Subsidence and Carbon Loss in drained tropical peatlands
(Hooijer et al., 2012) – A Critical Review
Paramananthan, S.1, Ramadason, K.2 and Tan, Y.A.2
1
Param Agricultural Soil Surveys
Petaling Jaya, Selangor
2
Malaysian Palm Oil Board
Bangi, Malaysia
BACKGROUND
The US Environmental Protection Agency (EPA), in January 2012 published an analysis
of the life-cycle greenhouse gas (GHG) emissions associated with palm oil based biodiesel and
renewable diesel. The results of the analysis indicate that, when compared with petroleum
diesel baseline, palm oil-based biofuels reduce GHG emissions by 17% and 11% respectively,
and thus do not meet the statutory 20% GHG emissions reduction threshold for the Renewable
Fuel Standard (RFS) program (EPA 2012).
Based on the EPA’s analysis, one of the major sources of GHG emissions was emissions
resulting from drained organic peat soils preceding the development of new palm oil
plantations. The EPA used a peat soil emission factor of 95 tonnes of carbon dioxide (CO2)
per hectare of drained peat soil, based on Hooijer et al. (2012), to help estimate the total GHG
emissions from the expansion of peat soil drainage due to development to oil palm.
In order to ensure that the EPA has taken into account the best available information on
this important emissions factor for the life-cycle GHG analyses of palm oil-based biofuels, the
Agency asked RTI International to facilitate an independent peer review. The purpose of this
review was to request additional scientific input about the Agency’s assessment of the average
annual GHG emissions from tropical peatlands resulting from the development of the land for
production of palm oil for use in EPA’s life-cycle GHG analysis of palm oil-based biofuels.
RTI selected five peer reviewers who are experts in GHG emissions from peat soils to review
EPA’s application of peat soil emissions factor and to provide feedback on the use of this factor.
Each of the experts were asked to respond to five questions that seek to address the relevance
and appropriateness of the emission factor.
SELECTION OF THE FIVE PEER REVIEWERS
The EPA requested recommendations for the peer-review candidates from various
organizations including the Embassies of Malaysia and Indonesia. A list of 21 was compiled
and submitted to RTI. RTI based on their own resources, added additional candidates.
Candidates (4) who were involved in the Hooijer et al. (2012) were excluded. Based on a
number of other criteria, the following were selected:
•
•
•
•
•
Scott Bridgham, Ph.D. – Professor University of Oregon
Kristell Hergouale, Ph.D. – Scientist, Centre for International Forestry Research
Monique Leclerc, Ph.D. – Regents Professor, University of Georgia
Supiandi Sabiham, Ph.D. – Professor Bogor Agricultural University
Arina Schrier, Ph.D. – Climate and Environmental International Consultancy – Owner
Subsidence and Carbon Loss in drained tropical peatlands (Hooijer et al., 2012) – A Critical Review
EPA reiterates that these 5 selected candidates were selected from recommendations
provided for by:
•
•
•
•
Ambassador for Indonesia – one candidate
Ambassador for Malaysia – one candidate
International Council on Clean Transportation – one candidate
Recommended by RTI – two candidates
RTI summarized the panel’s response below:Three (Bridgham, Schrier, Leclerc) out of the five reviewers agreed that the
emission factor used in EPA’s analysis of palm oil-based biofuels is an appropriate
coefficient to use based on current scientific understanding, but emphasized that
the emission factor should be re-evaluated as meta-analyses of existing research
are conducted and/or as additional research becomes available. Two reviewers
stated that EPA has likely overestimated the carbon emissions. One (Hergouale)
of these two reviewers recommend using the peat soil emission factors published
by the IPCC (Drösler et al., 2013), while the other reviewer (Sabiham)
recommended using peat soil emission factors published by Melling et al. (2007).
The five questions that each of the reviewers was asked to comment on were:
Charge Questions
Question 1 – Overarching Charge Question
Given the three criteria outlined in the Technical Work Product and the estimates
available in the literature, did the U.S. Environmental Protection Agency (EPA)
choose the most appropriate value for the peat soil emission factor? If not, please
provide a recommendation on the most appropriate peat soil emission factor to use
in EPA’s analysis, with a detailed explanation.
