Remediation of Pb/Cd-Contaminated Forest Soils by Compost and
Mycorrhizae: will it be a solution to the Forest Dieback?
, Yapa PI
, Nissanka SP
and Perera SP
Postgraduate Institute of Agriculture, University of Peradeniya, Sri Lanka
Faculty of Agricultural Sciences, Sabaragamuwa University of Sri Lanka, Sri Lanka.
Faculty of Agriculture, University of Peradeniya, Sri Lanka.
Rubber Research Institute, Agalawatta, Sri Lanka
Horton Plains is an upper montane forest in Sri Lanka that acts as the most important water
catchment of the island. Soil pollution and forest dieback (FDB) have become the major threat for this
sensitive natural ecosystem. Increasing vehicle emissions in the nearby cities and the rain polluted with Pb
and Cd falling on forest soils has been the key focus of the study. In the experiment, twenty-four permanent
plots were established within an area of 61-80% dieback severity and three soil amendments through addition
of (a) compost, (b) montane mycorrhizae, and (c) compost and montane mycorrhizae, alongside the control
made up the four treatments used in this study. Treatments were applied to five randomly selected Sysygium
saplings of approximate height of 1m and 0.015m diameter breast height (DBH) residing in
each plot. Soil organic matter content (SOM) and Pb and Cd were compared from soil samples collected at
0.2m depth. These comparisons were done for samples collected at three different stages and during the
experimental period, the selected saplings were closely monitored and changes in health were accordingly
recorded. The soil analysis shows clear indications of Pb and Cd contamination which impairs plant
metabolism leading to dieback. Effect of standard compost and montane mycorrhizae on protecting saplings
from Pb and Cd was significant (p = <0.001). Moreover, compost and mycorrhizae appeared to be effective
in reducing the effect of Pb and Cd on sapling’s mortality. Significant decline of Cd (p = 0.01) and Pb (p =
0.01) with the increasing SOM level were observed.
Forest dieback, Compost, heavy metals.
The Upper Montane Forest in Horton Plains, Sri Lanka, as it was in 1947, was described as a low, dense,
slow-growing forest with a healthy and vigorous appearance . It is located on the highest plateau of Sri
Lanka, which lies between 1,500 and 2,524m average sea level  and the geographical location is in the
Central Highlands of the Central Province, 6’47 – 6’50’N, 80’ 46’- 80’50’E. Annual rainfall in the region is
about 2540 mm. Temperatures are low, with an annual mean of 13°C, and ground frost is common in
February . The landscape characteristically consists of gently undulating highland plateau at the southern
end of the central mountain massif of Sri Lanka. Soil order Ultisol is characterized by a thick, black, organic
layer at the surface. Horton Plains is the most important catchments area of the countryand tributaries of
three major rivers originate from within the reserve. The land area covered by this montane rain forest is
approximately 3,160 ha. There are 54 woody species, of which 27 (50%) are endemic to Sri Lanka.
Belonging to different size and age classes of these forest, have been dying due to a yet unknown factor. This
phenomenon was first observed in the Horton Plains National Park and the earliest reports of a significant
level of dieback in the forest were by . Estimations using recent satellite images combined with ground
surveys revealed that about 654 ha, equivalent to 24.5% of the forest in the park has been subjected to
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2012 International Conference on Future Environment and Energy
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dieback . One of the worst affected trees was Syzygium rotundifolium
followed by Cinnamomum
, Neolitsea fuscata
, Syzygium revolutum
and Calophyllum walkeri
. Also, seedling establishment
and forest regeneration in the area is slow . Healthy forest in the park amounts to about 2012 ha. The
extent of the damage to the forest from dieback appears to be so severe that the stand structure in affected
areas show dramatic changes. If this dieback continues with the current rate, the majority of the large trees
will disappear from the forest soon. The vital functions offered by this precious forest will then be subjected
to significant changes most probably towards the negative side. Work done by many researchers so far has
ended up with no significant clues about the causal agents and remedial measures for the dieback though
work done by . has indicated the contamination of soils in the Horton plains by Pb and Cd and possible
links of the soil pollution to forest dieback. Therefore, the main objective of the study is to check the ability
of SOM in effectively remediating Pb and Cd in the affected soils.
