The Availability of Copper in Soils Historically Amended with Sewage Sludge, Manure, and Compost

Metals in soils amended with sewage sludge are typically less available compared with those in soils spiked with soluble metal salts. However, it is unclear if this diff erence remains in the long term. A survey of copper (Cu) availability was made in soils amended with sewage sludge, manure, and compost, collectively named organic amendments. Paired sets of amended and control soils were collected from 22 fi eld trials where the organic amendments had aged up to 112 yr. Amended soils had higher total Cu concentrations (range, 2-220 mg Cu kg(-1); median, 15 mg Cu kg(-1)) and organic C (range, 1-16 g kg(-1); median, 4 g kg(-1)) than control soils. All samples were freshly spiked with CuCl2, and the toxicity of added Cu to barley was compared between amended and control soils. Th e toxicity of added Cu was signifi cantly lower in amended soils than in control soil in 15 sets by, on average, a factor of 1.4, suggesting that aged amendments do not largely increase Cu binding sites. Th e fraction of added Cu that is isotopic exchangeable Cu (labile Cu) was compared between control soils freshly spiked with CuCl2 and amended soils with both soils at identical total Cu concentrations. Copper derived from amendments was signifi cantly less labile (on average 5.9-fold) than freshly added Cu in 18 sets of soils. Th is study shows that Cu availability after long-term applications of organic amendments is lower than that of freshly added Cu2+ salts, mainly because of its lower availability in the original matrix and ageing reactions than because of increased metal binding sites in soil.