Cadmium in Wood Ash Used as Fertilizer in Forestry
Pasanen, Jaana; Louekari, Kimmo; Malm, Jukka (2001)
maa- ja metsätalousministeriö
2001
Julkaisusarja:
Publications of the Ministry of Agriculture and Forest 5/2001
Julkaisun pysyvä osoite on
http://urn.fi/URN:ISBN:952-453-043-0
http://urn.fi/URN:ISBN:952-453-043-0
Tiivistelmä
A drastic increase of extractable concentrations
of cadmium shown in some studies was
quite surprising, because according to the general
understanding of the behaviour of cadmium with
increasing pH the situation should be the opposite.
Especially humus layer has a large capacity
to retain cadmium and bind it in a less bioavailable
form, especially if pH increases. Depending
on the quantity and quality of ash used, pH can
increase 1-2 units after ash treatment. In general,
the mobility of cadmium tends to decrease and its
adsorption on soil increase when pH increases. It
has been generally known that the soil pH is one
of the most important factors determining not
only the short-term, but also the long-term variations
in the uptake of cadmium by organisms and
the vertical and horizontal mobility of cadmium
in the soil.
Human health aspects
Diet is the main source of human exposure to
cadmium. Cereals, vegetables and potatoes cause
most of the average dietary intake of cadmium
in Finland (9.5 μg/day). In some sub-populations,
long-term dietary intake of cadmium is 20-30
μg/day in Finland. Normally only 5 % of the
cadmium in food is absorbed in the gastrointestinal
tract. However, studies on humans and
experimental animals show that a deficiency of
iron, which is not uncommon among women
increases the rate of absorption (up to 10 %).
There is some evidence that cadmium concentration
of lingon berries and mushrooms is
slightly increased by ash fertilisation. Some mushroom
species accumulate more cadmium several
years after ash fertilisation. There is no evidence
that the level of cadmium in blueberries, in perch
of forest lakes would be affected by ash fertilisation.
The cadmium content in liver and kidney of
elk has increased over about 20 years, but reason
of this trend remains to be elucidated. The preliminary
assessment showed that the cadmium bal-
Environmental aspects
The toxicity of cadmium to terrestrial organism
shows a variable pattern. Plants grown in soil
are generally considered insensitive to the effects
of cadmium, although some exceptions do exist.
Cadmium exhibits moderate and low toxicity to
avian species in subacute exposure. Microorganisms
and terrestrial invertebrates show moderate
or high sensitivity to cadmium.
The effects of cadmium in ash on soil biota
are hard to prove and distinguish from the effects
of the increased pH of the soil. The increase of pH
after ash treatment seems to cause the most prominent
effects on biota. In general, soil microbes
seem to benefit at least from the short-term effects
of ash treatments; soil respiration and microbial
activity has increased. However, the microbial
community has changed. The short-term effects of
ash fertilization on soil fauna seem to be adverse,
although there are some species which benefit
from the increase of soil pH.
No clear trend of increase of cadmium content
in mushrooms was noticed, although according to
one recent study a slight increase of cadmium, not
statistically significant, could be seen in mushrooms.
The general trend of increasing concentrations of
cadmium in berries is also lacking, although a slight
increase of cadmium concentrations in lingon- and
cloudberries and stems of blueberries after ash treatment
could be noticed in two studies.
Ash fertilization increased total concentrations
of cadmium in peat and mineral soils. In
peat soils cadmium concentrations even doubled.
However, the fate and impacts of easily mobile
and bioavailable extractable fractions of cadmium
are still unclear. The extractable content of cadmium
seemed to increase at least some years
after ash fertilization. However, no direct conclusions
can be drawn on the basis of the limited
number of available studies. In fact, available data
on extractable background cadmium concentrations
and concentrations after ash treatment do not enable
a quantitative risk assessment to be carried out.
ance of forest soil is not dramatically changed by
ash fertilisation.
The consumption of “forest foods”: i.e. berries,
mushrooms and liver and kidney of elk by
an average Finnish consumer is low. The average
intake of cadmium from berries and elk liver and
kidney is considered negligible. The intake of cadmium
from forest mushrooms by average consumer
is approximately 0.5 μg/day.
In the reasonable worst case scenario for recreational
hunters and their family members, it
was assumed that they annually eat a few meals
containing liver or kidney of elk. Based on food
consumption studies, it was estimated that they
eat twice as much forest mushrooms as the average
consumer and twice as much forest berries
as the average inhabitant of Northern Finland.
In addition, it is assumed that these foods come
from ash fertilized areas. The dietary intake of
cadmium is clearly higher among this consumer
group (about 17.4 μg/day) as compared with an
average Finnish consumer (9.5 μg/day). As such
this intake is considered to be safe. However, in
case recreational hunters or their family members
are also heavy smokers and have iron deficiency,
which increases the absorption of cadmium, their
body burden of cadmium might reach critical
level and cause adverse health effects.
These main factors of total cadmium exposure:
high dietary intake of cadmium (due to consumption
of “forest foods”), increased absorption
and smoking are considered in the risk assessment.
