Biogeochemical cycles of carbon, nitrogen, phosphorus

The circulation of chemical elements (substances) in the biosphere is called biogeochemical cycles.

The exchange of chemical elements between living organisms and the inorganic environment is called the biogeochemical cycle, or biogeochemical cycle.

Living organisms play a decisive role in these processes.

The elements necessary for life are conventionally called biogenic (life-giving) elements, or nutrients. There are two groups of nutrients:
macrotrophic substances include elements that form the chemical basis of the tissues of living organisms. These are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur.

Microtrophic elements include elements whose content in living organisms is insignificant. They are often referred to as micronutrients. These are iron, manganese, copper, zinc, boron, sodium, molybdenum, chlorine, vanadium and cobalt. Micronutrient deficiencies can have a profound effect on living organisms (in particular, limit plant growth), as can nutrient deficiencies.

Biogenic elements, due to participation in the cycle, can be used repeatedly. The reserves of biogenic elements are unstable: some of them are bound and included in the living biomass, which reduces the amount remaining in the ecosystem environment. And if plants and other organisms did not eventually decay, the supply of nutrients would be depleted and life on Earth would cease. Hence, we can conclude that the activity of heterotrophic organisms, primarily decomposers, is a decisive factor in maintaining the cycle of biogenic elements and preserving life.

Biogeochemical cycle of carbon

Consider the biogeochemical cycle of carbon. The natural source of carbon used by plants for the synthesis of organic matter is carbon dioxide, which is part of the atmosphere or is dissolved in water. The main links of the carbon cycle are shown in the figure.

In the process of photosynthesis, carbon dioxide is converted by plants into organic matter that serves as food for animals.

Breathing, fermentation and combustion of fuel return carbon dioxide to the atmosphere.

The carbon reserves in the atmosphere are estimated at 700 billion tons, and in the hydrosphere – at 50,000 billion tons. According to calculations, as a result of photosynthesis, the increase in plant mass on land and in water is 50 and 180 billion tons, respectively.

Biogeochemical cycle of nitrogen

The circulation of nutrients is usually accompanied by their chemical transformations. Nitrate nitrogen, for example, can be converted into protein, then converted into urea, converted into ammonia and synthesized again into the nitrate form under the influence of microorganisms. Various mechanisms, both biological and chemical, operate in the biochemical nitrogen cycle.

Biogeochemical cycle of phosphorus

One of the simplest cycles is the phosphorus cycle. The main reserves of phosphorus contain various rocks, which gradually (as a result of destruction and erosion) give their phosphates to terrestrial ecosystems. Phosphates are consumed by plants and used to synthesize organic substances. During the decomposition of animal carcasses by microorganisms, phosphates are returned to the soil and then reused by plants. In addition, part of the phosphates is carried out with the water flow into the sea. This ensures the development of phytoplankton and all food chains with the participation of phosphorus. Some of the phosphorus contained in seawater can return to land again in the form of guano – seabird excrement. Where they form large colonies, guano is obtained as a very valuable fertilizer.

Some organisms can play an extremely important role in the phosphorus cycle. Molluscs, for example, filtering water and extracting small organisms from there, their remains, capture and retain large amounts of phosphorus. Despite the fact that the role of mollusks in the food chains of coastal marine communities is small (they do not form dense clusters with a high biomass, their nutritional value is low), these organisms are of paramount importance as a factor allowing to preserve the fertility of the sea zone where they live. Shellfish populations are like natural batteries, only instead of electricity they accumulate and retain phosphorus, which is necessary to sustain life in coastal areas of the seas. In other words, the population of these organisms is more important for the ecosystem as an “intermediary” in the exchange of matter between living and nonliving nature (community and biotope).

This example is a good illustration of the fact that the value of a species in nature does not always depend on indicators such as its abundance or raw material qualities. This value can manifest itself only indirectly and is not always found out with a superficial examination.