2. Materials and Methods
Twenty-four permanent experimental plots of 20 m
× 20 m were demarcated using GPS (Global
Positioning System) points with a 20 cm accuracy to represent an affected area in the Horton Plain National
Park. Randomized Complete Block Design (RCBD) was used with six blocks to replicate each and every
treatment six times. Plot locations were selected to cover a 61 – 80 % dieback of trees and to maintain soil
and topography as constant as possible. Four soil amendments (a).compost-2kg/sapling, (b). compost and
montane mycorrhizae-4kg/sapling. (c).montane mycorrhizae-2kg/sapling including a control were used for
the study while taking Syzygium rotundifolium as the indicator plant. An Investigation of harmful elements
such as Pb and Cd in the soil samples were measured by wet ash method  and the extractants were
analyzed for the above elements by Atomic Absorption Spectrophotometry . In addition, the soil organic
matter content was determined using the method of total organic C by Walkley and Black described by .
The soil samples were collected from 0.20m depth and 0.3m-0.5m away from each sapling representing three
different time periods. Furthermore, Death rates of the saplings were calculated by keeping records of the
selected saplings throughout the experimental period and counting the deaths at the end of the trial.
3. Results and Discussion
The results shown in this paper are based on the work done during the two-year study period within the
61-80% dieback areas selected in the Horton Plains National Park (HPNP), Sri Lanka. Soil organic matter
content and heavy metals such as Pb and Cd were compared first among treatments under three stages of
sampling. In addition, the data collected were compared with the death rate of the saplings.
Soil organic matter
Soil organic matter (SOM) level in the study area of Horton Plains has not reached upper levels in the
range, up to 12%, as expected in tropical moist evergreen forests . In ordinary tropical moist evergreen
forests, SOM content varies around 6% . Relatively low plant nutrient levels in montane forests are not
unusual according to past studies (e.g., (.). For each 1000m rise in altitude, there is a 7
C drop in
temperature . This has a dramatic effect on plant and animal distribution in this ecosystem. With the
elevation of about 2524m, Horton Plains is cold (mean annual temperature 15
C) and contains a very
specific vegetation which is much more sensitive to the changes in the environment than normal tropical
forests . Under the prevailing conditions in the montane environment –low sunlight, low temperature,
shallow soil depth and so on, production of SOM is weaker in the Horton Plains than in an ordinary tropical
forest . As far as the SOM content is concerned, there are significant differences among the treatments at
soil sampling stage 1 (p = <.001), stage 2 (p = <.001), and stage 3 (<.001) in the 0.2m depth (Fig 1(a) ). The
soils treated with compost and compost + mycorrhizae mixture showed the higher values of soil organic
matter though soils treated with mycorrhizae only and the control showed the lowest at all three stages.
Fluctuation of SOM levels in the area may be linked with temperature, rainfall, soil depth and addition of
organic debris from the aggressively growing undercover vegetation such as Strobilanthus spp. The
function of SOM springs from its effects on soil structural stability (its action as a bonding agent between
primary and secondary mineral particles leads to enhanced amount, size and stability of aggregates) and soil
water retention (as a water adsorbing agent, it enhances water acceptance and availability) and, hence, on
infiltration and percolation. At the same time, SOM controls soil nutrients that affect biomass. 
emphasized that soil structural stability is influenced by the type of organic matter, as well as its amount.
Therefore, in some cases, high SOM content is not accompanied by high structural stability.  pointed out
that some fungi exude oxalic acid, which enhances dispersion and breakdown of aggregates. Humic
substances are the components of SOM which play the key role in detoxifying the soil from pollutants such
as Pb and Cd residues of Agro-chemicals from surrounding areas . Unsatisfactory levels of SOM exhibit
the poor activity of humic substances and resultant soil pollution. It should also be noted that even a milder
form of soil contamination in the Horton Plains cannot be afforded since the montane vegetation is highly
sensitive to the changes in the environment.
Heavy Metals in soil (Pb and Cd)
The level of soil Pb and Cd has gone up to 106 and 7.29 ppm respectively. The maximum allowable limit
of Pb is 100 ppm while it is 3ppm for Cd . Even the smallest amount of both Pb and Cd may impose
severe damages on plant’s metabolism leading to dieback . Results from soil analysis clearly indicated
contamination of soil from these two trace elements in Horton Plains. Treatments used for the study have
significantly influenced the soil Pb at sampling stages 1 (p=0.01) and 2 (p=0.004) but there is no significant
influence detected at stage-3 (p=0.79) (Fig 2(a)) and the highest Pb content was observed in the control.
Cadmium content in the soils of the study area is not significantly different with the treatments at stage-1
(p=0.18), -2 (p=0.35), and -3 (p=0.51) though the highest is observed in the control (Fig 2(b)). Mycorrhizae
significantly increase the absorption of various elements from the soil including heavy metals such as Pb and
Cd . Therefore, it could be assumed that mycorrhizae are responsible for the reduction of Pb and Cd in
the soil treated with mycorrhizae. Soil microorganisms play a vital role in maintaining overall soil quality.