Three Criteria: Hooijer et al. (2012) – 95 tonnes CO2 ha-1 yr-1
IPCC (Drösler et al., 2014) – 40 tonnes CO2 ha-1 yr-1
Melling et al., 2007 – 41 tonnes CO2 ha-1 yr-1
Question 2 – Potential Adjustment of Emissions Factor from Hooijer et al. (2012)
Some commenters have raised questions about particular values used in the
Hooijer et al. (2012) study (e.g., organic carbon content and peat bulk density).
Would you recommend that EPA use the overall approach and data published in
Hooijer et al. (2012) but use a different value for: (a) organic carbon content, (b)
peat bulk density, (c) the percent of subsidence due to oxidation, or (d) another
parameter (please specify)? Please explain your recommendation and provide
supporting documentation.
Question 3 – Directionality of estimate
EPA recognizes that the Hooijer et al. (2012) study that forms the foundation of
our estimate of peat soil emissions was conducted under specific circumstances.
For example, it was conducted in a limited number of plantations on the island of
Sumatra. For the reasons listed in the TWP, we believe this is the best available
estimate of peat soil emissions, but we recognize that numerous factors could cause
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Subsidence and Carbon Loss in drained tropical peatlands (Hooijer et al., 2012) – A Critical Review
this estimate to be higher or lower than the average emission factor for peat soils
drained for oil palm across Southeast Asia. Please discuss whether the emission
factor value used by EPA (95 tCO2/ha/yr) is likely to overestimate, underestimate
(and if so by how much) or provide a plausible estimate of average greenhouse gas
(GHG) emissions from peat soil drainage for oil palm across Southeast Asia. In
particular, please discuss whether the following factors are likely to make EPA’s
emission factor an overestimate or an underestimate:
a. Variation in the type of peat soil (mineral content, carbon content, depth, extent
of degradation, etc.).
b. Precipitation regime (annual rainfall, timing of rainfall, etc.).
c. Differing water management practices at plantations.
d. Different types of plantations (e.g., oil palm versus acacia).
e. The approach used by Hooijer et al. (2012) to estimate emissions during the
first five years after drainage.
f. Omission of methane and nitrous oxide emissions.
g. Omission of emissions due to fire (As discussed in the TWP, omission of this
factor will cause EPA’s emission factor to underestimate emissions, but we
welcome comments about how large this underestimation may be).
h. Omission of incidentally drained peat swamps adjoining the plantations.
Question 4 – Intergovernmental Panel on Climate Change (IPCC) report
IPCC (2014) lists a Tier 1 emission factor of 40 tCO2/ha/year for tropical drained
oil palm plantations. This value does not include emissions for the first 6 years
after drainage. However, studies have shown that a pulse of higher emissions
occurs right after drainage. The IPCC report also gives a default DOC emission
factor of 3 tCO2/ha/yr. In addition, the IPCC gives guidance on quantifying
emissions from fires. The report gives a default emission factor of 1,701 gCO2/(kg
dry matter burned) for tropical organic soil and a default dry matter consumption
value of 155 t/ha for prescribed fires in the tropics.1
a. Would it be appropriate for EPA to use the IPCC Tier 1 default emission factor
of 40 tCO2/ha/year, or is it scientifically justified to use a different number
based on more detailed information?
b. Should the emission factor that EPA uses include the emissions pulse that
occurs in the first several years immediately following drainage?
c. Should EPA include DOC and fire emission factors in the overall emission
factor? If so, are the IPCC emission factors appropriate to use, or are there
better estimates for EPA’s purpose?
d. There are also erosion losses of particulate organic carbon (POC) and
waterborne transport of dissolved inorganic carbon (primarily dissolved CO2)
derived from autotrophic and heterotrophic respiration within the organic soil.
The IPCC concluded that at present the science and available data are not
sufficient to provide guidance on CO2 emissions or removals associated with
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Subsidence and Carbon Loss in drained tropical peatlands (Hooijer et al., 2012) – A Critical Review
these waterborne carbon fluxes. Do you agree that the science on these factors
is not sufficient for EPA to consider losses of POC and dissolved inorganic
carbon in its peat soil emission factor?
