Effects of lead and cadmium nitrate on biomass and substrate utilization pattern of soil microbial communities

Abstract

A study was conducted to evaluate the effects of different concentrations of lead (Pb) and cadmium (Cd) applied as their nitrates on soil microbial biomass carbon (C mic) and nitrogen (N mic), and substrate utilization pattern of soil microbial communities. The C mic and N mic contents were determined at 0, 14, 28, 42 and 56 days after heavy metal application (DAA). The results showed a significant decline in the C mic for all Pb and Cd amended soils from the start to 28 DAA. From 28 to 56 DAA, C mic contents changed non-significantly for all other treatments except for 600 mg kg −1 Pb and 100 mg kg −1 Cd in which it declined significantly from 42 to 56 DAA. The N mic contents also decreased significantly from start to 28 DAA for all other Pb and Cd treatments except for 200 mg kg −1 Pb which did not show significant difference from the control. Control and 200 mg kg −1 Pb had significantly lower soil microbial biomass C:N ratio as compared with other Pb treatments from 14 to 42 DAA, however at 56 DAA, only 1000 mg kg −1 Pb showed significantly higher C:N ratio compared with other treatments. No significant difference in C:N ratio for all Cd treated soils was seen from start to 28 DAA, however from 42 to 56 DAA, 100 mg kg −1 Pb showed significantly higher C:N ratio compared with other treatments. On 56 DAA, substrate utilization pattern of soil microbial communities was determined by inoculating Biolog ECO plates. The results indicated that Pb and Cd addition inhibited the functional activity of soil microbial communities as indicated by the intensity of average well color development (AWCD) during 168 h of incubation. Multivariate analysis of sole carbon source utilization pattern demonstrated that higher levels of heavy metal application had significantly affected soil microbial community structure.