Saturday, 15 July 2006

Changes in Phosphorus Fractions of a Mediterranean Calcareous Sandy Soil Following Long-Term Application of P Fertilizer.

Dang Thanh Vu1, Caixian Tang1, and Roger D. Armstrong2. (1) La Trobe Univ, Plenty Road, Bundoora, Melbourne, Australia, (2) Primary Industries Research Victoria, Natimuk Road, Horsham, Australia

There have been many short-term field experiments in which the changes in Phosphorus (P) fractionations have studied. The fate of phosphorus derived from granular fertilizers in the present study has been examined in long-term field experiment on a calcareous sandy soil (Calcarosol) in a semi-arid environment (annual rainfall = 325 mm) at Walpeup, Victoria, Australia. The experimental plots were established in 1940 and had rotation of fallow-wheat-oat before 1956 and a fallow-wheat system since 1960. Five rates of granular single-superphosphate fertilizer was applied to each crop at rates of 0, 3, 6, 9 and 12 kg P/ha. Soil samples were taken from 0-10 cm before sowing in 2005. Soil P was sequentially extracted with distilled water, 0.5 M NaHCO3 (pH 8.5), 0.1 M NaOH, 0.5 M H2SO4¬, and 1 M HCl and H2SO4 /H2O2¬. Inorganic P in each fraction was determined colorimetrically, and total P using ICP-AES. Organic P fractions was the difference between total P in extract and inorganic P fractions. The soil samples were also analysed for resin-P and Olsen-P. A glasshouse experiment was conducted to examine the utilisation of the different pools of P in this soil by two different crops: wheat (Triticum aestivum L. cv. Yitpi) and chickpea (Cicer arietinum L.). Following 8 weeks growth, P in shoots and roots were analysed. The rhizosphere soil was sampled and analysed for P fractions to examine the changes of P fractions after cropping. Application of P fertilizer increased significantly all inorganic soil P (Pi) fractions (P<0.05) except HCl-Pi. By contrast, the proportion of organic P (Po) in the extracts tended to decrease with increasing rates of P fertilizer applied to the soil. Residual P, inorganic P and organic P fractions were accounted for an average of 66%, 27% and 8% of total soil P, respectively. The amount of resin-extractable P (11.8-41.9 mg/kg soil) was similar to the amount of water- plus bicarbonate-extractable Pi (12.4-43.4 mg/kg soil). Olsen P was strongly correlated with water extractable P fraction and resin-P (r2 = 0.98). Thus, the long-term application of P fertilizer resulted in an increase of residual P and inorganic P fractions. All Pi fractions in the rhizosphere soil sharply decreased following 8 weeks growth of either wheat or chickpea, indicating that both species were able to access all inorganic pools, and that the residual P fraction could be a buffer for more labile P pools. In contrast, all Po fractions decreased significantly (P<0.05) except water-extractable Po, which slightly increased after the first cropping. This indicates that organic P was mineralised to replenish solution Pi. The chickpea plant showed a better growth in comparison with wheat and took up more P where no P was added to the soil, which was probably due to greater root exudation and lower rhizosphere pH of chickpeas. Increasing P fertiliser rate increased total uptake by both species but increased more in chickpea. It appears that the long term application of fertilizer P to the calcareous sandy soil resulted in a build-up of residual P and Pi fractions. However, these fractions and high proportion of Po fractions could be a potential source for plant available P. Legume such as chickpea appear to have better access to “non-labile” P pools than wheat in this calcareous sandy soil.

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