Question 5 – Additional input
Please provide any additional scientific information that you believe the EPA
should consider regarding the Agency’s assessment of the average annual GHG
emissions from draining tropical peatlands for palm oil cultivation for use in EPA’s
lifecycle GHG analysis of palm oil-based biofuels.
INTRODUCTION
The EPA’s acceptance of the paper on the ‘Subsidence and Carbon Loss in drained
tropical peatlands’ (Hooijer et al., 2012) with its conclusion of 95 tCO2 eq. ha-1 yr-1 over the
25-year life-cycle of a crop like oil palm and Acacia plantations in Sumatera needs firstly to be
evaluated critically. Hooijer et al. (2012) report that in tropical peatland areas are different
compare to the Fenlands of the UK, the Netherlands, Venice Lagoon in Italy, the Everglades
and Sacramento Delta in the United States and Lake Hula in Israel where a total subsidence of
200 to 600 cm occurred over 40 to 130 years bringing surface levels close to or below sealevels. In all these cases peat oxidation is reported to be the main cause of subsidence. They
also report that the net carbon losses and resultant CO2 emissions from peatlands drained for
agriculture in Malaysia and Indonesia range from <40 t CO2 ha-1 yr-1 (Melling et al., 2005;
Murdiyarso et al., 2010; Herchoualc’h and Verchot, 2011) to >60 t ha-1 yr-1 at watertable depths
of around 0.7 m. Hooijer et al. (2012) state that the uncertainty in the rate of carbon emission
from drained tropical peatland is caused partly on the reliance on measurements of gaseous
CO2 emissions that are difficult to conduct and interpret. They add that unless CO2 emission
studies are carried out on a large scale (i.e. a large number of measurements conducted over a
range of environmental conditions (in terms of watertable, vegetation cover and temperature),
data uncertainty is considerable (Couwenberg et al., 2010; Murdiyarso et al., 2010; Jauhiainen
et al., 2012). A few studies quoted estimates of CO2 emissions from oil palm plantations on
peat have been based on fewer than 50 observations including replicates at single locations
(Murayama and Bakar, 1996; Melling et al., 2005). Few studies have estimated net CO2
emissions resulting from peat oxidation alone, excluding root respiration. Also, gas flux
measurements do not account for carbon losses in discharge water (DOC and POC) that leave
the peatlands in drainage water (Alkhatib et al., 2007; Baum et al., 2007; Moore et al., 2011).
Hooijer et al. (2012) claim that measurements of land subsidence in combination with data on
peat characteristics, provide a direct approach to carbon loss assessment that is relatively
straightforward to conduct in the field and to interpret. All impacts, on the carbon stock, they
claim are integrated over time without requiring instantaneous measurements, thereby
providing a more accurate value for total carbon loss even if the individual loss components
(CO2, CH4, DOC and POC) cannot be separated using this method.
MEASUREMENTS/METHODOLOGY USED BY HOOIJER ET AL., 2012
The measurements and methodology used by Hooijer et al. (2012) is summarised in Table
1. It is clear from this table that while over 200 sites were measured, the sites selected do not
reflect the current practices used by the oil palm and acacia plantations planted on peatlands
today. This will be elaborated below.
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Subsidence and Carbon Loss in drained tropical peatlands (Hooijer et al., 2012) – A Critical Review
Table 1. Summary of locations and measurements made by Hooijer et al. (2012).
Crop/
Vegetation
Location and
No. of Sites
(218) (GPS
Coordinates)
Study
Period
Drainage
Intensity
Initiation of
Data
Recording
Data Recorded
Subsidence
Measurements
Water-table
• 3-12 months after
installation.
• Discarded 3-14
sites 1 year after 2
years installation,
and monitoring.
• Used elevation
data from
company records
from elevation
surveys one year
after drainage.
2-12 weekly
intervals.
• 2, 4-17 and 18
years intervals.
• 22 locations.
