The plants, besides ten macro and six micronutrients, require a few other elements. All of which participate in its organization and metabolism. They flow from the environment into and out of the plant in a cyclic manner. This flow of nutrients from abiotic to biotic components of the ecosystem and biotic to abiotic components of the ecosystem this cycle is called the biogeochemical cycles. Some important cycles are given below:
- Carbon cycle : Total carbon of the body of living organisms is obtained from CO₂ of atmosphere. Autotrophs take CO₂ from atmosphere for the synthesis of their food. The autotrophs are eaten by herbivorous which in turn are eaten by consumers for building their body. Thus by a study of food chain or food web we find that all animals depend for their food on plants directly or indirectly. All organic compound are oxidised to CO2 and water to be reutilized by plants. Coral reefs, coal, petroleum are all parts of CO₂ cycle and are formed in nature by living organisms.
- Nitrogen cycle : The atmosphere is the source of nitrogen but unless this is converted to NO3 form, plants cannot absorb it. Some plants can directly take atmospheric nitrogen. Protoplasm is mostly made up of nitrogenous substances. Decomposition of proteins is called putrefaction. NH3 so released is converted first to NO₂ and then to NO3. This conversion is termed nitrification. Although most of the nitrogen is absorbed by plant as nitrate, ammonia can be absorbed in low concentration up to some extent. Leguminous roots have symbiotic nitrogen fixing bacteria in their roots. Denitrifying bacteria also play an important role in nitrogen cycle. They oxidize NO3 to free nitrogen thus decreasing soil fertility.
- Phosphorus cycle : Phosphorus is also an important component of nucleic acids, ATP, NADP and phospholipids, etc. The biotic component of the ecosystem procures phosphorus from the abiotic component, (e.g., atmosphere and soil) and the decomposition of organic matter replenishes this phosphorus of atmosphere. Soil derived from lake or ocean beds are usually rich in phosphates due to the decomposition of marine animals. Inorganic phosphate usually occurs in the insoluble form. It is dissolved by excretions of micro-organisms and is absorbed by plants from soil. Plants later convert it to the organic form. Rocks are another source of phosphorus.
- Oxygen cycle : Oxygen is required by all living organisms for respiration. It occurs in the gaseous form in the atmosphere to the extent of 20-21%. It also occurs as CO2, H2O and in the form of various oxides and salts. The land organisms take it directly from the atmosphere, whereas aquatic and marine forms take it by the process of diffusion of oxygen from water. It is also utilized in various non- biological processes to produce various oxides. sulphates, nitrates etc. The major replenishment of oxygen is carried out by the green plants by photosynthesis. Oxygen evolved in the process of photosynthesis comes directly from the splitting of water molecules.
- Water cycle : Water is an essential constituent of all living organisms It constitutes 65-95% of the body weight of an organism. The source of water in the atmospheres is due to the precipitation of water vapours from the surface of lakes, rivers, and oceans. There is a continuous process of circulation of water from sea to land and again from land to sea. Plants obtain water from their surroundings by conducting tissues. Most of the water which is absorbed by the plants is lost through transpiration process. Animals obtain water through their food and drink. It is recycled in the environment through sweat and excretion.
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