Effects of heavy metal accumulation in apple orchard soils on the mineralization of humified plant residues

Abstract

A laboratory incubation experiment was conducted to determine the effects of heavy metals accumulated in apple orchard soils on the mineralization of humified plant residues. Apple orchard soils with varying concentrations of Cu, Pb, and As were incubated for 70 d at 25°C with and without humified orchardgrass prepared in the laboratory. The largest amounts of total Cu, Pb, and As found in the soils used were 1,010, 926, and 153 mg kg^-1 soil, respectively, and the amounts of Cu and Pb extractable with 0.1 M CaCl₂ were each negatively correlated with the soil pH. Although the amount of mineralized C from the humified orchardgrass added to the soils did not show a close relationship to the total concentrations of heavy metals, it was negatively correlated with the amount of 0.1 M CaCl₂ -extractable Cu and positively with the soil pH throughout the 70-d period of incubation. It was correlated with the amount of 0.1 M CaCb-extractable Pb only during the early period of incubation, while not with the amount of 0.1 M CaCl₂ -extractable As. The results indicated that in apple orchard soils with heavy metal accumulation the C mineralization of humified plant residues was inhibited by the pH-dependent water-soluble and exchangeable heavy metals, especially Cu.