• 19 drained 4 to
7 years earlier.
• 3 drained 2
years earlier.
• One location
• 2, 4-17, 18
years after
drainage.
20072010
• Width 5-8 m
• Depth 3 m
• Intensity 500800 m apart
• 3-8 years
after
drainage
• Average 6
years
• 14-19 years
after
drainage
• Average 18
years
• One year
• July 2009 – June
2010
• Company records.
• Monitored 2
weekly intervals.
2-12 weekly
intervals.
-
-
2-12 weekly
intervals.
Acacia
Riau
(125)
0.595/
102.334
Oil Palm
Jambi
(42)
1.566/
103.601
20072010
• Width 5-8 m
• Depth 3 m
• Intensity 500800 m apart
Forest
Riau
(51)
0.595/
102.334
20072010
• Drained (not
known) (site
2 km from
Acacia site)
Bulk Density
5
-
Peat Maturity/
Thickness
Ash Content
Visual
223 sub –
samples (Loss
on Ignition)
Visual
None
-
-
Subsidence and Carbon Loss in drained tropical peatlands (Hooijer et al., 2012) – A Critical Review
NEED
FOR
PEAT
CHARACTERISTICS
SUBSIDENCE
MEASUREMENTS
AND
PEAT
The paper lacks detailed peat characteristics in the sites where the measurements were
made leading to the possibility of misrepresentation/misinterpretation of the data. A basic
factor in site characterization of peat is peat maturity (sapric/hemic/fibric) and the description
of the peat profile – particularly on tropical peats. Unlike temperate peats tropical peats often
have large pieces of wood – undecomposed or decomposed. The presence of wood makes soil
sampling for bulk density and subsidence measurements difficult. Most International
Classification such as the IUSS WRB 2014 and the USDA’s Keys to Soil Taxonomy (Soil
Survey Staff, 2010) fail to recognize this difference. The Malaysian Soil Taxonomy
(Paramananthan, 2010a, b) does. Consequently, recent work has shown that these International
Classifications fail to fully describe and map tropical peats (Wüst et al., 2003; Veloo et al.,
2014a, b). These works have shown that peat maturity and the presence of wood influence the
performance of oil palm cultivation on peats. It is further suggested that these characteristics
may also influence GHG measurements on tropical peats. Paramananthan (2015, in press)
suggest that the wide variation in data on performance, GHG emissions from tropical peats is
due to inadequate site characterization. The technical journals continue to prolong is
inadequacy by not requiring publications in their journals to fully characterized their organic
soils instead of simply calling them ‘peats’.
Thus, Hooijer et al. (2012) do not fully characterize their sites. Their Figure 1 suggests
that the upper 60 cm is sapric – possibly due to the fire that was used in land clearing as stated
in the paper or due to fire which took place in Southeast Asia resulting in most peats having
only sapric materials in the upper 50-60 cm (Paramananthan, 2014). Most publications suggest
that most tropical peats are fibric (IPCCC, 2012). This is not true in the upper 100 cm of the
over 1 million hectares of tropical peats which the senior author has surveyed both in Malaysia
and Indonesia they were mostly sapric. Most workers fail to apply the rubbing test as required
by Soil Taxonomy (Soil Survey Staff, 2010). The test requires that large fragments (>2 cm
diameter) be removed and the fine fraction rubbed before estimating the fibre content. Under
tropical conditions with its high ambient soil temperature and watertables below 100 cm in dry
periods, the fine fraction in peats decompose, but retain their fibric form as pseudomorph until
they are rubbed. Hooijer et al. (2012) applied a visual test to determine peat maturity. This is
inherently wrong. We need to follow a set of correct methodologies, if not, we will continue to
prolong our misinterpretations.
