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EC number: 234-522-7
CAS number: 12007-92-0
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
not relevant for this risk assessment. The ecological relevance of
enzymatic assays is questionable for several reasons:
- The enzymatic activities are measured
at conditions that are not representative for in situ conditions.
- Several assays are conducted in pH
buffered soil suspensions (some tests even at pH>10) and since the
metal-enzyme interaction is pH dependent, this might obscure the
relationship with effects in the soil.
- Almost all assays use saturating
substrate concentrations (typically several mM), a condition that is
unlikely to occur in situ. The in situ effect of metals on an enzymatic
reaction may be rather insensitive to the enzyme capacity (as measured
with the enzyme assays) if substrate supply is the rate-limiting step.
- The colorimetric reaction that is
often required in enzymatic assays can also be subject to effects of
metals (Nannipieri P. et al ,1997). and not all studies have
experimentally verified this, i.e. some of the reported effects may be
information on enzymatic processes is summarized
in Annex IV from the “Boron effects assessment in the terrestrial
compartment” report prepared by ARCHE (2010) is
proposed to be used as supporting information only.
Relevancy of the
Only data from
observations in natural and (OECD) artificial standard soil media have
been used for the derivation of the PNEC.
The data used in the
effect assessment should ideally be based on organisms and exposure
conditions from. This would, however, considerably reduce the amount of
data that can be used. Therefore, data based on soils collected
outsidehave also been used. Two options may be followed here:
using all reliable data derived for
non-European soil and
only using data for non-European soils
with properties relevant for the EU conditions. These EU conditions are
defined as 10thand 90thpercentiles of soil
properties of the EU soils. These boundaries for the EU soils are based
on the GEMAS-project (Geochemical Mapping of Agricultural and Grazing
Land Soil project), which provides a harmonized and directly comparable
dataset on soil properties (pH, organic matter content, clay content and
effective) and metals in 2211 samples of arable land soil (0-20 cm) and
2118 samples of grazing land soil (0-10 cm) at the EU scale (average
sampling density of 1 site per 2500 km2, i.e. grid of 50 x 50
km). Only the ecotoxicity data were retained in case they were within
the following bounderies: pH (0.01M CaCl2): 4.3 – 7.4;
organic matter: 1.6 – 10.0% and clay: 6.0 – 37.0%.
Option 1 is selected
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 & McCallum, 1988; Gestring &
Soltanpour, 1987; Van Laer et al., 2010). The effects found of soil
properties on boron toxicity in soil are however rather limited (≤
factor 10) (see below),
Taking into account all soils, including
the most sensitive ones is the most conservative approach
Relevancy of the
Studies on the
ecotoxicity of boron have been performed with various compounds, such as
boric acid (H3BO3), anhydrous sodium tetraborate
(Na2B4O7), and hydrated sodium
For the purpose of this evaluation, all endpoints are converted to
concentrations of elemental boron (B) using the relative molar mass.
What comprises “chronic
exposure” is a function of the life cycle of the test organisms. A
priori fixed exposure durations are therefore not relevant. 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 (e.g. ISO, OECD,) protocols.
Typically chronic test
durations for the higher plants are within the range of 4 (e.g. the root
elongation test based on ISO 11269-1 (1995)) and 21 days (e.g. the shoot
yield test based on ISO 11269-2 (1995)). OECD n° 208 (plant seedling
emergence and growth test, 2006) recommended a test duration of between
14 and 21 days after emergence of the seedlings. Testing with soil
invertebrates have a typical acute exposure duration of 7 to 14 days for
the oligochaetes Eisenia fetida/Eisenia andrei. Assessment
of the chronic effects of substances on sub-lethal endpoints such as
reproduction on oligochaetes has a typical exposure duration of 3 to 6
weeks for the standard organism Enchytraeus albidus (OECD, 2000;
ISO 16387). For another standard species Folsomia candida survival
and reproduction is typically assessed after 28 days of exposure (ISO
11267, 1999). Test durations using soil micro-organisms for the OECD 216
(carbon transformation test) andthe OECD 217 (nitrogen transformation
test) last 28 days..
