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Toxicity to soil microorganisms

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Endpoint:
toxicity to soil microorganisms
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Though not specifically mentioned in the publication, it seems that no replicates were used in this study.
Qualifier:
no guideline followed
Principles of method if other than guideline:
pot experiments (2-liter pots, sandy loam, 1.0 kg of wet soil, 2.34 % organic matter content, incubation in the dark)
GLP compliance:
no
Analytical monitoring:
no
Vehicle:
no
Test organisms (inoculum):
soil
Total exposure duration:
7 wk
Moisture:
70%
Key result
Duration:
7 wk
Dose descriptor:
NOEC
Effect conc.:
1 550 other: mg/kg soil w.w.
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
other: number of microbial populations

Of the populations studied, none were significantly inhibited, but stimulations were noticed after 2 and 7 weeks with nitrite-forming microbes and with the fungi and cellulolytic microbes by isobutylidenediurea (IBDU) when applied at a rate of 1550 mg/kg wet soil (equivalent to about 1215 kg N / ha). The increase of the fungal population and of the number of cellulolytic propagules in the soil suggests that these groups might be directly involved in the degradation of IBDU. No significant increase of the Nitrobacter-population could be detected. IBDU did not significantly enhance the ammonifying microorganisms.  All fertilizers reduced slightly and temporarily the urease, phosphatase and saccharase activity of the treated soils even when applied at the low dose. However, these reductions never surpassed 35 percent of the corresponding control values and the enzymatic activities gradually increased to control values at the twentieth week. Only 38 percent of the amount of carbon added, was recuperated as carbon dioxide. This indicated that IBDU was hydrolyzed into two urea molecules and one molecule of isobutyraldehyde and that this latter compound is not rapidly further metabolised. IBDU liberated immediately a small amount of ammonium and after 7 weeks about 72 percent of the amount of N added were detected as ammonium and, mainly, as nitrate (urea: 98 percent). Nitrite concentrations in soil were never increased. As to the pH, no significant shifts were observed with the low dose. When added at the high dose, a slow increase of 1.5 units followed by a return to the pH of the control soil was observed (no further details given in the publication).

Endpoint:
toxicity to soil microorganisms
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: No appropriate negative control was used to evaluate IBDU effects. Summary lacking experimental details. The use of sand and peat topdressing material may have masked effects.
Principles of method if other than guideline:
Field test
GLP compliance:
no
Test organisms (inoculum):
soil
Remarks:
2 years

Four natural organic fertilizers, alone or in combination with isobutylidenediurea (IBDU) were compared with IBDU alone for their effect on soil/root microbial populations associated with bermudagrass grown on a golf course putting green in southern Florida/USA. No significant differences in microbial populations were observed over a 2-year period.

Description of key information

NOEC (7 wks): 1550 mg IBDU/kg wet soil

Key value for chemical safety assessment

Additional information

Vertraete et al. (1974) studied the influence of different slow-release fertilizers on some natural soil microbiological populations and compared the effect to those of urea. The influence of the fertilizers was evaluated by applying the compounds at a low and at a high dose, corresponding respectively with the application of 173 kg N/ha and 1215 kg N/ha (222 and 1550 mg IBDU/kg wet soil). Of the populations studied, none were significantly inhibited, but stimulations were noticed with Fungi, and with ammonifying- and nitrite-forming microorganisms.

Four natural organic fertilizers, alone or in combination with isobutylidenediurea (IBDU) were compared with IBDU alone for their effect on soil/root microbial populations associated with bermudagrass (Cynodon sp.) grown on a golf course putting green in southern Florida/USA. No significant differences in microbial populations were observed over a 2-year period (Elliott and Des Jardin 1999).