Abstract
The effects of additions of various organic materials and nitrogen fertilizers on legume bacteria, blue-green algal populations and free-living nitrogen-fixing soil microorganisms in a 30 yr old field experiment have been investigated. Soil pH was found to be an important regulating factor for the occurrence and activity of nitrogen-fixing microorganisms. Nitrogen-fixing processes were retarded in the soil treated with sewage sludge compared to soils with a similar pH, indicating that other unfavourable conditions had build up in the soil through the sewage sludge additions, likely elevated metal concentrations. Relatively high numbers of Rhizobium leguminosarum biovar trifolii were found in all treatments although no legumes have been grown since the start of the experiment. Numbers of rhizobia in the soils were related to soil pH and all isolates were equally effective. Delayed nodulation was observed in plants inoculated with bacteria isolated from plots treated with sewage sludge. No increase in heavy-metal resistance was found in these bacteria when studied on agar plates. The occurrence and nitrogen-fixing activity of blue-green algal populations were correlated with soil pH. Algae were present in all treatments, except for the ammonium sulphate fertilized treatment (pH 4.4). Maximum nitrogen-fixing activity of the blue-green algae when grown under optimum conditions occurred after 1.5 months, except for the soil treated with sewage sludge, where maximum activity was found after 5 months, when it was ca 100 times lower than in the other treatments. There was no indication that this slowly established population was better adapted to growth in the presence of elevated heavy-metal concentrations when grown in liquid media. Nitrogen-fixation activity of the free-living aerobic nitrogen-fixing bacteria was correlated with soil pH and was reduced in the sewage sludge treated soil, compared to soils with similar pH. The implications of the results for control of metal contamination of soils is discussed.
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