Saturday, 15 July 2006

Phosphorus Release and Bioavailability Form Agri-Waste Amended Soils under Chloride and Sulphate Salts.

Zahoor Ahmad, Haytham El Sharkawi, Toshimasa Honna, Sadahero Yamamoto, Muhammad Irshad, and Faridullah n/a. Tottori Univ, Laboratory of Soil Science, Dept of Bio-Environmental Sciences, Faculty of Agriculture, Tottori, Japan

Agricultural lands are the ultimate places for the disposal of Agricultural wastes produced in thousands of tones every year. Apart from heavy metal contamination it caused serious non-point source of Phosphorus (P) loading on water bodies in many areas of the word. The ameliorative effect of P on plant growth under saline environment has broadened the scope of this hazard. Knowledge about the P release and uptake under sulphate (SO4) salinity is very limited as compared to chloride (Cl) salts. Three different studies were conducted to evaluate the release and bio-availability of P from different P sources under chloride and sulphate salinity. First pot experiment was conducted with NaCl and Na2SO4 (75 and 150 mmolc L-1) salt saturated soils. Phosphorus (P) was amended at the rate of 100 kg ha-1 in the form of composted livestock manure (Po), KH2PO4 (Pi) and Po+Pi (Pc) along with the control. Wheat was used as a test crop. Chloride salt severely suppressed the plant grow than SO4. Chloride salt was found more toxic for P uptake than the SO4 salt and recovery percentage was also lower in Cl salt treated pots. Water extractable P in post harvested soil was higher in SO4 salt treated pots. Combination of organic and inorganic P sources (Pc) gave better plant growth and higher P uptake than each source applied alone. However, phosphorus recovery was equally higher in Pc and Po sources than Pi treatment. In second laboratory experiment, soil was amended with three P sources and subjected to one salt and nine subsequent water extractions. Irrespective of the P sources, P release was substantially increased in the soils pre-saturated with salts as compared to the non-saline soil. Sulphate (SO4) salts released higher amount of P in subsequent water extractions than chloride (Cl) and a synergistic effect has been observed between Na and SO4 ions for P release. Phosphorus release differed for the salt types and P sources as Na2SO4 > NaCl > K2SO4 > KCl and livestock composted manure (Po) ≈ KH2PO4 (Pik) > Control > Ca(H2PO4)2 (Pic), respectively. Biological available P (extracted with NaHCO3) was higher in Cl than SO4 salt treated soils while P sources did not differed significantly. Most of the Al and Fe associated P fractions (extracted with NaOH) were recovered in control followed by Pik and Pic. A substantial amount of stable Ca associated P fractions were extracted with HCl however P sources and salt types did not differ. Third study was conducted to evaluate the release of P in the leachate by Cl and SO4 salt irrigations. Soil was amended with live stock manure, poultry droppings and sludge composts. Saline irrigation was done with Cl and SO4 salts (60 mmolc L-1) and leaching fraction of 0.2-0.3 was maintained. Sulphate irrigation substantially increased the P concentration in the leachate as compared to Cl salt irrigation. Phosphorus concentration in the leachate was higher in the pots amended with sludge compost followed by animal compost and P concentration gradually reduced in each subsequent leaching. It can be inferred from the studies that SO4 salt has more ionic strength to replace the PO4 ions than the Cl and P release depends upon the type of P source rather than the quantity of P applied.

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