2. Forms and Behaviors of Phosphorus in Soils Soil P exists in inorganic and organic forms and both are important to plants as a source of this nutrient element (Schachtman et al., 1998; Brady, 2001). Each form is a continuum of many P compounds, existing in equilibrium with each other and ranging from solution P to very stable or unavailable compounds. Inorganic P is usually associated with calcium (Ca), aluminum (Al), and iron (Fe) compounds of varying solubility and availability to plants. Depending on soil pH, P can rapidly be fixed thus forming complex compounds, which makes P is unavailable to plants. The maximum P availability in soils for plant uptake is obtained between the soil pH 6.0-7.0 (Brady, 2001). The recovery of P in fertilizer is usually 10-30% (Roberts, 1995). In acid soils Fe, Al and Mn and their hydrous oxides are usually present and reactions with H2PO4- immediately occur thus forming insoluble hydroxyl phosphate. If soluble P is added to this kind of soils the concentration of P in soil solution increases, however, with a set of reactions its concentration quickly decreases (Ruaysoongnern and Keeratikasikorn, 1998). While in the alkaline soils with pH 8.0 or above P becomes fixed with Ca and forms the most insoluble tri-calcium phosphate at a later stage (Brady, 2001). Conversion of unavailable to available forms of soil P usually occurs too slowly, therefore, there will be a shortage of P supply for plant requirements. However, these kinds of transformations of P in soils are controlled by the physical and chemical conditions of soils, which include soil pH, clay content, exchangeable Al, and soil organic matter content, etc.
Organic P compounds range from readily available forms, undecomposed plant residues and microbes in the soil to stable compounds that have become part of soil organic matter. The amount of organic P present in soils varies from 20 to 80% of the total P (Prasad and Power, 1997; Schachtman et al., 1998). Phosphorus cycling and availability in soils is controlled by a combination of biological (i.e. mineralization-immobilization) and chemical (i.e. adsorption-desorption and dissolution-precipitation) processes. Biological processes in the soil such as microbial activities, soil-root interrelationship, soil arthropods etc tend to control the mineralization and immobilization of organic P. Phosphorus immobilization by microorganisms, turnover of microbial P and mineralization of microbial byproducts seem to be the major processes regulating P cycling and its availability from plant residues in soils (McLaughlin et al., 1988a). Arbuscular mycorrhizal (AM) association plays an important role in the plant P nutrition (Lopez-Gutierrez et al., 2004). This kind of mutualistic symbiosis between plant and fungus is localized in a root or root-like structure in which energy moves primarily from plant to fungus and inorganic resources including P absorbed move from fungus to plant (Allen, 1991) can contribute greatly to plant P nutrition even at P deficient conditions (Joner and Jacobsen, 1995). AM contribution for nutrients especially P acquisition may be direct or indirect. The direct effect is the consequence of the production of extracellular phosphatases and the access to distant P sources otherwise not available to plants (Joner and Johansen, 2000). This provides additional benefits of P supply to plants in acid soils (with low pH below 5.0 to 5.5), when the P fixation becomes common due to increased solubility of Al. Fungi become dominant at pH below 5.5, which enables arbuscular mycorrhiza to grow and develop faster than that under pH above 5.5 and to support the plant to acquire soil P. The indirect effect is due to its extraradical hyphae that are capable of absorbing and translocating nutrients, can explore more soil volume and induce physiological changes that favor the establishment of P solubilizing and mineralizing microorganisms in the micorrhizosphere.
Organic P compounds range from readily available forms, undecomposed plant residues and microbes in the soil to stable compounds that have become part of soil organic matter. The amount of organic P present in soils varies from 20 to 80% of the total P (Prasad and Power, 1997; Schachtman et al., 1998). Phosphorus cycling and availability in soils is controlled by a combination of biological (i.e. mineralization-immobilization) and chemical (i.e. adsorption-desorption and dissolution-precipitation) processes. Biological processes in the soil such as microbial activities, soil-root interrelationship, soil arthropods etc tend to control the mineralization and immobilization of organic P. Phosphorus immobilization by microorganisms, turnover of microbial P, and mineralization of microbial byproducts seem to be the major processes regulating P cycling and its availability from plant residues in soils (McLaughlin et al., 1988a). Arbuscular mycorrhizal (AM) association plays an important role in the plant P nutrition (Lopez-Gutierrez et al., 2004). This kind of mutualistic symbiosis between plant and fungus is localized in a root or root-like structure in which energy moves primarily from plant to fungus and inorganic resources including P absorbed move from fungus to plant (Allen, 1991) can contribute greatly to plant P nutrition even at P deficient conditions (Joner and Jacobsen, 1995). AM contribution for nutrient especially P acquisition may be direct or indirect. The direct effect is the consequence of the production of extracellular phosphatases and the access to distant P sources otherwise not available to plants (Joner and Johansen, 2000). This provides additional benefits of P supply to plants in acid soils (with low pH below 5.0 to 5.5), when the P fixation becomes common due to increased solubility of Al. Fungi become dominant at pH below 5.5, which enables arbuscular mycorrhiza to grow and develop faster than that under pH above 5.5 and to support the plant to acquire soil P. The indirect effect is due to its extraradical hyphae that are capable of absorbing and translocating nutrients, can explore more soil volume and induce physiological changes that favor the establishment of P solubilizing and mineralizing microorganisms in the micorrhizosphere.
In tropical cropping systems, a better understanding of P transportations and P cycling during decomposition of incorporated organic residues is important (Salas et al., 2003). Much attention has been paid to determining plant, biological and environmental factors influencing the release and mineralization of N from plant residues (Vanlauwe et al., 1996). However, little information exists on the incorporation of plant residues on P cycling, especially on biological transformations associated with the release of P from residues and subsequent accumulation and turnover of organic P during decomposition.