The corresponding urinary levels of cadmium
and the critical urinary levels that are associated
with health effects are compared. It is concluded
that for the risk group, the urinary Cd level
is about 2-3 μg/l. This is also the level of urinary
excretion of cadmium associated with risk of
cadmium-induced kidney dysfunction and bone
effects, which are due to the increased excretion
of cadmium. It is noteworthy, that the number of
people (among the recreational hunters and their
family members) affected by all the three risk factors
(and having the urinary Cd level of 2-3 μg/l)
is very small.
The dietary intake of cadmium by some recreational
hunters and farmers is about two-fold
as compared with the average Finnish consumer.
On the basis of current data, not the ash fertilization
but the food consumption pattern increases
the cadmium exposure of this population group.
There is a need for more data on the effect of ash
fertilization on the Cd-content of mushrooms and
liver and kidney of elk to confirm or refute the
preliminary estimates presented in this report.
Recommendations
In order to assess more reliably the effects of cadmium
in wood ash used as fertilizers in forestry
more comprehensive carefully planned long-term
ash fertilization studies are needed, where extractable
cadmium concentrations are measured in
soil. Based on the current limited data on extractable
cadmium concentrations in forest soil, no
direct conclusions of the impact of ash fertilization
can be drawn. Also long-term studies on
leaching of cadmium from forest soil are needed.
More accurate estimates on the Cd balance
(total long-term inputs and outputs) in the forest
soil are needed. These can be done on the basis of
the above mentioned studies on extractable cadmium
concentrations and leaching studies. The
instructions and application practices (e.g. frequency
of ash treatment, amount of ash applied
per hectare, maximum allowed concentration of
cadmium in the ash) of ash fertilisation should be
designed taking into account the long-term balance
of cadmium in forest soil.
In addition, in order to assess the long-term
trend of cadmium content in berries and mushrooms
more long-term studies on the effect of ash
fertilization are needed. Thus, surveillance of cadmium
concentrations in berries and mushrooms
e.g. in experimental areas founded by the Finnish
Forest Research Institute should be continued.
The effect of ash fertilisation on the level of
cadmium in liver and kidney of elk and hare
should be studied. It seems reasonable to retain
the current recommendations given to the rec8
reational hunters for avoidance of frequent consumption
of liver and kidney of the elk and hare.
Despite the uncertainties in this preliminary
risk assessment, it can be recommended that a
maximum allowed concentration of cadmium in
wood ash used as fertilizer in forest be established.
In addition, the use of ash without liming
effect e.g. peat ash with low pH should be considered
very carefully. It can be assumed that
the extractable cadmium concentrations in soil
and the related environmental and health risks
are likely to increase, when ash with low liming
capacity is used.
of cadmium shown in some studies was
quite surprising, because according to the general
understanding of the behaviour of cadmium with
increasing pH the situation should be the opposite.
Especially humus layer has a large capacity
to retain cadmium and bind it in a less bioavailable
form, especially if pH increases. Depending
on the quantity and quality of ash used, pH can
increase 1-2 units after ash treatment. In general,
the mobility of cadmium tends to decrease and its
adsorption on soil increase when pH increases. It
has been generally known that the soil pH is one
of the most important factors determining not
only the short-term, but also the long-term variations
in the uptake of cadmium by organisms and
the vertical and horizontal mobility of cadmium
in the soil.
Human health aspects
Diet is the main source of human exposure to
cadmium. Cereals, vegetables and potatoes cause
most of the average dietary intake of cadmium
in Finland (9.5 μg/day). In some sub-populations,
long-term dietary intake of cadmium is 20-30
μg/day in Finland. Normally only 5 % of the
cadmium in food is absorbed in the gastrointestinal
tract. However, studies on humans and
experimental animals show that a deficiency of
iron, which is not uncommon among women
increases the rate of absorption (up to 10 %).
There is some evidence that cadmium concentration
of lingon berries and mushrooms is
slightly increased by ash fertilisation. Some mushroom
species accumulate more cadmium several
years after ash fertilisation. There is no evidence
that the level of cadmium in blueberries, in perch
of forest lakes would be affected by ash fertilisation.
The cadmium content in liver and kidney of
elk has increased over about 20 years, but reason
of this trend remains to be elucidated. The preliminary
assessment showed that the cadmium bal-
Environmental aspects
The toxicity of cadmium to terrestrial organism
shows a variable pattern. Plants grown in soil
are generally considered insensitive to the effects
of cadmium, although some exceptions do exist.
Cadmium exhibits moderate and low toxicity to
avian species in subacute exposure. Microorganisms
and terrestrial invertebrates show moderate
or high sensitivity to cadmium.
The effects of cadmium in ash on soil biota
are hard to prove and distinguish from the effects
of the increased pH of the soil. The increase of pH
after ash treatment seems to cause the most prominent
effects on biota. In general, soil microbes
seem to benefit at least from the short-term effects
of ash treatments; soil respiration and microbial
activity has increased. However, the microbial
community has changed. The short-term effects of
ash fertilization on soil fauna seem to be adverse,
although there are some species which benefit
from the increase of soil pH.
