Boron

Endpoint summary

Description of key information

Relevance of terrestrial toxicity studies

The toxicity data on terrestrial organisms are from ecotoxicity tests that study relevant ecotoxicological parameters such as survival, growth, reproduction, and emergence. Relevant endpoints for soil micro-organisms focused on functional parameters (such as respiration, nitrification, mineralization) and microbial growth. Enzymatic processes are considered for this risk assessment as the ecological relevance of enzymatic assays is questionable.

 

Only data from observations in natural and (OECD) artificial standard soil media have been used for the derivation of the PNEC. Ideally, data used in the effect assessment should be based on European organisms and exposure conditions. This would, however, considerably reduce the amount of data that can be used. Therefore, all reliable data derived for non-European soil were considered: there are studies that show a tendency of increased boron toxicity in soils with low organic matter content, low clay content and pH < 7.5 (Aitken and McCallum, 1988; Gestring and Soltanpour, 1987; Van Laer et al., 2010). The effects seen on boron toxicity in soil from soil properties are however rather limited (≤ factor 10).

 

Other criteria that were considered during the review of existing data were the relevance of the test substance, as well as the test duration. The duration should be related to the typical life cycle and should ideally encompass the entire life cycle or, for longer-lived species, the most sensitive life stage. Retained exposure durations should also be related to recommendations from standard ecotoxicity protocols (e.g. ISO, OECD, ASTM).

 

More detailed information on each of the mentioned relevancy/reliability criteria is provided in the Background Document “Environmental effects assessment of boron”, which is attached in the technical dossier in IUCLID Section 13.

 

Summary of chronic terrestrial data:

Relevant and reliable chronic no-effects values were identified for thirty-nine terrestrial species or microbial processes: Triticum aestivum, Zea mays, Avena sativa, Medicago sativa, Hordeum vulgare, Brassica napus, Folsomia candida, Calamagrostis canadensis, Enchytraeus luxuriosus, Trifolium pratense, Lycoperiscon esculentum, Enchytraeus crypticus, Hypoaspis aculeifer, Daucus carota, Beckmannia syzigachne, Phleum pratense, Agropyion dasystachyum, Agropyion riparium, Agropyion smithii, Brasica oleracea, Brassica rapa, Bromus marginatus, Cucumis sativa, Glycine max, Koeleria macrantha, Linum usitatissimum, Lolium perenne, Latuca sativa, Festuca rubra, Onychiurus folsomi, Eisenia andrei, Substrate induced nitrification, Raphanus sativus, Pecilus cupreus, Nitrogen transformation, Schizachyrium scoparius, Allium cepa, Eisenia fetida and Caenorhabditis elegans. EC10/NOEC values are available for different monocotyledon and dicotyledon plants belonging to 8 different families. Long term NOEC values are available for 6 different invertebrate taxonomic groups, including soft-bodied and hard-bodied invertebrate species. Reliable long term EC10 values are included for two microbial processes affecting the nitrogen cycle.

No-effect levels for dissolved boron ranged between 7.2 mg B/kg soil dw and 86.7 mg B/kg soil dw, i.e., a difference of a factor of 12 between the most and least sensitive species. The zebrafish plant Z.mays was the most sensitive trophic level. The least sensitive species was the nematode C.elegans. A Species Sensitivity Distribution (SSD) has been developed for the assessment of boron in the terrestrial compartment, using the reliable species-specific chronic toxicity effect levels that have been generated in various research studies. 

 

 

Boron-specific considerations - essentiality

Boron is a naturally occurring element that is essential to a variety of organisms. In plants, the necessity for a variety of metabolic processes (e.g. nitrogen metabolism, nucleic acid metabolism and membrane integrity and stability) has been known for several decades to be an essential micronutrient for terrestrial plants (Butterwick et al., 1989; Eisler, 2000). Additional information on fate and mobility in different types of plants, as well as implications on the use of B-fertilizers is provided in the Background Document “Environmental effects assessment of boron”, which is attached in the technical dossier in IUCLID Section 13.

Because boron is a necessary plant micronutrient, it is intentionally added in some instances where required by crop plants, but is limited in the natural soil. It therefore seems more appropriate to use a PNEC for agricultural soil that protects the agricultural uses of the soil, rather than a PNEC derived to protect non-agricultural or non-industrial soil. A potential approach would entail the derivation of a PNEC for agricultural soil based on toxicity, but also with consideration of the risk of deficiency. For natural soils, the presumption is that locally-adapted species will not be adversely affected by boron deficiency, so only boron toxicity is relevant for deriving a PNEC.

 

The concentration-response curve for an essential element like boron is likely to be U-shaped for most species, with adverse effects observed at high and low concentrations, while no adverse effects are observed at the intermediate concentrations (Lowengart, 2001).

