Enhancing Phosphorous Availability to Canola (Brassica Napus L.) Using P Solubilizing and Sulfur Oxidizing Bacteria (Report‪)‬

Introduction The two types of phosphorous (P) in the soil are organic and inorganic. On average, the amounts of P in the earth crust and agricultural soils are 0.12 and 0.06%, respectively. Different parameters such as soil pH, calcium concentration, amount of organic matter, type and amount of clay, soil moisture, soil texture, root density and exudates can affect the availability of soil P to the plant (Tisdale et al., 1993; Barber, 1995). Parameters including high pH, high amount of CaC[O.sub.3], little amount of organic matter and drought decrease P availability to plants in the calcareous soils of Iran, with arid and semiarid climates. Using P fertilizers, especially superphosphate, as a very common method of providing plant P requirement, is not very efficient in calcareous and alkaline soils. Because under such conditions high amounts of P are turned into insoluble products and become unavailable to the plant, as only 20% of the fertilizer is soluble in the first year of use (Tisdale et al., 1993). Application of P fertilizer has increased significantly to enhance crop yield production and as a result of using organic matter improperly. Rock phosphate, which is the main source of P fertilizers production, is not recyclable and its mines are found in North of Africa, Iran, USA, Russia, China, and Morocco, which produce 75% of the all world rock phosphate (van Kauwenbergh, 2001). Use of rock phosphate as a source of P fertilizer, which is a simple and in the meanwhile not expensive method, is recommendable for acidic soils, because in calcareous soils, high pH and high amount of CaC[O.sub.3], decreases the fertilizer solubility (Chein et al., 1996; Abd-Elmonem and Amberger, 2000). Different researchers have indicated that it is likely to increase P availability in soil. For example, acidizing rock phosphate, mixing rock phosphate with sulfur and organic matter and using rock phosphate with microorganisms including P solubilizing bacteria, sulfur oxidizing bacteria and arbuscular mycorrhiza are among the methods, used for enhancing P availability (Chien, 1996; Vessey, 2003). Plant residues can be used as a source of C for soil fungi and heterotrophic bacteria, which produce organic acids enhancing P availability in the rock phosphate through protonization and chelating. Acid strength, the amount of soluble calcium and type and properties of chelating ligands are among the parameters affecting P availability (Chein et al., 1996). Phosphate solubilizing microorganisms (10% of total soil microorganisms), which include a large number of soil micro-flora (Whitelaw, 1997; Sundara, 2001), can solubilize inorganic phosphate (including soil phosphate) with the production of inorganic (carbonic and sulfuric) and organic (citric, butyric, oxalic, malonic, lactic and etc.) acids and phosphatase enzyme (Whitelaw, 1997; Sundara et al., 2001). The activities of such microorganisms are affected by different soil parameters including soil fertility, temperature, moisture, organic matter, and soil physical properties (Kim et al., 1998). Sing and Kapoor (1992) indicated that mixing rock phosphate with P solubilizing bacteria, increased wheat yield between 32 to 42%, relative to the control treatment. In the same experiment, treatments of control, rock phosphate, Bacillus circulans and Cladosporium herbarum inoculants, mixture of rock phosphate and B. circulans, and mixture of rock phosphate and C. herbarum resulted in the wheat dry weights of 2.017, 2.04, 2.54, 2.76, 2.7 and 2.89 g/pot, respectively. Sundara et al. (2002) illustrated the great effects of P solubilizing bacteria on sugar beet yield when combined with rock phosphate. Scofield et al. (1981) evaluated the use of 1:5 rock phosphate and elemental sulfur with Thiobacillus tiooxidance as a source of P fertilizer (biosuper) in three calcareous soils in the greenhouse. They found that similar to superphosphate the biosuper fertilizer also increased trifolium yield and P uptake. The