Influence of Microorganisms on Phosphorus Bioavailability in Soils

Abstract

Phosphorus is the most important plant growth-limiting nutrient in soils besides nitrogen. The total phosphorus content of arable soils varies from 0.02-0.5% with an average of 0.05% in both inorganic and organic forms (Barber 1995). While most mineral nutrients in a soil solution are present in millimolar amounts, phosphorus is only available in micromolar quantities or less (Ozanne 1980). This low availability is due to the high reactivity of phosphorus with calcium, iron and aluminum. Alkaline soils contain different calcium phosphates like hydroxyapatite and fluoroapatite, while acidic soils include amorphous iron-and aluminum phosphates, variscite (AlPO 4 · 2H 2 O), strengite (FePO 4 · 2H 2 O) and similar minerals (Barber 1995). An important portion of inorganic soil phosphate is adsorbed onto iron and aluminum oxides and hydroxides, clay minerals and organic substances which contain iron or aluminum complexes. Organic phosphorus compounds have to be mineralized or enzymatically cleaved to become available to plants. All these forms are not directly available to plants. As a result, higher plants depend on diffusion processes and a continuous release from insoluble sources to meet their phosphorus demand. As de-composers of organic matter as well as mobilizers of inorganic phosphates-in association and competition with higher plants-soil microorganisms influence the availability of phosphorus for plants to a great extent. 2 Microbial Effects on Rhizodeposition As a result of the low phosphorus mobility in soils, plants only have access to phosphorus a few millimeters around their roots. This volume is highly affected by root deposits and intensively settled by microorganisms, which