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. Typical applied doses are 1-2 kg
B/ha/yr, but may range from 0.5 to 7.6 mg B/ha depending on local
conditions (Shorrocks, 1997; Borax 2002). This may be in the form of
formulated fertilizers broadcast to agricultural soils or sprays applied
directly to the plant or vicinity of the plants. In these instances, it
might be 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. This is consistent with the
REACH Guidance Document (2008) distinctions in developingfor
agricultural, natural/grassland, and industrial soil.
A potential approach would therefore be to derive 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.
Boron is a naturally occurring element that is essential to a
variety of organisms. In plants, it is necessary for a variety of
metabolic processes (e.g. nitrogen metabolism, nucleic acid metabolism
and membrane integrity and stability) and has been known to be an
essential micronutrient for terrestrial plants for several decades
(Butterwick et al., 1989; Eisler, 2000). Shorrocks (1997) documented the
use of boron applications for 132 crops in over 80 countries,
demonstrating the widespread nature of agricultural use of boron.
Evidence exists that it
is essential for nitrogen fixation in some species of algae (Smyth and
Dugger, 1981), fungi and bacteria (Saiki et al., 1993; Fernandez et al.,
1984), some diatoms and algae and macrophytes (Eisler, 2000). Required
levels may vary, especially among plants, such that essential levels for
one species may be toxic to another (Eisler, 1990).Work
with rainbow trout and zebrafish has shown that embryo-larval
development was adversely affected in waters deficient in boron (Rowe et
al., 1998, Eckhert, 1998). Fort et al. (1998) reported that abnormal
development in frog embryos (Xenopus laevis) was observed when
larval stages were exposed to 0.003 mg-B/L or less. Boron does not
appear to be essential for all species, however. Evaluation of
essentiality in some animals is limited by the innate boron content in
plant-based animal feeds.
The concentration-response curve for 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). The attached document
"U-Shaped Toxicity Pattern_Plant yield as influenced by soil boron
concentrations" illustrates such a pattern for plants (Gupta et al.,
1985) although the response has been normalized to 100 %, making the
curve an inverted-U shape.
Plant and animal species vary in the concentrations associated
with deficiency and toxicity. Monocotyledons (e.g. corn and grasses)
require about one-quarter as much boron as dicotyledons (e.g. tomatoes,
carrots, clovers, beets) (Gupta 1985; Butterwick et al., 1989). The
mobility of boron within the plant may help explain the observed
deficiency and toxicity patterns. Boron is more mobile in plants that
produce the simple sugars known as polyols (e.g. sorbitol and mannitol)
than in species that do not produce polyols. In polyol-producing
species, boron is translocated from one part of the plant to another and
so may reach the meristem and affect growth. In the absence of polyols,
boron is relatively immobile within the plant (Brown et al., 2002). A
polyol-producing plant may be both more tolerant of boron deficiency and
more sensitive to higher boron concentrations because of the mobility of
boron within the plant. This is important in agricultural applications
of boron, which may be applied as a soil treatment or as foliar spray.
Agricultural application of boron depends on the plant and
cultivar, as well as the local soil. Recommended application rates range
from 0.5 to 7.6 kg B/ha (Borax, 2002), but typically are in the range of
1 to 2 kg B/ha (Shorrocks, 1997). If one assumes typical soil densities
of 1700 kg/m³ and a mixing depth of 20 cm (default values used in the
EUSES model), an application rate of 1 to 2 kg B/ha results in an
estimated added soil concentration of 0.3 to 0.6 mg B/kg soil. Mortvedt
et al. (1992) estimated soil concentrations of 0.16 to 2.0 mg B/kg soil
for several crops with application rates of 0.45 to 5.7 kg/ha. The
intentional application of borates to achieve such soil concentrations
for agricultural crops should be acknowledged in the risk assessment
of boron in soils
Boron may be considered a typical metalloid having properties
intermediate between the metals and the electronegative non-metals.
Boron has a tendency to form anionic rather than cationic complexes
(Keren and Bingham, 1985). Boron does not undergo oxidation reduction
reactions or volatilisation reactions in soils (Goldberg, 1997). Boron
chemistry is of covalent boron compounds and not of B3+ions
because of its very high ionisation potentials.
Boron oxide, B2O3reacts with water to form
boric acid, H3BO3. Boric acid is moderately
soluble (4.9g 100mL-1water at 20°C). It acts as a weak Lewis
acid by accepting a hydroxyl ion to form the borate anion.
