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Toxicity to soil microorganisms

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Studies of the effects of ammonium sulfate on three nitrogen-fixing soilbacteria and on total soil bacteria have been carried out. Theabundance of nitrogen-fixing cyanobacteria in a Spanish rice field wasreduced significantly following a single ammonium sulfate applicationeven at the lowest application of 82.5 kg/ha (calculated from 17.5 kgN/ha) (Fernández Valiente et al., 2000). In another experiment,biological nitrogen-fixing ability in a field under crop rotation insouthern Sweden was reduced (nitrogen-fixing legume bacteria) oreliminated (nitrogen-fixing blue green algae) by over thirty years annualapplication of 377 kg/ha ammonium sulfate (calculated from 80 kg N/ha)(Martensson and Witter, 1990). The lowering of soil pH by ammoniumsulfate was the main cause of the reduction in the nitrogen-fixingcapacity of the soil. In this experimental field, total soil biomass was reduced by almost 50 % relative to unfertilized control plots (Witter,Martensson and Garcia, 1993), although base respiration rate was unaffected.

Conclusion:

Nitrogen fixation and total soil biomass (but not soil baserespiration rate) can be affected by ammonium sulfate applied at 82.5kg/ha or more.

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