Evaluating the Effects of Metals on Microorganisms in Flooded Paddy Soils Using the SEM/AVS-Based Approach and Measurements of Exchangeable Metal Concentrations

We examined possible adverse effects of heavy metals on microbial activity, biomass, and community composition using the simultaneously extracted metals (SEM)/acid-volatile sulfide (AVS)-based approach and measurements of exchangeable metal concentrations in three paddy soils (wastewater-contaminated soil, mine-contaminated soil, and noncontaminated soil) incubated for 60 days under flooded conditions. Incubation under flooding increased pH and decreased Eh in all samples. AVS increased when Eh decreased to approximately −200 mV for the mine-contaminated and noncontaminated soils, while the wastewater-contaminated soil originally had a high concentration of AVS despite its air-dried condition. Addition of rice straw or alkaline material containing calcium carbonate and gypsum increased AVS levels under flooded conditions. We observed no apparent relationship between soil enzyme activity (β-d-glucosidase and acid phosphatase) and concentrations of SEM, [∑SEM − AVS], and exchangeable metals. Bacterial and fungal community composition, assessed using polymerase chain reaction-denaturing gradient gel electrophoresis (DGGE) analysis targeting rRNA genes, was largely influenced by site of collection and incubation time, but metal contamination did not influence community composition. We observed significant negative correlations between biomass C and [∑SEM − AVS] and between biomass C and ∑SEM, suggesting that [∑SEM − AVS] and ∑SEM might reflect the bioavailability of organic matter to microorganisms in these soils.