 

Boron-specific considerations - bioavailability in soils

Boron toxicity to plants and many soil micro-organisms is a function of the bioavailability of the dissolved boron species in the soil solution and the ability of the soil to buffer boron concentrations in the soil solution. The bioavailability of metals and other inorganic substances in soil can be strongly affected by soil properties and slow equilibration reactions (ageing) after application to the soil. Various environmental factors can influence boron availability in soils, including pH, soil texture, organic matter content, soil moisture, and temperature. As boron is either neutral or negatively charged under environmentally relevant conditions, cation exchange capacity is not expected to play a relevant role.

 

Boron availability to invertebrates depends on the relative amounts taken up by the organism by dermal adsorption and/or ingestion, although the relative importance of each route has not been determined (Vijver et al., 2001).

 

Soil properties

The amount of boron adsorbed by soil varies greatly with the contents of various soil constituents. Boron is adsorbed onto soil particles, with the degree of adsorption depending on the type of soil minerals present, pH, salinity, organic matter content, iron and aluminium oxide oxy/hydroxy content, and clay content (Hingston, 1964; Sims and Bingham, 1968; Bingham et al., 1970; Bingham, 1973).

 

There are few studies that compare boron toxicity for the same endpoint in different soils (Aitken & McCallum, 1988; Gestring and Soltanpour, 1987; Liang and Tabatabai, 1977; Liang and Tabatabai, 1978). The available results indicate a significant variation in boron toxicity thresholds among soils and show a tendency of increased boron toxicity in soils with low organic matter content, low clay content and pH < 7.5. The information is, however, too limited to allow conclusions on soil properties controlling boron toxicity in soils.

 

Van Laer et al.(2010) studied the effect of soil type and ageing on the toxicity of boric acid to root elongation of barley seedlings in a set of 17 soils covering a large range in pH (4.4-7.8), organic carbon (0.14-30.7 %), clay content (2-59 %) and background boron concentration (1.2-32 mg B/kg). The percentage clay and organic carbon (log) were positively correlated with the log ED50 values of the root elongation test.

 

Ageing reactions

The rate of boron adsorption on clay minerals is likely to consist of a continuum of fast adsorption reactions and slow fixation reactions. Short-term experiments have shown that boron adsorption reaches an apparent equilibrium in less than one day (Hingston, 1964; Keren et al., 1981). Long-term experiments have shown that fixation of boron increased even after six months of reaction time (Jasmund and Lindner, 1973).

Studies on the residual effect of boron application after a single application also indicated decreasing boron toxicity to plants with increasing time since application (Gupta and Cutcliffe, 1984; Gestring and Soltanpour, 1987). Van Laer et al. (2010) studied the effect of a 5-month ageing period on the toxicity of boric acid to barley root elongation and microbial nitrification, but observed only a negligible effect of ageing on boron toxicity. It was therefore decided to take into account the data for all equilibration times as replicates and calculate a geomean value for each soil for PNEC derivation.

In contrast to this negligible ageing effect of added B, Van Laer et al. (2010) observed a large difference between soil partitioning between added boron and naturally present boron in these soils, and concluded that ageing after spiking for 5 months does not affect boron availability in the same way as natural geogenic or field equilibrated boron. The difference in boron speciation between added boron and native boron may be related to boron incorporation into silicate structures or boron in biomass.

 

Hence, the available data support the use of added-boron, rather than total boron, as the basis for derivation of a PNEC for soil; because of the large difference in bioavailability between boron naturally present in soils and added soluble B, risks of added soluble boron are assessed by using the added risk concept.

 

Conclusion

1. Taking all information into account, it was concluded that bio-availability correction is not possible for the moment and presumably having a limited impact. It was therefore decided not to implement normalization models for soil properties in the PNEC derivation for boron in soils because:
2. There is a relatively limited effect of soil properties on boron toxicity (≤ factor 10 difference among soils).
3. In contrast to the moisture content during the barley root elongation test, moisture content is not constant under field conditions. Moreover, excess soluble boron will leach out with percolating rainwater under normal field conditions.
4. A normalization model is only available for plants (barley root elongation) and insufficient data are available for the derivation of such models for invertebrates and soil micro-organisms.
5. All toxicity data derived in different soils are therefore taken together and the PNEC derivation is based on one species mean value for each endpoint.
6. The ageing effect on boron toxicity of added boron was negligible and is therefore not taken into account in the PNEC derivation. It was decided to take into account the data for all equilibration times as replicates and calculate a geomean value for each soil.
7. The available data point to a significant difference in bioavailability between boron naturally present in soils and added soluble B. Therefore, the added risk concept is followed for assessing the risks of boron added to soils.

More detailed information on boron bioavailability and ageing processes is provided in the Background Document “Environmental effects assessment of boron”, which is attached in the technical dossier in IUCLID Section 13.

