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Toxicity to soil microorganisms

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Studies of the effects of ammonium sulfate on three nitrogen-fixing soil bacteria and on total soil bacteria have been carried out. The abundance of nitrogen-fixing cyanobacteria in a Spanish rice field was reduced significantly following a single ammonium sulfate application even at the lowest application of 82.5 kg/ha Ammonium sulphate (= 98.7 kg/ha Ammonium hydrogencarbonate, calculated from 17.5 kg N/ha) (Fernández Valiente et al., 2000). In another experiment, biological nitrogen-fixing ability in a field under crop rotation in southern Sweden was reduced (nitrogen-fixing legume bacteria) or eliminated (nitrogen-fixing blue green algae) by over thirty years annual application of 377 kg/ha ammonium sulfate (= 451 kg/ha ammonium hydrogencarbonate, calculated from 80 kg N/ha) (Martensson and Witter, 1990). The lowering of soil pH by ammonium sulfate was the main cause of the reduction in the nitrogen-fixing capacity 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 base respiration rate) can be affected by ammonium sulfate applied at 98.7 kg/ha or more.