The Phosphorous Cycle

THE PHOSPHORUS CYCLE

Phosphorus exists almost exclusively as phosphate i.e. orthophosphate esters. In the lithosphere, it is most common as apatite [3Ca₃F(PO₄)₃ x Ca(FeCl)₂] and as flurapatite [Ca₅F(PO₄)₃]. In animals, it is found as hydroxyapatite [Ca₅OH(PO₄)₃ in bones. It is not found in atmosphere. No gaseous form of phosphours exists. In soils, it may be present up to order of magnitude of 1gm per Kg of the soil. Less than 5% is available to the organisms in soluble form. In aqueous environment, it is extremely small as compared to soil i.e. 0.01 to 0.07 mg per liter.

Phosphorus is one of the most important constituents of several important compounds always present in organism. It occurs both in organic (nucleic acids, nucleopoteins, phospholipids, etc.) and inorganic (phosphate) forms in the living organisms. Phosphate also forms an essential portion of the ATP molecules. The hydrolysis of phosphate from ATP to ADP forms the basis of most energy transfer reactions within biological system. Animals possessing bones have large amount of phosphorus in its inorganic form. However, phosphorus is added to soil through chemical fertilizers, excrete and organism-resides. Though there is plenty of phosphorus present in the soil in unavailable inorganic forms, most of the plants obtain it only as orthrophosphate ions (soluble inorganic forms). However, mycorrhizae, when present, help the plants in obtaining phosphorus.

However, phosphorous is not an abundant component of the biosphere and often limits microbial growth. Its availability is restricted by its tendency to precipitate in the presence f bivalent metal ions (Ca²⁺, Mg²⁺) and ferric (Fe³⁺) at neutral to alkaline pH. Large and slowly cycled reversoirs of phosphate occur in marine and other aquatic sediments. Small and actively cycled reservoirs of phosphate are dissolved phosphate in soils and waters and phosphate in the living and dead organic matter.

The cycle of phosphorus is schematically represented in Figure below and can be well studied two heads:

I. Mineralization: Conversion of Organic Phosphorus into Insoluble Inorganic Phosphates

Many soil microorganisms produce enzymes that attack many of the organic phosphorus compounds in the soil and release inorganic phosphate. This process is comparable to the mineralization of organic nitrogen compounds. The enzymes involved in these reactions are collectively called ‘phosphatase or phytase’ which have a broad range of substrate specificity.

Inositol hexaphosphate + 6H₂O                      ®                    Inositol + 6PO₄^3-                                                                                                                     Phytase  

Microorganisms compete with plants for available PO₄^3- in the soil. They are involved in both mobilization and immobilization of phosphate in the soil.

The productivity in many habitats is PO₄^3- limited. The availability of phosphate in the soil and water depends on seasonal fluctuations. The flooding of soils enhances release of phosphate by this mechanism. Insoluble forms of salts of Iron, magnesium and aluminum is changed into soluble form in an anaerobic condition.

In Fe(PO₄)₃                 Fe³⁺                             ®                                Fe⁺² (more soluble form)                                                                                                            ^{reduce (Anaerobic condition)}

The availability of phosphate greatly influenced biomass production especially in marine habitat e.g. algal and cyanobacterial blooms. It causes eutrophication in fresh water habitats such as ponds and lakes.

II. Solubilization: Conversion of Insoluble Inorganic Phosphates into Soluble Phosphates

The availability of phosphorus depends on the degree of Solubilization by various organic and inorganic acids produced by microorganisms in soil. These are the solubilized form of insoluble inorganic phosphates which are taken in by the plants. Fungi, e.g., Aspergillus, Pencillium, Fusarium are the most important of the soil microorganisms which produce substantial amounts of these acids; others are the bacteria, namely, Thiobacillus (T. thiooxidans, T. ferroxidans), Nitrosomonas, Bacillus (B. megaterium), Pseudomonas, Micrococcus, Flavobacterium, etc.

The overall conversion of insoluble inorganic phosphates into soluble inorganic phosphates by the action of acids can be exemplified via reactions as under:

Figure 1: Chemical reaction for phosphate solubilization

Figure 2: Schematic representation of Phosphorous cycle

The action of acids to convert insoluble phosphates into soluble ones is generally called ‘solubilization’ and particularly takes place in close proximity of the root surfaces where sugar from root-exudates are converted of microorganisms into organic acids.

Super Bio-phosphate: It is a biofertilizer which supplies S and P sources to plants. The elemental sulphur is mixed with thermophilic bacteria, Thiobacillus thioxidans and sprayed in the field rich in insoluble phosphate rocks. In presence of moisture, the bacteria oxidize sulphur and produce sulphuric acid. The acid then mineralizes the phosphate rocks to release soluble phosphate.