Earthworm and organic amendment effects on microbial activities and metal availability in a contaminated soil from China

We tested the effect of an addition of organic matter (OM: 10% of a mixture of coconut bran and cattle dung) and/or earthworm inoculation (15 g fresh weight kg−1 soil) in a soil polluted by metals on microbial activities and concentrations of metal DTPA extractable fractions. The experiment, conducted under laboratory conditions, lasted for 60 days. Soil organic C and total N in control were 15.3and 1.47 mg kg−1, and total Zn, Cd, Pb and Cu contents, 405, 0.639, 439 and 394 mg kg−1, respectively. The native earthworm species Amynthas morrisi exhibited 91.5% mortality in the un-amended polluted soil, whereas only 20% of Eisenia fetida individuals died. In the OM treatment the native species performed much better showing reproduction and a higher soil ingestion rate than E. fetida. In both amended and non-amended soils, casts exhibited higher concentrations of total organic C (+15.7 to 46.5%) and N(+13.3 to 59.3%) and alkali hydrolysable N (+78.0 to 133%), but not dissolved organic C. Microbial enzymatic activities were significantly increased when OM was added to the soil (+142 to 456%), with the sole exception of acid phosphatase activity. The addition of earthworms had contrasting effects on microbial activities: N-acetyl glucosamine activity was enhanced (+559 to 829%) while no significant difference was noted for other measured enzymatic activities. Introduction of earthworms in OM amended treatments tended to decrease all activities (−30.6 to −59.3%) although they were still higher than in the non- amended soil, especially for β-glucosidase (+182 to 230%). We noted no significant differences between the effects of the two earthworm species. Increased microbial activities resulting from the addition of organic matter did not substantially alter the availability of Zn, Cd, Pb and Cu assessed by their association with DTPA. Earthworm increased the availability of Zn (up to +31%), Cd (+ 78 to 193%) and decreased Pb (down to −16.4%) in the non-amended treatment. In the amended treatment earthworms induced an increase in Cd (up to 18.8%), but a decrease in Zn availability. Total Cd available concentrations in the experimental units (in soil and casts when earthworms were present) increased from 36.2% in control soil to up to 46.1% after 60 days depending on treatments, while no significant changes were observed for other metals. This rather important change obtained for Cd in such a short amount of time indicates a possible environmental risk.