Comparison of the effects of Cu, Pb, and As on plant residue decomposition, microbial biomass, and soil respiration

Comparisons were made among the effects of Cu, Pb, and As on the plant residue decomposition, microbial biomass, and soil respiration using soils artificially enriched with Cu, Pb, and As. A total of eight metal treatments, consisting of a control, single metal treatments (10 mmol Cu kg(-1), 20 mmol Cu kg(-1), 10 mmol Pb kg(-1), and 10 mmol As kg(-1)), and combined metal treatments (5 mmol Cu kg(-1) + 2.5 mmol Pb kg(-1) + 1 mmol As kg(-1), 10 mmol Cu kg(-1) + 5 mmol Pb kg(-1) + 2 mmol As kg(-1), and 20 mmol Cu kg(-1) + 10 mmol Pb kg(-1) + 4 mmol As kg(-1)) were prepared. The inhibition of orchard grass decomposition was estimated by the difference in the contents of organic C and total N of soils treated with individual metals with and without orchard grass amendment following incubation under field conditions for 3 years. In the metal concentration range studied, Cu inhibited severely the orchard grass decomposition, Pb slightly inhibited it, while As had no notable effect on it. The amounts of microbial biomass C and N and the soil respiration were measured for the soils with and without orchard grass amendment at the end of the field incubation. When compared on a molarity basis, the toxicity was in the order of Cu>>Pb>As with respect to both the microbial biomass and the soil respiration, regardless of the orchard grass amendment. Furthermore, the addition of heavy metals tended to increase the respiration per unit of biomass C in the soils without orchard grass amendment. Nevertheless, the biomass specific respiration showed a highly negative exponential correlation with the amount of biomass C.