Additional information

The available ecotoxicity database for the effect of boron on soil organisms is large and covering series of trophic levels and species. Therefore, the use of the statistical extrapolation method is preferred for PNEC derivation rather than the use of an assessment factor on the lowest NOEC, as specified by the Guidance document on information requirements and chemical safety assessment Chapter R.10.3.1.3. The PNEC is based on the 50% confidence value of the 5th percentile value of the effect NOEC/EC10 data (HC5,50) and an additional assessment factor taking into account the uncertainty on the HC5,50 (thus PNEC = HC5,50/AF). The advantage of this statistical extrapolation method is that it uses the whole sensitivity distribution of species in an ecosystem to derive a PNEC instead of taking only the lowest long-term NOEC. Bounded Klimisch 1 and 2 toxicity data that were considered relevant for PNEC derivation were used to calculate the geometric species mean value per endpoint (see overview below). The most sensitive endpoint was selected for the assessment factor or statistical extrapolation (SSD) approach.

The lognormal distribution was selected as the best fitting distribution based on the Anderson-Darling test. The HC5 at the 50th % confidence limit (together with 5th and 95th confidence limits) derived from the lognormal distribution, is 11,3 (8.8 – 13.5) mg B/kg (based on added boron concentrations).

Overview of the selected geometric species mean value for the most sensitive endpoint (based on added boron concentrations)

Species Name	Species Name	Taxa	Added geomean NOEC/EC10 (mg B/L)
Plants	Triticum aestivum (Wheat)	Root yield	16.7
Plants	Zea mays (Corn)	Shoot yield	7.2
Plants	Avena sativa (oat)	Shoot biomass	11.0
Plants	Medicago sativa (Alfalfa)	Shoot yield	11.4
Plants	Hordeum vulgare (Barley)	Root elongation	13.2
Plants	Brassica napus (Canola)	Shoot biomass	13.9
Invertebrates	Folsomia candida (springtail)	Juvenile production	15.4
Plants	Calamagrostis canadensis (Bluejoint marsh reed)	Seedling emergence	16.6
Invertebrates	Enchytraeus luxuriosus (worm)	Reproduction	17.0
Plants	Trifolium pratense (Red clover)	Seedling emergence	17.1
Plants	Lycoperiscon esculentum (Tomato)	Seedling emergence	20.6
Invertebrates	Enchytraeus crypticus (worm)	Reproduction	22.6
Invertebrates	Hypoaspis aculeifer (mite)	Reproduction	22.7
Plants	Daucus carota (Carrot)	Seedling emergence	24.8
Plants	Beckmannia syzigachne (American sloughgrass)	Seedling emergence	24.8
Plants	Phleum pratense (Timothy)	Seedling emergence	26.0
Plants	Agropyion dasystachyum (Northern wheatgrass)	Root length	28.0
Plants	Agropyion riparium (Streambank wheatgrass)	Root yield	28.0
Plants	Agropyion smithii (Western wheatgrass)	Shoot length	28.0
Plants	Brasica oleracea (cabbage)	Root length	28.0
Plants	Brassica rapa (Turnip)	Root yield	28.0
Plants	Bromus marginatus (Mountain bromegrass)	Shoot yield	28.0
Plants	Cucumis sativa (Cucumber)	Root length	28.0
Plants	Glycine max (Soybean)	Root length	28.0
Plants	Koeleria macrantha (june grass)	Root yield	28.0
Plants	Linum usitatissimum (Flax)	Root yield	28.0
Plants	Lolium perenne (Perennial ryegrass)	Root length	28.0
Plants	Latuca sativa (Lettuce)	Seedling emergence	28.9
Plants	Festuca rubra (Red fescue)	Seedling emergence	30.1
Invertebrates	Onychiurus folsomi (springtail)	Juvenile production	31.0
Invertebrates	Eisenia andrei (earthworm)	Growth (juvenile dry weight)	36.1
Microbial	Substrate induced nitrification	Substrate induced nitrification	41.3
Plants	Raphanus sativus (Radish)	Shoot yield	42.0
Invertebrates	Pecilus cupreus (carabid beetle)	feeding rate	47.5
Microbial	Nitrogen transformation	N transformation	48.1
Plants	Schizachyrium scoparius (Little bluestem)	Seedling emergence	48.9
Plants	Allium cepa (Spanish onion)	Shoot length	56.0
Invertebrates	Eisenia fetida (earthworm)	Reproduction	70.1
Invertebrates	Caenorhabditis elegans (Nematode)	Reproduction	86.7

 

 

Data are not corrected for ageing or soil properties. Because of the absence of a significant ageing effect, toxicity data for various equilibration periods (ageing) are treated as replicates and a geomean value is calculated for this soil-endpoint combination. The toxicity data derived in different soils are also taken together considering the very limited effect of soil properties on boron toxicity in soil and the PNEC derivation is based on one species/soil mean value for each endpoint. Because of the large difference in bioavailability between boron naturally present in soils and added soluble boron, the PNEC is based on added boron concentrations.