Availability of heavy metals in compost-amended soil

Composted municipal wastes can be applied to cropland to supply nutrients a nd improve soil physical properties, but farmers are concerned about heavy metal availability. Three municipal composts were applied at 0, 25, 50 and 100 x 10(6) g ha(-1) in 1994 to an unlimed and limed (pH 6.5) Dyke clay (cl ayey, mixed, mesic Typic Rhodudults), and burley tobacco (Nicotiana tabacum L.) was planted in 1994 and 1995. The composts were municipal solid waste (MSWC), wastewater biosolids (WBC) and co-composted municipal solid waste/w astewater biosolids (COC). Leaf samples were collected three times in each year and analyzed for heavy metal concentration. Soil samples were collecte d three times in each year, extracted with DTPA and extracts were analyzed for heavy metal concentration. With the exception of Cd in 1994 cured burle y leaves, Cd, Ni, and Pb concentrations were generally undetectable for all treatments in both years. When leaf Cu and Zn concentrations in cured leav es from COC and WBC in 1994 were regressed against amounts of Cu and Zn app lied with respective compost treatments, mean leaf Cu and Zn were significa ntly higher in COC (215 mg Cu kg(-1) and 738 mg Zn kg(-1)) than in WBC (173 mg Cu kg(-1) and 499 mg Zn kg(-1)) treatments. Because compost Cu and Zn c oncentrations in MSWC (53 mg Cu kg(-1) and 96 mg Zn kg(-1)) were much lower than in other composts, cured leaf Cu and Zn in MSWC treatments were not c ompared with cured leaf Cu and Zn in COC or WBC treatments. DTPA-extractabl e Zn, Pb, Cd and Cu concentrations increased with increasing soil pH at the 100 x 10(6) g ha(-1) rates of COC addition in September, 1994. Higher meta l composts were associated with higher metal extractability than were lower metal composts: when equal rates of metal addition to soil from different composts were compared, DTPA-extractable Cu and Pb concentrations in Septem ber 1994, were significantly higher in COC than in WBC treatments, and DTPA -extractable Cd concentration was significantly lower in MSWC treatments th an in COC or WBC treatments. (C) 1999 Elsevier Science Ltd. All rights rese rved.