Towards robust subsidence-based soil carbon emission factors for peat soils
in south-east Asia, with special reference to oil palm plantation – (Couwenberg
and Hooijer, 2013) – A critical review
S. Paramananthan
e-mail: passparam@gmail.com
INTRODUCTION
Organic soils or peat are distinguished from mineral soils by the presence of large
amounts of organic matter. In the lowland tropics, these organic soils are naturally found in
waterlogged depressions where these organic materials accumulate. Boreal and temperate
organic soils are formed by the accumulation of low growing plants such as moss, sphagnum.
On the other hand, tropical lowland organic soils are formed by the accumulation of leaves,
twigs and logs from tropical wetland forest. Consequently, tropical lowland organic deposits
contain much more organic carbon compared to temperate organic deposits. Tropical organic
deposits which can attain depths or more than 10 metres are thus globally recognized as carbon
sinks.
Globally total organic deposits are estimated to be about 400 Mha and tropical organic
deposits about 653,500 km2 or 50% (Joosten, 2009). Organic deposits in Southeast Asia
(Indonesia, Malaysia and Papua New Guinea) are estimated to occupy 24.8 Mha or 6 to 8% of
total global organic deposits (Joosten, 2009). Indonesia has about 20 Mha while Malaysia
about 2.7 Mha (Rieley et al., 1996). Organic deposits are an important ecosystem that are
estimated to store 48-52% organic carbon (Anderson, 1998). Turenen et al. (2002) estimated
that global organic deposits store 220-460 pg of carbon and can hence significantly influence
atmospheric CO2 concentrations (Hilbert et al., 2000).
Due to scarcity of good suitable land and increasing population, large areas of organic
soils or peatlands have been cleared for oil palm and pulpwood (mostly Acacia) plantations in
Southeast Asia. Current estimates of these industrial plantations are around 3.1 Mha but this
is predicted to double by 2020 (Miettinen et al., 2012). Both the United States and the
European Union require dependable emission estimates to determine whether palm oil
produced from oil palm grown on peatlands meets the sustainability criteria for renewable
biofuels. Thus Cowenberg and Hooijer (2013) report that reliable carbon emission factors for
drained tropical peat soils are needed to help countries such as Indonesia and Malaysia to
measure, report and verify (MRV) their greenhouse gas emissions from land use and land-use
change within the framework of climate mitigation programmers (Reducing Emissions from
Deforestation and Forest Degrading, REDD+; and take Nationally Appropriate Mitigation
Actions (NAMAs). Unfortunately reliable emissions data on Greenhouse Gases (GHGs)
currently available are unreliable, highly variable and to a large extent questionable. Joosten
and Cowenberg (2009) Page et al. (2011), Cowenberg and Hooijer (2013) report that deriving
emission factors from tropical peatlands converted to plantations is a scientific challenge.
Direct measurements of gas fluxes are expensive, complicated and have technical constraints.
Consequently they add that very few scientifically sound and indisputable greenhouse gas flux
measurements are currently available. Detailed ‘Tier 3’ (i.e. advanced) model-based
approaches (IPCC 2006) are not yet feasible due to lack of accurate data for all relevant
ecosystem fluxes in and out of peat soils.
1
In an effort to partly bridge this knowledge gap, two papers have been published recently:
•
Hooijer, A., Page., S., Jauhiainen, J., Lee, W.A., Lu, X.X. Idris, Arif Anshari, G., 2012.
Subsidence and Carbon loss in drained tropical peatlands. Biogiosciences, 9, 1053-1071.
•
Cowenberg, J & Hooijer, A., 2013. Towards robust subsidence based soil carbon,
emission factors for peat soils in south-east Asia with respect to oil palm plantations.
Mires and Peat 12:1-13.
The objective of this paper is to provide a critical review of Cowenberg and Hooijer
(2013). The review on Hooijer et al., 2012 has been prepared earlier.
METHODOLOGY/MESUAREMENTS USED BY COWENBERG AND HOOIJER
2013
The study sites used by Cowenberg and Hooijer (2013) are located in two oil palm
plantations and a group of Acacia plantations in Jambi and Riau Provinces in Sumatera,
Indonesia (Table 1). These sites are located on part of large peat domes. The two oil palm
sites were drained 4-7 (OP5 sites) and 15-20 (19OP site) years and located near the Berbak
National Park, Jambi Province. The Acacia plantations were located in the Kampar Peninsula
and were drained for 3-8 years (6A site). All sites had peat depths >6 m. Fire was used in the
land clearance in the oil palm site but not in the Acacia site. The Acacia site (6A) and part of
the oil palm site 19OP were presented in Hooijer et al. (2012).
Table 1. Study sites information used by Cowenberg and Hooijer (2013).
