Biogenic carbon cycle in the biosphere and anthropogenic impact

V.R. Williams wrote that the only way to give something finite the properties of the infinite is to force the finite to rotate along a closed curve, i.e., to involve it in a cycle. This is exactly what happens in ecosystems.

Biogenic circulation occurs at the ecosystem level and consists of soil nutrients, water, and carbon accumulating in plant matter and being spent on the construction of the body and the life processes of both the plants themselves and consumer organisms. Decomposers break down organic matter into mineral components, which are once again accessible to plants and re-engaged by them in the flow of matter.

Thus, the third basic principle of ecosystem functioning—the acquisition of resources and the disposal of waste—occurs within the framework of the circulation of all elements.

Let us consider such a cycle for the main components that make up the biosphere.

Carbon, contained in the form of CO2 in the atmosphere (23.5·1011 t), serves as raw material for plant photosynthesis, and is then consumed along with their matter by consumers of different trophic levels. During the respiration of plants, animals, and decomposers, as well as during the decomposition of dead matter in the soil, CO2 is released, in the form of which carbon returns to the atmosphere.

Most of the carbon involved in the cycle is contained in the oceans. The amount of carbon dioxide in the atmosphere depends mainly on the carbon contained in the oceans in the form of carbonates. The ocean absorbs excess carbon dioxide from the air, resulting in the formation of carbonate and bicarbonate ions. There is also a reverse process, during which carbon dioxide is released from the oceans into the atmosphere. Thus, the oceans, which maintain the concentration of CO2 in the atmosphere at a constant level, play the role of a kind of buffer. It is believed that this mechanism ensured the relative constancy of the carbon dioxide content in the atmosphere until the factor of industrialization intervened.

In distant geological epochs (hundreds of millions of years ago), a significant part of the photosynthesized organic matter was not used by either consumers or decomposers, but accumulated and was gradually buried under mineral sediments. Located in the ground for millions of years, this detritus (dead plant and animal remains) was transformed into oil, natural gas, and coal under the influence of high temperature and pressure. Now we extract this fossil fuel in huge quantities to meet the energy needs of our industrial society and, by burning it, we complete the carbon cycle. When burning, the carbon of the fuel is released in the form of CO2, the concentration of which in the air increases sharply: its influx significantly exceeds the absorption capacity of plants. This is fraught with serious climatic consequences, which will be discussed in the section dedicated to the engineering protection of the atmosphere.

Organisms need various chemical forms of nitrogen for the formation of proteins and genetically important nucleic acids like DNA. Most green plants require nitrogen in the form of nitrate ions (NO3-) and ammonium ions (NH4+). Gaseous nitrogen (N2), which makes up 78% of the Earth's atmosphere by volume, cannot be used directly by plants, humans, or most other organisms. Gaseous nitrogen can be converted into water-soluble compounds containing nitrate ions and ammonium ions, which are assimilated by plant roots in the process of the nitrogen cycle.

The conversion of atmospheric gaseous nitrogen into chemical forms that are assimilable by plants is called nitrogen fixation. It is carried out mainly either by blue-green algae and certain types of bacteria in the soil and water, or by bacteria of the genus inhabiting small nodules on the roots of alfalfa, clover, peas, beans, and other leguminous plants. A certain contribution to nitrogen fixation is made by lightning discharges, during which gaseous nitrogen and oxygen in the atmosphere are converted into nitrogen oxide and dioxide. These gases interact with water vapor and are converted into nitrate ions, which fall to the Earth's surface in the form of nitric acid dissolved in atmospheric precipitation and in the form of nitrate salt particles.

Inorganic nitrate ions and ammonium ions, absorbed by plants from soil moisture, are converted by them into proteins, DNA, and other necessary nitrogenous organic compounds. Animals cover most of their needs for nitrogenous nutrients by eating plants or other herbivorous animals.

Specialized bacteria—decomposers—convert the nitrogenous organic compounds of biological waste (excrement and dead organisms) into inorganic substances such as gaseous ammonia (NH3) and water-soluble salts containing ammonium ions (NH4+). Other special groups of bacteria then convert these inorganic forms of nitrogen into nitrate ions in the soil and into gaseous nitrogen, which, entering the atmosphere, completes the cycle.

Phosphorus in the form of phosphate ions (PO43- and HPO42-) is an essential nutrient for both plants and livestock animals. It is a component of DNA molecules that carry genetic information; ATP and ADP molecules, in which chemical energy required for organisms and used in cellular respiration is stored; fat molecules that form cell membranes in plant and animal cells; as well as substances that are part of the bones and teeth of animals.

The phosphorus cycle is shown in Fig. 2.11.

In this cycle, phosphorus slowly moves from phosphate deposits on land and shallow ocean sediments to living organisms and then back again.

Phosphorus deficiency in the soil and features of its natural cycle

The primary source of phosphorus for plants is phosphate rock, which releases the element through slow weathering and erosion. The released phosphorus dissolves in soil moisture and is absorbed by the root system. At the same time, in most soils, its natural reserves are minimal: phosphate compounds are extremely poorly soluble in water and are present only in certain types of rock.

Further movement of the element occurs along the food chain — animals obtain phosphorus by eating plants or other herbivorous individuals. A significant portion of phosphorus returns to the soil, rivers, and the ocean floor in the form of insoluble phosphate sedimentary rocks along with excrement and decomposing organic residues. Partial compensation of the elements on land occurs through the excrement of fish-eating poultry, however, this volume is incomparable to current losses.

Every year, an incomparably larger amount of phosphates is washed from the land surface into the ocean due to natural factors and human economic activity. Natural replenishment of the element occurs only during geological processes lasting millions of years. The uplift and drying of sections of the ocean floor form new islands and continents, after which the weathering of exposed rocks triggers the cycle of phosphorus release again.