Phosphorus and Metal Fractions in Paddy Soils Under Different Fertilizer Management

ABSTRACT

For sustainable nutrient management in rice, it is important to understand the long-term effects of fertilizer or manure application on the chemical forms of elements in soils under cultivation. This experiment was carried out to characterize sequentially extracted forms of phosphorus (P) and selected heavy metals (copper [Cu], zinc [Zn], nickel [Ni] and lead [Pb]) in a paddy soil after long-term application of cow manure, oil-seed cake and inorganic fertilizers. The P forms studied were H₂O-P, NaHCO₃-P, NaOH-P, and HCl-P. The soil treated with organic fertilizers had greater accumulation of all forms of P. The total P in the soil increased up to 5.8-fold in cow manure, 4.6-fold in oil-seed cake, and 3.6-fold in the inorganic fertilizer, compared with unamended control. Water-soluble P was highest in the soil treated with inorganic fertilizer, but the HCl-P and NaOH-P constituted the largest P fractions in the soils. Regardless of the amendment, extracted P decreased in the order of HCl > NaOH > NaHCO₃ > H₂O. Marked changes were also noted for heavy metals due to long-term manure or oil-seed cake application. Nickel, Pb, and Zn were dominant in carbonate and residual fractions, whereas Cu was mostly present in the organic form. Soil treated with organic fertilizer contained more Cu and Zn, whereas variations in Ni and Pb were not statistically significant. Potassium, Ca, and Mg increased substantially with organic fertilizers. The lower amounts of readily extractable elements in the soil surface could reflect their loss to the waterways or exhaustion by rice plants.

KEYWORDS:

• Fractionation
• heavy metal
• fertilizer
• paddy soil
• phosphorus

This research has been continued by the financial assistance of Japanese Ministry of Education, Culture, Sports, Science and Technology to the first author, as a visiting Associate Professor.