COMPARISON OF ACACIA TO OIL PALM
The two plantation crops used in the study, Acacia and oil palm require different types
of management in terms of land preparation, drainage design and intensity, fertilizer regimes
and harvesting methods and harvesting cycles/frequency. One can safely assume that both
areas were logged before clear felling and burning before the crops were planted. Hence one
can also assume that during logging drainage was implemented to facilitate logging. Logging
does not require good water management but often deep drains are dug to lower the water-table
to facilitate the logging. Moreover, since fire was used in the oil palm site as a means of land
clearing, one can safely assume that the peat at least to a depth of one metre was dry at the time
of the fire. It is therefore possible that the surface sapric layer reported was due to the fire used
in land clearing. Further, the type of water management used should have been different for
oil palm and Acacia. Acacia is a dicot with a taproot system while oil palm is a monocot and
hence have completely different rooting systems and different water management regimes.
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Subsidence and Carbon Loss in drained tropical peatlands (Hooijer et al., 2012) – A Critical Review
Figure 1. Cross section along typical study transect in Sumatera, 6 years after drainage,
showing variation in peat depth, average water level, land use and monitoring location
density (Hooijer et al., 2012).
DRAINAGE DESIGN
The drainage design in both the oil palm and Acacia was similar. The layout and intensity
used in the two areas is summarised in Table 1. It is most likely that this drainage design and
intensity report was inherited from the logging phase. No mention of control structures is
reported in the paper. Figure 1 in the paper indicates that in the dry season, the watertable was
at one metre depth. It appears that the two plantations do not follow current Best Management
Practices used today.
Since 1986 after United Plantations published their paper, Gurmit et al. (1986) the
drainage layout and intensity and types of drains used as part of the BMPs for oil palm follow
this (Figures 2 and 3). Further prior to planting, compaction is practised (Figure 4). It is clear
therefore, that the oil palm plantation and possibly the acacia plantation did not follow the
current BMPs which includes a NO BURN Policy. It is not clear why the acacia plantation
carried out repeated elevation survey after drainage was implemented. Thus, based on Figure
5, the authors concluded that the acacia plantation subsided an average of 75 cm in the first
year and 67 cm in the 2nd to 4th year giving a total subsidence of 142 cm in the first five years.
This in our humble opinion, is erroneous. Subsequent subsidence was 5 cm yr-1. It was
assumed that the oil palm subsidence was the same. No mention of cultural practices of
compaction prior to planting of oil palm or acacia is mentioned. The weir which was
constructed was two years after the initial drainage. If water control was to be practiced, the
weirs should have been there from the beginning. It appears therefore that the total subsidence
of 212 cm over 18 years is a gross overestimate. The wide variations in the watertable depths
0.47 m to 0.98 m for acacia and 0.33 to 1.03 m for oil palm clearly shows that the watertable
was NOT managed well in both the plantations. The subsidence rate of 5 cm yr-1 in an oil palm
plantation in Johor, Malaysia for 14-28 years old palm (Woosten et al., 1997) was without
water controls as reported by Salmah (1992).
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Subsidence and Carbon Loss in drained tropical peatlands (Hooijer et al., 2012) – A Critical Review
Type and size of drains
Type of drain
Width (m)
Depth (m)
Top
Bottom
Field
1.0-2.0
0.5-0.6
0.9-1.0
Collection
1.8-2.5
0.6-0.9
1.2-1.8
Main
3.0-6.0
1.2-1.8
1.8-2.5
Source: Gurmit et al. (1986)
Figure 2. Layout plan and size of the drainage system in peat areas.
Source: Gurmit et al. (1986)
Figure 3. System of consolidation of harvesting paths and planting rows in peat
areas.
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Subsidence and Carbon Loss in drained tropical peatlands (Hooijer et al., 2012) – A Critical Review
Figure 4. Compaction of peat before planting (Source: Proceedings of the
Workshop on Standard Operating Procedures (SOP) for Oil Palm Cultivation On
Peat (MPOB 2010).
