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Toxicity to soil microorganisms

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Description of key information

Ferrous slags are not inhibitory to key metabolic activities of soil microorganisms. 

Key value for chemical safety assessment

Long-term EC10 or NOEC for soil microorganisms:
10 000 mg/kg soil dw

Additional information

It was shown by laboratory and field studies that ferrous slags are not inhibitory to key metabolic activities of soil microorganisms i.e. respiration. Nitrogen metabolism and cellulose degradation were also shown not to be inhibited, but even slightly activated.

Laboratory guideline study

To test the effects of slags, ferrous metal, blast furnace (air-cooled – ABS) on the soil microflora as an indicator for conservation of soil fertility, the metabolic activity of microbial biomass and its nitrogen conversion potential were determined. The tests were performed according to C.22 (Soil Microorganisms: Carbon Transformation Test)(identical to OECD-Guideline No. 217) and C.21 (Soil Microorganisms: Nitrogen Transformation Test) of the EU-Regulation440/2008 (identical to OECD-Guideline 216). The metabolic activity of the microbial biomass was influenced only temporarily by the slag in the acclimatisation phase. The respiratory activity (carbon transformation test) in the soil mixtures treated with 10 g test item / kg of soil (dry matter) was negatively affected only at the start of the incubation period t0 (28% reduction). At the later time points, the differences between slag-incubated and control soils were less than ± 25 %. After 28 d, the microbial respiration of the soil was slightly increased in the treatment with 10 g/kg soil dry matter in comparison to the controls.

The nitrogen conversion (ammonification and nitrification) of lucerne meal (alfalfa) which was added to the soil, was not negatively influenced by the slags applied at a dose of 10 g / kg (dry matter). In general, the deviations of the NO3-- and NO2--nitrogen values of the treated samples from the untreated ones were <25%. In regard to nitrogen metabolism, ABS served as a fertilizer.

Slags, ferrous metal, blast furnace (air-cooled – ABS) had no relevant effect on the activity of the soil microflora even in the highest concentration tested (10 g/kg) for 28 d (SGS 2010).

Extended laboratory study

To evaluate the effects of steelmaking slags (SMS) on soil microorganisms, in laboratory experiments SMS was added to oxisol soil at concentrations of up to 21.2 t/ha. The pH varied between 4.8 (controls) and 7.4 (SMS, 21.2 t/ha, no pH controls done). Dry matter production, pH and metal uptake into Sorghum bicolor plants raised in parallel experiments in pot were plotted against each other and a correlation analysis was performed.

The dry matter production of Sorghum bicolor was highest when the concentration of SMS added to oxisol was approximately 9.6 t/ha. Uptake of Ca, Mg, Fe, Mn and Zn shows a flat maximum at 7 -10 t/ha in regard to the concentration of slag in the oxisol (controls not reported). The uptake of Cu was independent of the slag concentration whereas the Ni content was low but increased with increasing concentration of slag in the range tested, and in parallel to the pH.

Minimum of microbial respiration occured at a SMS concentration of approximately 10 t/ha. This minimum is explained by the authors by pH controlled availability of metal ions. Although this hypothesis cannot be verified, it is apparent that the inhibition of microbial respiration is not caused by high slag concentration in the soil, as the microbial respiration recovered at the highest slag concentration tested (Costa et al. 1992).

Field studies

To assess the applicability of slags as agricultural fertilizers in regard to soil microorganims, agricultural research done at the Sommerland Agricultural Research Station was reviewed. The metabolic potential of soil microorganisms had been measured as respiration rate and cellulose degradation with the ferrous slags ABS

(slags, ferrous metal, blast furnace, air-cooled) and BOS (slags, steelmaking, converter).

The chromium accumulation in soils in dependence of different fertilization regimes (controls, lime, ABS, BOS) was monitored for 15 years. Soil contained approximately 50 mg Cr(total)/kg soil in controls and fields fertilized with lime or ABS, and approximately 65 mg/kg soil in fields fertilized with BOS.

The agricultural yields was highest in slag treated fields (approximately 5 % above controls), The soil respiration in controls was 420 mg CO2/kg soil. It was highest in lime-treated fields (580 mg CO2/kg soil), and 490 and 550 mg CO2/kg soil in ABS- and BOS-treated fields, respectively. Microbial cellulose degradation was lowest in control soils (1.4 mg/cm2) and highest in the slags: ABS 2 mg/cm2, and BOS 2.3 mg/cm2.

The ferrous slags ABS (slags, ferrous metal, blast furnace, air-cooled) and BOS (slags, steelmaking, converter) did not inhibit the microbial respiration and cellulose degradation potential of soils.