Aqueous boron species other than B(OH)3and B(OH)4-can
be ignored for most practical purposes in soils (Keren and Bingham,
1985). In most soils with soil solutions in the pH range 4.0 to 9.0, the
uncharged B(OH)3predominates. The borate ion is expected to
form a variety of complex salts with suitable metal acceptor ions.
However, there is relatively little evidence for the existence of metal
borate complexes in solutions. Among the organic borates, the tendency
is for boron to replace carbon or nitrogen in three-fold coordination
(Keren and Bingham, 1985). In regions of low rainfall, the boron content
of the soil is usually high. Boron in these soils probably exists
largely as sodium-calcium borates. However, there is no information on
the kinetics of dissolution of these minerals in water or on the
composition of their products (Keren and Bingham, 1985).
affecting the bioavailability of boron 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. Investigations into properties,
including ageing, which modify boron availability to plants are
underway, but reports were not available in time to incorporate into
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).
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). Boron
adsorption can vary from being fully reversible to irreversible,
depending on the soil type and environmental conditions (IPCS, 1998). As
the pH is increased to about 9, the B(OH)4-concentration
increases rapidly and the amount of adsorbed boron increases rapidly
(Keren and Bingham, 1985). Hence, the critical range of extractable
boron levels leading to deficiency is generally higher in alkaline soils
than in acid soils (, 1999).
Boron reacts more strongly with clay than sandy soils (Keren and
Bingham, 1985). Clay soils buffer boron in the soil solution better than
sandy soils. The magnitude of the boron adsorption onto clay minerals is
affected by the exchangeable cation (Keren and Gast, 1981; Keren and
Mezuman, 1981; Keren and O'Connor, 1982; Mattigod et al., 1985).
Calcium-rich clays adsorb more boron than sodium and potassium clays
(Keren and Gast, 1981; Keren and O'Connor, 1982; Mattigod et al., 1985).
Higher organic matter content increases the B-sorption capacity of
soils (Yermiyahu et al., 1995). Adsorbed boron and boron adsorption
maxima have been highly significantly correlated with organic carbon
content (Elrashidi et al., 1982; Gupta, 1968). The uptake of boron by
plants can be markedly affected by the presence of other plant nutrients
in soils. The most well known of these is the effect of Ca (Gupta et
There are few studies that compare boron toxicity for the same
endpoint in different soils (Aitken & McCallum, 1988; Gestring &
Soltanpour, 1987; Liang & Tabatabai, 1977; Liang & 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.
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 & Cutcliffe, 1984; Gestring & Soltanpour, 1987).
Van Laer et al. (2010)
studied the effect of ageing on the toxicity of boric acid to barley
root elongation and microbial nitrification by measuring toxicity
immediately after spiking soils and after ageing the soil for 1 or 5
months in closed containers (no leaching). Ageing at 20°C for up to 5
months in a closed container had only a slight effect on the boron
toxicity: EC10 values increased by an average factor of 1.3 (range 0.4 –
3.5) after 1 month of ageing and 1.4 (range 0.5 – 3.3) in the 5 months.
Because of this
negligible effect of ageing on boron toxicity, it was 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. Measured background soil
solution boron concentrations in 3 soils were 0.2 to 0.9 mg B/l,
equivalent to Kd values of 3 to 15 L/kg (Van Laer et al., 2010).
Measured boron concentrations exhibiting effects on barley (EC10) in the
added-boron soils ranged between 5 and 104 mg B/L across 17 natural
soils (mean: 26 mg B/L). The median Kd value for added boron was 0.4
L/kg. Therefore, it was 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. Van Laer et al. (2010) noted
that the amount of boron naturally present in soils, as measured by aqua
regia soluble boron, did not correspond to EC10 values based on added
boron, even with ageing. They found the aqua regia-soluble boron to
range from 1 to 32 mg B/kg in several natural soils, but these did not
cause barley toxicity, even though EC10 values from freshly spiked soils
ranged from 3 to 38 mg B/kg. This supports the used 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 will be assessed by
using the added risk concept.
Taking all information into account, it was decided not to
implement normalization models for soil properties in the PNEC
derivation for boron in soils because:
There is a relatively limited effect of soil properties on boron
toxicity (≤ factor 10 difference among soils).
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.
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.
All toxicity data derived in different soils are therefore taken
together and the PNEC derivation is based on one species mean value for
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.
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 will be followed for
assessing the risks of boron added to soils.
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