Biogeochemistry of phosphorus, iron, and trace elements in soils as influenced by soil-plant-microbial interactions

Interactions among soil, plant, and microorganisms are most prevalent in rhizosphere, the region adjacent to plant roots where soil-chemical, -physical, and -biological properties differs from that of the bulk soils as a result of the interactive processes. The boundary where the rhizosphere ends and the bulk soil begins, however is not easy to define and varies with plant species and age and morphology of roots. At any given time, the rhizosphere accounts for only a very small fraction of the soils in the root zone. Yet, all of the surface soils, at one time or another, may be under the influence of roots. Its chemical composition is dynamic and complex and its microbial community is dense and diverse and with presently available methodology chemical and microbiological properties of the rhizosphere are impossible to determine in situ. Present knowledge of the rhizosphere biogeochemistry is based largely on ex situ experiments employing test tube cultures and hydroponics, mostly under aseptic conditions. The rhizosphere is enriched with organic substances of plant and microbial origins that include organic acids, sugars, amino acids, lipids, coumarins, flavonoids, proteins, enzymes, aliphatics, aromatics, carbohydrates and other substances. Among them, the organic acids are the most significant as substrates for microbial metabolism and precursors of chemical reactions. In the rhizosphere, organic acids of microbial origin are virtually of the same composition as those synthesized by plants and enter the rhizosphere as cell lysis, root leakage and secretion of root exudates. The most commonly found organic acids in the rhizosphere are acetic, butyric, citric, fumaric, lactic, malic, malonic, oxalic, propionic, and succinic acids. Chemical processes involving organic acids release not readily available plant nutrients such as iron, phosphorus, and trace elements and induce weathering of soil minerals through reduction, dissolution, and complexation. reactions.