Site
Province
Indonesia
6A
Riau
5OP
Jambi
19OP
Jambi
GPS
Coordinates
Years since
drainage
(range)
0°35’42”N
102°20’2.4”E
1°40’48”S
103°49’40.8”E
1°42’32.4”S
103°53’52.8”E
6
(3-8)
5
(4-7)
19
(15-20)
Peat
Thickness
(m)
mean ISD
No. of
Subsidence
Poles
No. of Soil
Pits
9.0±2.6
125
19
6.3±0.7
17
8
7.7±0.7
34
10
Table 2. Frequency of Subsidence and Bulk Density Measurements.
Site
Commenced
(Period)
Frequency of Measurements
W.T. and B.D.
Acacia (6A)
September 2007 (3 years)
Monthly
Oil palm (5OP)
June 2009 (3 years)
Fortnightly
Oil palm (19OP)
March 2009 (3 years)
Fortnightly
Subsidence rates and water table depths were measured fortnightly for the oil palm sites
monthly for Acacia sites for three years (Table 2). For the subsidence and water table
measurements subsidence monitoring poles and perforated PVC poles were used respectively.
For the bulk density measurements soil pits were used. The water was pumped out and the soil
samples collected using metal cylinders 8 cm diameter and length. Samples were collected
horizontally even dried at 105°C and once constant weight was obtained, the dry bulk density
(DRB) was determined. In the oil palm plantations, the peat was sampled to at least 1.5 m
depth at 0.1 or 0.2 m intervals. In the Acacia Plantation this was done to 1.2 m at 0.15-0.3 m
2
intervals. Three replicates were at 0.1 m apart were collected. The carbon loss was calculated
from the subsidence rates and peat dry bulk density below the water table following the method
developed by van den Akker (in Kuikman et al., 2005) and recently applied in the Netherlands
and Switzerland by van den Akker et al. (2008) and Leifeld et al. (2011). The basic assumption
is that, after the end of the consolidation phase which follows immediately after drainage,
compaction and oxidation above the water table are the only causes of surface height loss. In
contrast to other methods, this approach does not require estimation of the relative
contributions of these two processes to subsidence. Under unchanged land use with associated
water tables maintained by regulation (water management practices) and periodic deepening
of canals, processes in the upper ‘oxic’ peat layer may be assumed to be in a steady state.
As compaction and oxidation affect the upper layer, leading to subsidence and
progressive elevation of the watertable relative to the peat surface, regular lowering of the
watertable (by regulation of canal watertables or deepening of drainage canals) transfers the
undecomposed peat from the anoxic layer below the watertable to the upper oxic layer. This
process takes place continuously at a steady rate and the thickness of the oxic layer remains
constant. As the processes of peat subsidence and adjustment of watertables continue, undecomposed peat is progressively incorporated into the oxic layer, becoming more and more
oxidized towards the surface. The result is a defacto constant peat profile in the upper layer,
with the peat going from a relatively un-decomposed condition just above the watertable to a
strongly decomposed (oxidized) state at the surface. Because of this dynamic equilibrium the
upper oxic peat layer need not be considered when deriving carbon losses from the peat column,
even though it is here that actual losses take place. Instead, peat (and carbon) losses can be
calculated from only two variable: surface height loss and characteristics of the low peat layer.
𝑉𝑜𝑥 = 𝑆𝑡 × 𝐷𝐵𝐷 ······································ (1)
Vox
= annual peat loss (km-2 y-1)
St
= Total surface height loss (my-1)
DBD = Dry bulk density of the peat layer below the watertable (kg m-3)
𝐶𝑙𝑜𝑠𝑠 = 𝑆𝑡 × 𝐶𝑣𝑜𝑙 = 𝑆𝑡 × 𝐷𝑅𝐵 × 𝐶𝑑𝑤 ············· (2)
The amount of carbon lost, Closs (kg m-2 y-1) is calculated by multiplying surface height
loss St by the volumetric carbon density of the peat below the watertable, Cvol (kg m-3) Equation
2. Volumetric carbon density is the product of the dry bulk density and the carbon
concentration in the peat on the dry weight basis Cdw (kg kg-1)
i.e.