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Subsidence and Carbon Loss in drained tropical peatlands (Hooijer et al., 2012) – A Critical Review
Figure 5. Top: average subsidence rates as measured at 14 locations in Acacia plantations,
over the first 9 years after drainage. Bottom: as measured at a larger number of drained peatland
locations in Sumatra (this study), Malaysia (from Wösten et al., 1997, based on DID Malaysia,
1996), Mildred Island in the California Sacramento Delta (Deverel and Leighton, 2010) and
Florida Everglades. The Everglades record is averaged from three records presented by
Stephens and Speir (1969); as the first two years after completing the drainage system in 1912
were missing from the subsidence record, which started in 1914, we added a subsidence of 22.5
cm yr-1 for those years, which is the average subsidence rate over 1,914 and 1,915 and therefore
almost certainly an underestimate of actual initial subsidence. Also shown are long-term
calculated subsidence rates for SE Asia, applying both the relation determined for Florida
Everglades (Stephens et al., 1984), assuming a water depth of 0.7m and an average temperature
of 30°C, and the relation found for SE Asia in this paper (Hooijer et al., 2012).
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Subsidence and Carbon Loss in drained tropical peatlands (Hooijer et al., 2012) – A Critical Review
BULK DENSITY
Bulk density in peat is very difficult to measure due to the presence of wood. Moreover
at depths below one metre (below the permanent watertable), it is near impossible. Our
experience in Malaysia is that even with two water pumps working, we often face a losing
battle with the water rising very fast. The very low bulk densities reported in the paper even
in the surface layers of 0.15 g cm-3 near the surface to 0.075 g cm-3 at depth below 0.5 m are
very low figures especially in the oil palm where harvesting would already have been done.
Below 1.0 m, values of 0.073 g cm-3 for acacia and 0.078 g cm-3 for oil palm are extremely
low. One wonders how the oil palm or acacia were harvested. Many of our peats in Sarawak
have undecomposed wood that needs to be chain sawed to dig drains (Figure 6). These logs
do not decompose and makes oil palm less suitable on such soils – both in terms of yield and
cost of development (Veloo et al., 2014a, b).
Figure 6. Presence of wood in Tropical Lowland Peats (Source: Paramananthan, 2008b).
Figure 7a. Leaning of palms on peat.
Figure 7b. Excessive leaning of palms on peat.
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Subsidence and Carbon Loss in drained tropical peatlands (Hooijer et al., 2012) – A Critical Review
SUBSIDENCE RATES
The effect of drainage and subsidence of the peat on crops such as coconuts and oil palm
is well recorded. Subsidence results in leaning palms with exposed roots (Figure 7a & b).
These observations were mainly in the early plantings before United Plantation’s pioneer work
of Gurmit et al. (1986) proposed controlled drainage to minimise subsidence of the peat. Thus
many of the data on subsidence reported in the literature (Woosten et al., 1997; DID Malaysia,
1996; Mohammad et al., 2009) often quote values which may not be applicable today. For
example the values of 4.6 cm yr-1 for 14 to 28 year old palm in Johor were high as these areas
were drained for flood mitigation (Salmah, 1992). In the old days, the concept of deep drains
to remove and lower the watertable received priority in peat areas. Today, the experience of
United Plantation prevails and water control and management are more important and critical.
On another point, one cannot use an universal value for subsidence (and decomposition) as
mentioned by Hooijer et al. (2012). The values of 3 cm yr-1 for the Everglades (Stephens and
Speir, 1969) over more than 50 years. Temperate and boreal peats do not decompose the whole
year. Unlike tropical peats, due to their higher temperature with no seasonal change will give
higher decomposition rates with the same type of management. Thus, it would be unfair and
unscientific to apply a single default rate for both temperate and tropical peats. Added to this
also the fact that growth rates and hence litter production and carbon sequestration is
continuous and higher in the tropics compared to temperate and boreal areas. We in the Tropics
decompose more but also replace more. Thus, it is not surprising to note that Stephens et al.
(1984) calculated that if the Everglades was in the Tropics, it would subside 8 cm yr-1, a value
higher than the 4.0-5.5 cm yr-1 expected in tropical peatlands after an initial phase of rapid
subsidence. Kool et al. (2006) based on measurements in Central Kalimantan in an area subject
to logging report they recorded a subsidence of 2.2 m to 4.0 m but the actual decomposition
was only 2 cm to 47 cm. It is apparent from this data during logging which takes place prior
to oil palm or acacia cultivation the peat dome collapses as large drains are dug with little or
no control. Thus by the time the oil palm is planted, this major subsidence has already taken
place. Thus it is possible that Hooijer et al. (2012) have interpreted the early subsidence rate
of 142 cm to be due to oil palm. This is probably due to this logging.
