Influence of heavy metals on C and N mineralisation and microbial biomass in Zn-, Pb-, Cu-, and Cd-contaminated soils

Abstract

Eight soil samples were taken along a gradient of metal contamination, from a heavy metals contaminated pasture that was polluted by the disposal of wastes of metallurgic industry that occurred 70 years ago. The concentrations of heavy metals (Zn, Cd, Pb and Cu) were estimated by total contents and DTPA-extractions. The total contents of the heavy metals ranged from 108 to 1231 mg Zn kg −1, 2.5 to 5.2 mg Cd kg −1, 31 to 466 mg Pb kg −1 and 33.5 to 107.5 mg Cu kg −1. The DTPA extractable fractions were highly correlated with the total contents of metals in the soils. Therefore, total metal contents and DTPA extractable fractions both presented similar correlations with the physico-chemical and biological characteristics of soils. Organic C, total N, microbial biomass C and N, ranged from 19.7 to 72.3 g C kg −1, 0.9 to 5.4 g N kg −1, 197 to 1230 mg C kg −1 and 62 to 122 mg N kg −1. They were positively correlated with the metal content of the soils, while the ratio of biomass C/organic C, which varied from 1.28 to 1.82%, was unaffected by heavy metal contamination. The ratios of biomass C/biomass N, which ranged from 3.2 to 10.3%, were highly positively correlated with soil metal contents, indicating variation in the microbial community in relation to contamination. On the other hand, microbial activities such as respiratory intensity, specific respiration, the respiration rate, net N mineralisation and the intensity of net N mineralisation of biomass N, which varied from 19.3 to 54.7 mg C-CO 2 g −1 organic C, 0.02 to 0.24 mg C-CO 2 mg −1 C biomass, 2.4 to 8.9 mg C-CO 2 g −1 organic C per day, 15 to 46 mg N mineralised mg −1 total N, 0.13 to 0.88 mg N mineralised mg −1 N biomass, were all negatively correlated with the heavy metal contents of the soils.