Chemical Fractionation and Behavior of Heavy Metals In Upland Field after Long-Term Application of Pig Farm Manure.

Heavy metal contamination in agricultural fields by excessive application of livestock manure has become evident.  The application of pig farm manure (PFM) not only results in the accumulation of heavy metals in the plow layer, but also enhances transfer of heavy metals toward deeper layers.  Soils were sampled at fields under chemical fertilizer (CHF), 53 Mg-PFM ha^-1year^-1 (SM) and 160 Mg-PFM ha^-1 year^-1 (TM) application during 13 years.  We examined the adsorption characteristics of Zn, Cu and Mn by six-step sequential extraction procedure.  Also, adsorption isotherms of Zn in the soils applied chemical fertilizer and PFM were determined.

Adsorption isotherm of Zn was fitted with Freundlich isotherm.  Distribution coefficients for adsorption isotherms of Zn, K_F(dm³ kg^-1), increased as the amount of applied PFM increased.  Greater K_F suggests higher Zn adsorption capacity of the soil in the plow layer. 

Metal-organic-complex-bound (Me-Org), exchangeable-with-CH₃CO₂Na (NaOAc) and water soluble-exchangeable (EXCH) Zn fractions of soils from 0-60cm deep layers under the TM and SM were significantly greater than those under the CHF.  On the other hand, H₂O₂-extractable-organically-bound (H₂O₂-Org) fractions of the same layers were less than those under the CHF.  Outer-sphere complex form with organic matter can not retain Zn strongly.  Following to PFM decomposition, Me-Org, heavy metals associated with humic acid and fulvic acid in the soil, and H₂O₂-Org, heavy metals associated with persistent organic matter (ex. soil organic matter taken up by soil organisms) changed to be more mobile, EXCH and NaOAc extractable forms.

Even though adsorption capacity of Zn may increase with PFM application, the greater amount of mobile Zn is likely to be moved downward by water percolation and moved laterally by surface water runoff after excessive and continuous application of PFM to the soil over a long time.