They have been proved to be effective in detoxifying pollutants in the soil that include heavy metals such as
Pb and Cd. Soil microbes (e.g, mycorrhizae) on the other hand, maintain extremely useful symbiotic
associations with the forest vegetations which provide additional advantage for the plants to mine nutrients
and water .
Death rate of Syzygium rotundifolium saplings
It was clearly evident that the addition of standard compost and mycorrhizae has significantly controlled
the death of Syzygium rotundifolium saplings. Treatment effect on the death of saplings is significant (p=<
0.001) whilst the control clearly shows the highest death rate (Fig 1(b)). The standard compost consists of
humic and fulvic acids that are formed during the microbial decomposition of organic materials. These
specific molecules, known as humic substances, possess extraordinary capability of immobilizing soil
contaminants such as Pb and Cd. Additionally, dozens of fractions in compost help the plants to withstand
stressful conditions such as drought, nutrient imbalances, acidity and so on . In addition, standard
compost is a good reservoir of all forms of essential plant nutrients and growth factors of plants .
Mycorrhizae, on the other hand, act as a remarkable symbiotic mechanism for the plants to survive under
stressful conditions such as droughts, nutrient deficiency, soil contaminants such as Pb and Cd . Thus, it
could be argued that treating the Syzygium rotundifolium samplings with standard compost and mycorrhizae
until they become grownup trees might help to fill the gaps caused by the dieback in the forest.
3.4. Soil organic matter Vs Pb in the soil
The content of soil Pb is inversely proportional to the SOM content (Fig 3(a)) and the relationship was
statistically significant( p = <0.001). The findings indicate that the availability of Pb in the soil for plants in
the study area could be reduced by increasing SOM level. The nature of the decline of Pb with the increasing
SOM level seems to be linear-by-linear type. Immobilization of soluble Pb in the soil by the humic and
fulvic acid molecules present in SOM has been documented by several researchers (e.g., ).
Fig. 1: (a) Status of SOM% among the treatments; (b) Death rate of the saplings after 2 years with four different
3.5. Soil organic matter Vs Cd in the soil
The relationship between the availability of Cd in the soil and SOM content was significant at p = 0.01.
The nature of the relationship was a linear-by-linear as shown in fig. 3(b). According to the graph soil Cd
levels gradually decreased with the increasing SOM level. The results clearly show that the effect of
available soil Cd on montane vegetation could be reduced by improving SOM level. Action of humic and
fulvic acid on immobilization of Cd may have resulted these relationships . The results in general
indicate that the maintenance of SOM will help to mitigate Cd toxicity on forest vegetation.
Fig.2: (a) Status of Pb among treatments at four different stages of sampling; (b) Status of Cd among treatments at
four different stages of sampling
Fig.3: (a) Soil organic matter Vs Pb in soil; (b) Soil organic matter Vs Cd in soil; Soil organic matter content in the
soil Vs Death rate of saplings
3.6. Soil organic matter content in the soil and dieback of plants
Results show that the increase of SOM level helps to reduce the death of saplings. The relationship
between SOM level and the death rate of the saplings (Sysigium rotudifolium) was significant (p = 0.05). The
nature of the relationship seems to be linear-by-linear and it further indicates that by maintaining SOM level
somewhere above 4%, the death rate of the saplings could significantly be reduced (see fig 3(c)). Humic and
fulvic acid molecules in SOM effectively immobilize toxic metals such as Pb and Cd in the soil .
Y =3+ 2 / (1+X)
R2 = 91.7%
P = <0.001
Y= 7.6-2.3/ (1-
Y = 20.3 – 1.8 / (1-0.3X)
Soil pollution in the montane forest with Pb and Cd as affected by increasing vehicle emissions and
consequential polluted rain appears to be one of the key causes for the deterioration of the vegetation by
forest dieback. Extra sensitivity of the montane forest vegetation to the changes in the soil may have
triggered the impact of soil pollution. Enrichment of the polluted forest soil with standard compost and
montane mycorrhizae appears to be effective in saving the saplings of Syzygium rotundifolium (one of the
worst affected trees) from untimely death. Maintenance of SOM at satisfactory levels in the soil appears to
be effective in reducing the levels of both Pb and Cd in the soil.
This study was conducted with the financial support of Sabaragamuwa University of Sri Lanka and the
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