No clear trend of increase of cadmium content
in mushrooms was noticed, although according to
one recent study a slight increase of cadmium, not
statistically significant, could be seen in mushrooms.
The general trend of increasing concentrations of
cadmium in berries is also lacking, although a slight
increase of cadmium concentrations in lingon- and
cloudberries and stems of blueberries after ash treatment
could be noticed in two studies.
Ash fertilization increased total concentrations
of cadmium in peat and mineral soils. In
peat soils cadmium concentrations even doubled.
However, the fate and impacts of easily mobile
and bioavailable extractable fractions of cadmium
are still unclear. The extractable content of cadmium
seemed to increase at least some years
after ash fertilization. However, no direct conclusions
can be drawn on the basis of the limited
number of available studies. In fact, available data
on extractable background cadmium concentrations
and concentrations after ash treatment do not enable
a quantitative risk assessment to be carried out.
ance of forest soil is not dramatically changed by
ash fertilisation.
The consumption of “forest foods”: i.e. berries,
mushrooms and liver and kidney of elk by
an average Finnish consumer is low. The average
intake of cadmium from berries and elk liver and
kidney is considered negligible. The intake of cadmium
from forest mushrooms by average consumer
is approximately 0.5 μg/day.
In the reasonable worst case scenario for recreational
hunters and their family members, it
was assumed that they annually eat a few meals
containing liver or kidney of elk. Based on food
consumption studies, it was estimated that they
eat twice as much forest mushrooms as the average
consumer and twice as much forest berries
as the average inhabitant of Northern Finland.
In addition, it is assumed that these foods come
from ash fertilized areas. The dietary intake of
cadmium is clearly higher among this consumer
group (about 17.4 μg/day) as compared with an
average Finnish consumer (9.5 μg/day). As such
this intake is considered to be safe. However, in
case recreational hunters or their family members
are also heavy smokers and have iron deficiency,
which increases the absorption of cadmium, their
body burden of cadmium might reach critical
level and cause adverse health effects.
These main factors of total cadmium exposure:
high dietary intake of cadmium (due to consumption
of “forest foods”), increased absorption
and smoking are considered in the risk assessment.
The corresponding urinary levels of cadmium
and the critical urinary levels that are associated
with health effects are compared. It is concluded
that for the risk group, the urinary Cd level
is about 2-3 μg/l. This is also the level of urinary
excretion of cadmium associated with risk of
cadmium-induced kidney dysfunction and bone
effects, which are due to the increased excretion
of cadmium. It is noteworthy, that the number of
people (among the recreational hunters and their
family members) affected by all the three risk factors
(and having the urinary Cd level of 2-3 μg/l)
is very small.
The dietary intake of cadmium by some recreational
hunters and farmers is about two-fold
as compared with the average Finnish consumer.
On the basis of current data, not the ash fertilization
but the food consumption pattern increases
the cadmium exposure of this population group.
There is a need for more data on the effect of ash
fertilization on the Cd-content of mushrooms and
liver and kidney of elk to confirm or refute the
preliminary estimates presented in this report.
Recommendations
In order to assess more reliably the effects of cadmium
in wood ash used as fertilizers in forestry
more comprehensive carefully planned long-term
ash fertilization studies are needed, where extractable
cadmium concentrations are measured in
soil. Based on the current limited data on extractable
cadmium concentrations in forest soil, no
direct conclusions of the impact of ash fertilization
can be drawn. Also long-term studies on
leaching of cadmium from forest soil are needed.
More accurate estimates on the Cd balance
(total long-term inputs and outputs) in the forest
soil are needed. These can be done on the basis of
the above mentioned studies on extractable cadmium
concentrations and leaching studies. The
instructions and application practices (e.g. frequency
of ash treatment, amount of ash applied
per hectare, maximum allowed concentration of
cadmium in the ash) of ash fertilisation should be
designed taking into account the long-term balance
of cadmium in forest soil.
In addition, in order to assess the long-term
trend of cadmium content in berries and mushrooms
more long-term studies on the effect of ash
fertilization are needed. Thus, surveillance of cadmium
concentrations in berries and mushrooms
e.g. in experimental areas founded by the Finnish
Forest Research Institute should be continued.
The effect of ash fertilisation on the level of
cadmium in liver and kidney of elk and hare
should be studied. It seems reasonable to retain
the current recommendations given to the rec8
reational hunters for avoidance of frequent consumption
of liver and kidney of the elk and hare.
Despite the uncertainties in this preliminary
risk assessment, it can be recommended that a
maximum allowed concentration of cadmium in
wood ash used as fertilizer in forest be established.
In addition, the use of ash without liming
effect e.g. peat ash with low pH should be considered
very carefully. It can be assumed that
the extractable cadmium concentrations in soil
and the related environmental and health risks
are likely to increase, when ash with low liming
capacity is used.