Phosphorus and heavy metal attachment and release in sandy soil aggregate fractions

The presence of P and heavy metals in different forms or in association with different size fractions influences availability and discharge of these elements from watersheds. Understanding the association of P and heavy metals with size fractions can improve evaluation of leaching potential of P and heavy metals from soils. In this study, five aggregate-size fractions, ranged from 1.00 to 0.50 to <0.053 mm, were separated from seven Florida sandy soils by dry sieving. Each aggregate fraction was characterized by phosphate sorption, sequential fractionation of P, total, water- and Mehlich III-extractable con centrations of P and heavy metals. Size differences in sand, silt, and clay aggregates influence the amount and strength of element binding. Elemental attachment (particularly heavy metals) increased with decreasing aggregate sizes. Phosphorus and heavy metals in the sandy soils are readily transported to surface waters with suspended fine particles. Higher percentages of water-extractable, Mehlich III-extractable P, and heavy metals were found in both the 0.50- to 0.25- and 0.25- to 0.125-mm aggregate fractions, suggesting that P and heavy metals in these two fractions had higher release potential. The sequential fractionation of P suggested that the 1.00- to 0.50-mm fraction contained a larger percentage of Ca-bound P, whereas the 0.50- to 0.25-, 0.25- to 0.125-, and 0.125- to 0.053-mm fractions had higher ratios of labile P (H2O-P and NaHCO3-P). Phosphorus release from smaller aggregate fractions is faster with a higher P-1/P-168 ratio than from larger aggregate fractions because of larger amounts of water soluble P attached in the smaller aggregate fractions.