DOC AND POC
The paper by Hooijer et al. (2012) quote that flux measurements do not account for
carbon losses in drainage water (Alkhatib et al., 2007; Baum et al., 2007; Moore et al., 2011).
Further, they quote that efforts to calculate the net change in peat carbon stock from the
difference between all estimated fluxes into and out of the peat including changes in biomass
(Herchoualc’h and Verchot, 2011), have been inconsistent because of limited data available
and cumulative uncertainties associated with each component. Hooijer et al. (2012) describe
the peat at their sites at depths (>1 m) as going towards hemic. Peat at greater depth was nearly
always fibric, and often woody, except the lowest few metres where the peat was hemic or
sapric (why?) and sometimes described as muddy (hardly a scientific term), indicating mixed
mineral and organic content. It is clear from their descriptions that POC have settled from the
upper layers to the base of the peat which is in contact with the underlying substratum. Similar
observations have been reported by Paramananthan (2014) in Sarawak (Figure 8). Such
material is termed humilluvic material in Soil Taxonomy (Soil Survey Staff, 2010). This is
carbon which has literally moved from the surface layers to settle at the bottom and these were
not included in the calculations of Hooijer et al. (2012). Further, loss of DOC in the water in
the drains and POC which moves laterally and settles in the drains is not accounted for. It is
for this reason that the drains in the oil palm estate need to be desilted regularly. This
redistribution of POC and DOC could be substantial particularly in the initial drainage and
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Subsidence and Carbon Loss in drained tropical peatlands (Hooijer et al., 2012) – A Critical Review
continues in subsequent years at a lower rate. This will result in a lowering of BD in the surface
layers and this will be compensated by a collapse of the peat causing high bulk density and not
due solely to decomposition as claimed.
SUBSIDENCE RATES OVER OIL PALM
Hooijer et al. (2012) concluded based on their study that the subsidence rates in the first
five years after drainage to be 142 cm and subsequent to that to be 5 cm/year. They also
concluded that 75% of the cumulative subsidence was caused by peat oxidation during the first
5 years. They therefore concluded that peat oxidation after 18 years in oil palm was 92%.
Based on their calculations, the average rate of carbon loss over the first 5 years was 178 t CO2
eq. ha-1 yr-1 which reduced to 73 t CO2 eq. ha-1 yr-1 over the subsequent years potentially
resulting in an average loss of 100 t CO2 eq. ha-1 yr-1 over 25 years. With improved water
management, they add that a 20% reduction is possible.
Most oil palm estates on peat today consider replanting after 20 years and hence based
on their subsidence rates, we would have lost about 217 cm of peat thickness (142 + 5 x 15
cm). This implies that areas mapped as shallow (50-100 cm) peat would no longer have any
peat on the soil surface within 5 years. The changes in depth of the initial peat depth is shown
below.
Initial Depth of Peat
Shallow peat (50-100 cm)
Moderately deep peat (100-150 cm)
Deep peat (150-300 cm)
Very deep peat (>300 cm)
Result at the Subsidence of Hooijer et al. (2012)
Within <5 years, no more peat
Within 5 years, only 8 cm of peat left on the surface
and would no longer qualify to be called peat.
Less than 100 cm peat left after the 1st cycle (20
years) of oil palm and none left after the 2nd cycle.
We would have lost close to 417 cm of peat (217 +
100 + 100 cm) after the 3rd cycle of oil palms.
This obviously cannot be true because today after 3 cycles, we still have many oil palm
estates on deep peat.
Figure 8. Humilluvic carbon at the organic soil material/mineral soil material interface
(Source: Paramananthan, 2014).
