Cadmium bioavailability in surface soils receiving long-term applications of inorganic fertilizers and pig manure

Long-term field experiments were established in 1990 at two sites, one in northeast (17 years) and the other in south China (16 years) to better understand the effects of pig manure and inorganic fertilizers on heavy metal accumulation in soil. Experimental treatments were NPK and SNPK (Straw + NPK) fertilizers and two application rates of pig manure plus NPK at the site in northeast China and NK, NP, NK and PK fertilizers and pig manure with and without PK in the south. Unamended control plots were included at both sites. Both application rates of pig manure with NPK in the northeast for 17 years increased concentrations of aqua regia extractable Cd in the topsoil by about 17.0- and 18.9-fold but repeated applications of straw for 17 years did not result in a marked accumulation of Cd in the surface soil. Similar results were found in the southern plots treated with pig manure (with or without NPK). The distribution patterns of DTPA-extractable Cd in the northeast (BSES) were similar to that of aqua regia-Cd. HCl-extractable Cd in the topsoil treated with pig manure (with or without NPK) in the south exceeded that in the control soil by about 4.5- and 5.4-fold (RSES). Sequential extraction results show that in the northeast pig manure from intensively managed pig farms led to accumulation of Cd in both exchangeable and reducible soil fractions.