Changes in lead availability affect bacterial community structure but not basal respiration in a microcosm study with forest soils

This study investigates the effects of Pb during time on the bacterial communities of forest soils using water-extractable Pb concentrations in the soil solution as predictors of Pb bioavailability. In a microcosm experiment we applied increasing concentrations of Pb(NO3)2 solutions (0.5, 2, 8, 32 mM) to 5 forest soils of pHb5 and to a calcareous soil of pHN6.5. Sampling of the microcosms was performed after 3, 30 and 90 days of incubation. Community analysis included basal respiration rates and changes in the structure of the bacterial communities through T-RFLP fingerprinting. We also investigated functional stability in terms of resistance, expressed as the effects on basal respiration after 3 days of incubation, and of resilience, expressed as the recovery of bacterial community structure and of respiration rates after 90 days of incubation. Water-extractable Pb increased with time in most of the soils, in parallel with an increase of water-extractable dissolved organic carbon (DOC). The increased concentrations slightly affected bacterial community structure, although OTU (operational taxonomic unit) richness was not significantly reduced with Pb concentrations in any of the soils. The highest Pb treatment (32 mM) caused significant effects on basal respiration in some of the acidic soils, but no clear trend was observed in relation to increased Pb bioavailability with time. Resistance to Pb additions was evident in five of the six soils, but only two showed resilience after 90 days. This is the first study showing the effects of time on Pb bioavailability in soils and on the resulting reactions of the soil microbial communities.