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Subsidence and Carbon Loss in drained tropical peatlands (Hooijer et al., 2012) – A Critical Review
Loss of Peat Thickness after cycles of oil palm (20 years):
•
•
•
1st cycle
2nd cycle
3rd cycle
142 + 75 cm
217 cm + 100 cm
317 cm + 100 cm
=
=
=
217 cm
317 cm
417 cm
If the subsidence rates reported by Hooijer et al., 2012 (Table 2) are to be accepted, then
today both Malaysia and Indonesia would have probably reduced the peat hectarage by more
than 50% and the carbon stock in our peats left would probably have declined considerably.
Thus, all this talk about Malaysia and Indonesian peats contributing to high amounts of CO 2
emissions are probably no longer true. The scenario of Hooijer et al. (2012) also does not take
into consideration current best management practices which many estates in the region are
practising. The addition of carbon from the oil palm fronds and EFB as part of this BMPs also
replaces a lot of carbon lost via decomposition of the peat per se. However some CO2 would
be contributed by the decomposition of these by-products. Plans are afoot to remove about
50% of this by-product biomass and to use them to extract valuable chemicals.
Table 2. Subsidence rate and equivalent oxidation (summarised from Hooijer et al., 2012).
Location
% of Subsidence
Equated to
Oxidation
Source
Everglades, Florida
78
Stephens and Speir (1969)
Sacremento Delta
68
Deverel and Rojstaczer
(1996)
Deverel and Leighton (2010)
61
Woosten et al., 1997
DID Malaysia, 1996
50-70
Murayama and Bakar (1996)
Volk, 1973
Stephens et al., 1984
Brady, 1997
Johor, Malaysia
Berg, 2000
Jambi, Indonesia
92 (O.P.)
Hooijer et al., 2012
75 (acacia)
Sensitivity:
77 (O.P.)
132
Conclusion
Subsidence
50 years – Sub-tropical peat.
Also confirmed by flux.
Measurements Neller (1944).
Lab measurements Volk (1973)
Flux and carbon balance
measurements 70 years.
Shallow peat.
Double the subsidence rate
increase in temperature.
Initial high rate due to labile
carbon which is finite. Once this
is gone, recalcitrant carbon are
more resistant to decomposition.
18 years after drainage.
6 years after drainage.
or 73 t CO2 eq ha-1 yr-1 – steady state.
or 132 t CO2 eq ha-1 yr-1 – 1st 5 years rather than 178 t CO2 eq
ha-1 yr-1
Jauhiainen et al. (2012) –
chamber method.
80 t ha-1 yr-1
Acacia  Forest
Jambi, Indonesia
Riau, Indonesia
Remarks
1st 5 years – 142 cm
After 5 years – 5 cm-1 year
14
Subsidence and Carbon Loss in drained tropical peatlands (Hooijer et al., 2012) – A Critical Review
CONCLUSION
Thus one has to conclude that the findings of Hooijer et al. (2012) should be viewed with
scepticism. Firstly, they are comparing oil palm and acacia which are different crops with
different management practices and life-cycles. As mentioned by them, the first 5 years are
critical. Their data on subsidence for this period was based mainly on elevation surveys carried
out in the acacia plantations after logging and drainage was done and after the crop was
planted. Field experience of such surveys suggest errors can be 1-2 m different as when
walking in peat swamp one seldom steps on the ground but on stumps or logs scattered on the
surface. Why did the acacia plantation conduct repeated elevation surveys after planting and
drainage? This was not explained. Therefore the data for the first five years which was critical
is actually questionable secondary data. The authors suggest that primary compaction is almost
equal to decomposition. Kool et al. (2006) who concluded that subsidence does not mean and
is never equal to decomposition. Their work in Central Kalimantan suggest that during
logging, a 2.2 to 4.0 m subsidence in the peat only results in a 2.0 to 46.9 cm of decomposition.
Hence the high subsidence rate of 142 cm recorded by Hooijer et al. (2012) was probably due
to logging, not oil palm.
Consequent to the above discussion, we are of the opinion that the data collected by
Hooijer et al. (2012) are questionable in today’s scenario and thus should be rejected. We are
therefore of the opinion that the data produced by Melling et al. (2013) and Herchoualc’h and
Verchot (2011) of around 40 t CO2 ha-1 yr-1 is probably closer to the truth.
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