrient Inputs to Ecosystems
Important nutrients for life generally enter ecosystems by way of four processes:
(1). Weathering
Rock weathering is one of the most important long-term sources for nutrients. However, this process adds nutrients to ecosystems in relatively small quantities over long periods of time. Important nutrients released by weathering include:
- Calcium, magnesium, potassium, sodium, silicon, iron, aluminum, and phosphorus.
- All of the micro nutrients.
Carbon, oxygen and nitrogen are not transferred into ecosystems by weathering. The main source for these important elements is the atmosphere and the decomposition of organic matter.
(2). Atmospheric Input
Large quantities of nutrients are added to ecosystems from the atmosphere. This addition is done either through precipitation or by a number of biological processes.
- Carbon - absorbed by way of photosynthesis.
- Nitrogen - produced by lightning and precipitation.
- Sulfur, chloride, calcium, and sodium - deposited by way of precipitation.
The quantity of nitrogen added to ecosystems by lightning and rain annually ranges from 1 to 20 kilograms per hectare depending on geographical location. A value of 5 to 8 kilograms per hectare is typical for temperate ecosystems like deciduous forest or grasslands.
(3). Biological Nitrogen Fixation
Biological nitrogen fixation is a biochemical process where nitrogen gas (N2) from the atmosphere is chemically combined into more complex solid forms by metabolic reactions in an organism. This ability to fix nitrogen is restricted to a small number of species. This special group of life includes a few species of bacteria (that have symbiotic associations with legumes and some other types of higher plants), several species of actinomycetes (filamentous form of bacteria), and blue-green algae (cyanobacteria). The amount of nitrogen fixed biologically has been estimated to be around 170 million metric tons per year. This is approximately twice as much as the total nitrogen added to ecosystems from non-biological sources.
Symbiotic Fixation with LegumesLegume is the common name for all species of plants that belong to the family Fabaceae. This family of plants is composed of about 18,000 species. Included in this group of plants are agriculturally important plants such as clover, lupins, alfalfa, cassia, beans, peas, peanuts, soybeans, and lentils. An important defining characteristic of legumes is the fact that they form a symbiotic relationship with bacteria of the genus Rhizobium. In this relationship, the Rhizobium bacteria colonize plants roots causing the formation of nodules. From inside these nodules, the bacteria absorb nitrogen gas (N2) from the atmosphere and then transform it into forms of nitrogen (nitrate: NO3- or ammonia: NH3) that are usable by itself and the host plant. In exchange for the nitrogen, the symbiotic host plant feeds the bacteria with carbohydrates that it produces from photosynthesis.
Besides feeding the host plant, the nitrogen fixed by the bacteria can also be used by other organisms in an ecosystem in two ways. Some of the fixed nitrogen seeps out from plant roots and nodules into the soil. Plants in close vicinity can consume this leaked nitrogen. Secondly, when the host plant dies the nitrogen stored in its organic matter can be made available to other plants through decomposition.
Symbiotic Fixation with Non-LegumesRoughly 170 species of non-legumes are known to form symbiotic relationships with actinomycetes of the genus Frankia for nitrogen fixation. Tree and shrub species belonging to the alder and birch families (genus Alnus) are good examples of such organisms in terrestrial environments. Tiny floating water ferns belonging to the genus Azolla play an important role in biological fixing nitrogen in aquatic environments. In this symbiotic relationship, the water fern pairs up with the cyanobacterium Anabaena azollae. Several different species of bacteria and cyanobacteria have the ability to fix nitrogen without a symbiotic relationship. These species are often found in habitats as diverse as aquatic environments to the surface soils of hot deserts. In tidal marsh and mangrove ecosystems, cyanobacteria play an important role in supporting the high plant productivity associated with these environments.
About 160 species of non-legumes are known to develop nodules to house bacteria for nitrogen fixation. Some of the commonly known plant species that have this ability belong to the alder and birch family
Symbiotic Nitrogen Fixation Without NodulesSeveral significant nonlegume symbiotic associations exist that fix nitrogen that do not involve higher plants. Among the most important are those involving blue-green algae (cyanobacteria).
(4). Immigration
The immigration of motile animals into an ecosystem can sometimes add significant additions of nutrients to an ecosystem that are locked up in the biomass of the organisms. These nutrients are released when the organism dies or sheds its tissues.
Nutrient Outputs to Ecosystems
Important nutrients required for life leave ecosystems by way of four processes:
(1). Erosion
Soil erosion is probably the most import means of nutrient loss to ecosystems. Erosion is very active in agricultural and forestry systems, where cultivation, grazing, and clearcutting leaves the soil bare and unprotected. When unprotected, the surface of the soil is easily transported by wind and moving water. The top most layers of a soil, which have an abundance of nutrient rich organic matter, are the major storehouse for soil nutrients like phosphorus, potassium, and nitrogen.
(2). Leaching
Another important process of nutrient loss is leaching. Leaching occurs when water flowing vertically through the soil transports nutrients in solution downward in the soil profile. Many of these nutrients can be completely lost from the soil profile if carried into groundwater and then horizontally transported into rivers, lakes, or oceans. Leaching losses are, generally, highest in disturbed ecosystems. In undisturbed ecosystems, efficient nutrient cycling limits the amount of nutrients available for this process.
(3). Gaseous Losses
High losses of nutrients can also occur when specific environmental conditions promote the export of nutrients in a gaseous form. When the soil is wet and anaerobic, many compounds are chemically reduced to a gas from solid forms in the soil. This is especially true of soil nitrogen. Scientific studies in Netherlands have shown that about 80% of the nitrogen fertilizer applied to the soil for crop consumption may be lost through the process of denitrification.
(4). Emigration and Harvesting
Just as material may be introduced to ecosystems by migration, so too may it be lost. The emigration of animals, and the removal of vegetation by humans are both processes by which outputs can occur from an ecosystem.