Erth is a closed system, where the over all quantity of matter remains constant. Microorganisms need electron, energy and nutrients to grow. They are responsible for cyclic transformation of compounds, and therefore they are called biogeochemical agents. They carryout transformation of carbon, nitrogen, sulphur, phosphorus, iron etc. This cycling of elements is called biogeochemical cycling. Both biological and chemical process are involved in biogeochemical cycling.

The oxidation reduction reactions are mainly responsible for biogeochemical cycling of compounds. This changes the chemical and physical characteristics of different compounds. These cyclic turnover of elements are brought about by different types of microorganisms resulting into continuous change in chemical states of matter.

The life of earth depends on cyclic conversion of chemicals from inorganic state to organic (complex state) to the elemental state. The break in the cycle at any point would dramatically affect all life forms. Various processes carrying out these transformation include :

Mineralization : 

It is a process of conversion of complex organic compounds to simple inorganic forms. Many heteroprophic microbes play role in mineralization.
The resultant simple compounds are made available to plants and microbes. Energy is released in the process. The process of mineralization is very important as it increases soil fertility.

Carbon mineralization :

  • Organic carbon is mineralized to inorganic state.
  • Under aerobic condition the main products of carbon mineralization are CO₂ and water.
  • In absence of O₂ organic carbon is incompletey metabolized to produce organic acids, alcohols and gases.

Assimilation :

  • It is the process of conversion of substrate elements to protoplasmic elements. 
  • Microbes take up the simple materials from the environment (soil) and convert them into cellular materials. It is known as assimiliation or biosynthesis.
  • In this process synthesis of energy and cellular material takes place. Through assimilation microbes store the excess of simple inorganic chemicals, and prevent their loss due to erosion.

Immobilization :

It is a process in which the quantity of plant available nutrients are reduced in soil by microorganisms. The nutrient assimilation is an important method of immobilization.
The uptake of various elements like carbon, nitrogen, phosphorus, sulphur, etc. cause immobilization.

  Intermediary substances accumulate abundant quantities of CH4 and smaller amount of H₂ is evolved. The factors affecting mineralization are level of organic matter, temperature, moisture, pH, depth and aeration.
  All these factors affect the growth and metabolism of microbes and the process of mineralization.

  • In nitrogen mineralization ammonium, nitrite and nitrates are accumulated from organic nitrogenous compounds like proteins, nucleic acids, etc.
  • In phosphorus mineralization organic phosphorus present in nucleic acid, phytin, lecithin, etc. are converted to inorganic phosphorus.
  • Sulfur mineralization involves aerobic breakdown of sulfur containing amino acids : cystine, cysteine, methionine, and vitamins, thiamine, biotin, thioctic acid releasing sulfates. Whereas in absence of oxygen , H₂S and odoriferous mercaptans accumulate in soil.