𝐶𝑙𝑜𝑠𝑠 = 𝑉𝑜𝑥 × %𝐶 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑜𝑖𝑙
Note: Volumetric carbon density C (g cm-3)
𝐶𝑔𝑐𝑚−3 =
%𝐶
× 𝐷𝐵𝐷
100
Closs: 𝐶𝑎𝑟𝑏𝑜𝑛 𝑙𝑜𝑠𝑠 (𝐶𝑙𝑜𝑠𝑠 ) = 𝑆𝑢𝑠𝑖𝑑𝑒𝑛𝑐𝑒 𝑅𝑎𝑡𝑒 × 𝑉𝑜𝑙. 𝐶𝑎𝑟𝑏𝑜𝑛 𝑔 𝑐𝑚−3 × 10,000
(𝑀𝑔 ℎ𝑎−1 ) = 𝑚𝑦 −1
3
Example
C content of Peat = 60%
DBD
= 0.07 g cm-3
Subsidence Rate = 2.7 cm = 0.027 my-1
Volumetric carbon (C𝑑𝑤 ) =
𝑀𝑔 ℎ𝑎
−1
𝑦
−1
%𝐶
60
× 𝐷𝐵𝐷 =
× 0.07 = 0.6 × 0.07 = 0.042 g cm−3
100
100
𝐶𝑙𝑜𝑠𝑠 = 𝑆𝑡 × 𝑉𝑜𝑙. 𝐶𝑑𝑤
= 0.027 × 0.042 × 10.000 = 11.34 𝑀𝑔 ℎ𝑎−1 𝑦 −1
Monitoring of peat subsidence has long been used to assess carbon losses caused by
microbial oxidation of drained peat soils (Armentano & Menges, 1986; Kasimir – Klemedtsson
et al., 1997; Oleszcuk et al., 2008). However besides oxidation, other processes that do not
result in carbon loss also contribute to peat height loss. Therefore we need to know the relative
contribution of these different process if carbon losses are to be qualified reliably.
Unfortunately such data and hence reliable emission estimates especially on tropical peatlands
have been rare.
The factors which contribute to GHG emissions from organic soils include factors such
as:
•
Site location – boreal, temperate, tropical
•
Climate factor – temperature and rainfall amount and monthly distribution and annual
variation
•
Site characteristics –
–
–
peat maturity (sapric, hemic, fibric), presence/absence of wood
and nature of the wood
Depth and intensity of the drainage
Agronomic and other cultivation processes
METHODOLOGY USED
Cowenberg and Hooijer (2013) in their study adopted a modified and simpler method
proposed by van den Akker (in Kuikman et al., 2005). Compared to that used in the past for
sub-tropical peats (Stephens and Spier, 1969) and tropical peats (Driessen & Soepraptoharjo,
1974; Wösten et al., 1997; Hooijer et al., 2012). In warm and temperate climates (Van der
Molen & Smits, 1962; Schothorst, 1997, 1982; Ewing & Vepraskas, 2006). Most of the older
methods that used the subsidence approach did not separate the relative contributions of
compaction and oxidation. Thus applied the simpler van den Akker method which they claim
can determine the total net contribution to carbon loss by subsidence records that is applicable
to steady state conditions under continuous land use. This method has been also applied in
Netherlands and Switzerland (van den Akker et al., 2008; Leifeld et al., 2011). The basic
assumption used in this approach is that after the end of the consolidation phase (5 years) which
follows immediately after drainage, compaction and oxidation above the watertable are the
only causes of surface height loss. Therefore in contrast to other methods, this approach does
not require estimation of the relative contributions of these two processes to subsidence. As
compaction and oxidation affect the upper ‘oxic’ layer leading to subsidence and progressive
elevation of the watertable relative to the peat surface. Therefore regular lowering of the
4
watertable by deepening the canals transfers part of the undecomposed ‘anoxic’ layer below
the watertable to above the new watertable and to the ‘oxic’ layer. This process takes place
continuously at a steady rate and the thickness of the ‘oxic’ layer remains constant. Thus the
total carbon loss can be calculated by multiplying surface height loss by the volumetric carbon
density of the peat below the watertable. Volumetric carbon density is the product of the dry
bulk density and the carbon concentration in the peat on a dry weight basis.
Based on Cowenberg’s and Hooijer’s (2013) data and that by other workers a compilation
has been made using the some assumption of carbon density of (0.042) to determine the C
emissions (see Table 3). These calculation indicate a range of C emissions. Kool et al. (2006)
reported that logging in Central Kalimantan resulted in a height loss of 2.2 m to 4.0 m but that
actual decomposition was only 2-47 cm. Such high rates of height loss are common during
logging and possible the reason why Hooijer et al. (2012) obtained a subsidence of 142 cm in
the first-five years in Riau and Jambi under both acacia and oil palm. Oil palm or acacia are
hardly ever planted directly from forest. The forest is first logged often without any water
control and hence large subsidence rates are common. Hooijer et al. (2012) probably
misinterpreted their data to state that the 142 cm was after oil palm was planted. Cowenberg
and Hooijer (2013) data suggests that even the rate of 5 cm y-1 subsidence is too high.
Cowenberg and Hooijer (2012) only report a subsidence of 3.7 to 5.0 cm. Thus we have to
conclude that both Hooijer et al. (2012) and Cowenberg and Hooijer (2013) tend to
overestimate their subsidence rates and the C emissions. The wide range in subsidence values
are due to differences in the nature of the peat – maturity and presence/absence of wood. The
failure of both these studies to characterize their peat profiles in detail means that their values
are not accurate and hence are questionable.
