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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.


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.