REFERENCES
Andriesse, J.P., 1988. Nature and Management of Tropical Peat Soils. FAO Bulletin, vol. 59.
Food and Agriculture Organization of the United Nations, Rome.
Armentano, T.V. and Menges, E.S., 1986. Patterns of change in carbon balance of organic
soil – wetlands of the temperate zone. Journal of Ecology 74, 755-774.
Cowenberg and Hooijer, 2013. Towards robust subsidence-based soil carbon emission
factors for peat soils in south-east Asia with special reference to oil palm plantations.
Mires and Peat 12, 1-13.
Driessen, P.M. and Soepraptohardjo, M., 1974. Soils for Agricultural Expansion in Indonesia.
Bulletin 1 Soil Research Institute, Bogor, Indonesia, 63 pp.
Etik Puji Handayani, Meine van Noordwijk, Kamarudin Idris, Supiandi Sabiham Sri
Djuniwati, 2010. J. Trop. Soils, Vol. 15 No. 3 2010, 255-260.
Ewing, J.M. and Vepraskas, M.J., 2006. Estimating primary and secondary subsidence in an
organic soil 15, 20, and 30 years after drainage. Wetlands 26, 119-130.
Hasnol Othman, Ahmad Tarmizi Mohamad, Farawahida Mohamad Darus, Mohd. Hanif
Harun and Muhammad Pilus Zambri, 2011. Journal of Oil Palm Research, Vol. 23,
1078-1086.
Hilbert, D.W., Roulet, N.T. and Moore, T., 2000. Modelling and analysis of peatlands as
dynamical systems. J. Ecol. 88, 230-242.
5
Table 3. Long term stable subsidence rates.
Deverel and Leighton
(2010)
Years since
drainage
(measuring
duration)
>50
(na)
>50
(na)
>50
(na)
Andriesse (1988)
-
DID and LAWOOD
(1996)
>12
(17-21)
Wösten et al. (1997)
14-28
Johor Barat
Peninsular, Malaysia
Phase I
Phase II
Johor, Malaysia
>2
>15
Source
Stephens and Spier
(1969)
Stephens et al., 1984
DID, Sarawak,
Malaysia 2001
Mohamad Ahman
Tharmizi et al. 2009
(PIPOC 2009)
Maswar (2011)
Subsidence Rate
(cmy-1)
Volumetric C
Density (gcm-3)
(assumed)
Calculated C
Loss
(t ha-1 y-1)
Remarks
Everglades, Florida, USA
3
(0.042)
12.6
Sub-tropical peat
Everglades, Florida, USA
8
(0.042)
33.6
Recalculated for
Tropical Conditions
Sacremento, California,
USA
7.5
(0.042)
31.5
Southeast Asia
6
(0.042)
25.2
Estimate of stable longterm subsidence
0.5
0.5
2.7
3.8
0.042
0.042
12.0 (11.34)
16.0 (15.96)
Carbon content 60%
na
4.6
(0.042)
19.32
-
Sarawak, Malaysia
0.6
5
(0.042)
21.0
Estimates
Peninsular Malaysia
0.4
4.3
(0.042)
18.1
Location
Watertable Depth
(m)
>15
Indonesia
0.6
5.0
0.041
20.4 (20.5)
Hasnol Othman et al.
2011
10
(7-8)
Sarawak, Malaysia
Moderately deep (100-150
cm)
Very deep (>300cm)
0.3-0.5
(0.4)
0.3-0.5
(0.4)
7.17
8.44
0.034
0.022
24.38
18.57
Hooijer et al. 2012
Acacia
Oil Palm
Oil Palm
3-8 (2)
4-7 (3)
15-20 (3)
Riau, Sumatera
Jambi, Sumatera
Jambi, Sumatera
0.7
0.7
0.7
5.0
5.0
5.0
0.041
0.045
0.043
20.5
22.5
21.5
Cowenberg & Hooijer
2013
Acacia
Oil Palm
Oil Palm
3-8 (2)
4-7 (3)
15-20 (3)
Riau, Sumatera
Jambi, Sumatera
Jambi, Sumatera
0.7
0.56*
0.56*
5.0
3.9**
3.7**
0.041
0.045
0.043
20.3 (20.50)
17.6 (17.55)
15.9 (15.91)
Assume C
concentration of 55%
*Watertable depths
change?
**Subsidence rate
changed
Kool et al., 2006
Logging
Central Kalimantan
Not known not
controlled
Height loss:
2.2 m → 4.0 m
Decomposition:
2-47 cm
(0.041)
8.2 to 192.7
Not yet planted with oil
palm
Carbon Loss = Subsidence x Carbon Density x 10,000
t ha-1 y-1
6
PhD Thesis
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Subsidence and carbon loss in drained tropical peatlands. Biogeosciences, 9, 10531071.
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7
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8