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Subsidence and carbon loss in drained tropical peatlands

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Conversion of tropical peatlands to agriculture leads to a release of carbon from previously stable, long-term storage, resulting in land subsidence that can be a surrogate measure of CO2 emissions to the atmosphere. We present an analysis of recent large-scale subsidence monitoring studies in Acacia and oil palm plantations on peatland in SE Asia, and compare the findings with previous studies. Subsidence in the first 5 yr after drainage was found to be 142 cm, of which 75 cm occurred in the first year. After 5 yr, the subsidence rate in both plantation types, at average water table depths of 0.7 m, remained constant at around 5 cm yr-1. The results confirm that primary consolidation contributed substantially to total subsidence only in the first year after drainage, that secondary consolidation was negligible, and that the amount of compaction was also much reduced within 5 yr. Over 5 yr after drainage, 75 % of cumulative subsidence was caused by peat oxidation, and after 18 yr this was 92 %. The average rate of carbon loss over the first 5 yr was 178 t CO2eq ha-1 yr-1, which reduced to 73 t CO2eq ha-1 yr-1 over subsequent years, potentially resulting in an average loss of 100 t CO2eq ha-1 yr-1 over 25 yr. Part of the observed range in subsidence and carbon loss values is explained by differences in water table depth, but vegetation cover and other factors such as addition of fertilizers also influence peat oxidation. A relationship with groundwater table depth shows that subsidence and carbon loss are still considerable even at the highest water levels theoretically possible in plantations. This implies that improved plantation water management will reduce these impacts by 20 % at most, relative to current conditions, and that high rates of carbon loss and land subsidence are inevitable consequences of conversion of forested tropical peatlands to other land uses.
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Biogeosciences, 9, 1053–1071, 2012
www.biogeosciences.net/9/1053/2012/
doi:10.5194/bg-9-1053-2012
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Biogeosciences
Subsidence and carbon loss in drained tropical peatlands
A. Hooijer
1
, S. Page
2
, J. Jauhiainen
3
, W. A. Lee
4
, X. X. Lu
5
, A. Idris
6
, and G. Anshari
7
1
Deltares, P.O. Box 177, Delft 2600 MH, The Netherlands
2
Department of Geography, University of Leicester, UK
3
Department of Forest Sciences, University of Helsinki, Finland
4
Singapore Delft Water Alliance, National University of Singapore, Singapore
5
Department of Geography, National University of Singapore, Singapore
6
University of Jambi, Jambi, Indonesia
7
Center for Wetlands People and Biodiversity, Tanjungpura University, Pontianak, Indonesia
Correspondence to: A. Hooijer (aljosja.hooijer@deltares.nl)
Received: 4 July 2011 – Published in Biogeosciences Discuss.: 15 September 2011
Revised: 6 February 2012 – Accepted: 9 February 2012 – Published: 20 March 2012
Abstract. Conversion of tropical peatlands to agriculture
leads to a release of carbon from previously stable, long-
term storage, resulting in land subsidence that can be a sur-
rogate measure of CO
2
emissions to the atmosphere. We
present an analysis of recent large-scale subsidence moni-
toring studies in Acacia and oil palm plantations on peatland
in SE Asia, and compare the findings with previous stud-
ies. Subsidence in the first 5yr after drainage was found to
be 142cm, of which 75 cm occurred in the first year. Af-
ter 5 yr, the subsidence rate in both plantation types, at aver-
age water table depths of 0.7m, remained constant at around
5cmyr
1
. The results confirm that primary consolidation
contributed substantially to total subsidence only in the first
year after drainage, that secondary consolidation was neg-
ligible, and that the amount of compaction was also much
reduced within 5yr. Over 5yr after drainage, 75% of cu-
mulative subsidence was caused by peat oxidation, and af-
ter 18yr this was 92 %. The average rate of carbon loss
over the first 5yr was 178t CO
2eq
ha
1
yr
1
, which reduced
to 73t CO
2eq
ha
1
yr
1
over subsequent years, potentially
resulting in an average loss of 100t CO
2eq
ha
1
yr
1
over
25yr. Part of the observed range in subsidence and carbon
loss values is explained by differences in water table depth,
but vegetation cover and other factors such as addition of fer-
tilizers also influence peat oxidation. A relationship with
groundwater table depth shows that subsidence and carbon
loss are still considerable even at the highest water levels
theoretically possible in plantations. This implies that im-
proved plantation water management will reduce these im-
pacts by 20% at most, relative to current conditions, and that
high rates of carbon loss and land subsidence are inevitable
consequences of conversion of forested tropical peatlands to
other land uses.
1 Introduction
More than half (24.8Mha) of the global area of tropical peat-
land is in SE Asia (56%), mostly in Indonesia and Malaysia.
Owing to the considerable thickness (mean >5 m) of the
peat in these two countries they contain 77% of the entire
tropical peat carbon store (Page et al., 2011). In Peninsu-
lar Malaysia and the islands of Sumatra and Borneo, some
60% of peat swamps were partly or completely deforested
by 2007, usually accompanied by some form of drainage,
and only 10% remained in pristine condition (Miettinen and
Liew, 2010). Some 5Mha were under agricultural use in
2007, of which 45% (2.3Mha) was large-scale oil palm and
pulpwood (Acacia) plantations. A recent inventory by Miet-
tinen et al. (2012) shows that the area of large-scale indus-
trial plantations had increased to 3.15Mha by 2010 (12% in-
crease per yr since 2007; excluding smallholder plantations
and other forms of land conversion), and that this high ex-
pansion rate is likely to continue unless the implementation
of land use planning policies is changed.
Published by Copernicus Publications on behalf of the European Geosciences Union.
1054 A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands
It has long been known that drainage of peatlands
causes irreversible lowering of the surface (subsidence) as a
consequence of peat shrinkage and biological oxidation, with
the latter resulting in a loss of carbon stock. In peatland ar-
eas as different as the Fenlands of the UK, the Netherlands,
Venice Lagoon in Italy, the Everglades and SacramentoDelta
in the United States and Lake Hula in Israel, a total subsi-
dence of 200 to 600 cm occurred over 40 to 130yr, bringing
surface levels close to or below sea level (Schothorst, 1977;
Hutchinson, 1980; Stephens et al., 1984; Hambright and Zo-
hary, 1998; Gambolati et al., 2003; Deverel and Leighton,
2010). In all of these cases, peat oxidation is reported to be
the main cause of subsidence. In recent years, rapidly in-
creasing peat carbon losses from drained SE Asian peatlands
have been found to contribute substantially to global green-
house gas emissions (Hooijer et al., 2006, 2010; Couwenberg
et al., 2010; Murdiyarsoet al., 2010). Estimates of net carbon
losses and resultant CO
2
emissions from peatland drained for
agriculture range from < 40t CO
2
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 water table depths around 0.7m
(applying relations between water table depth and emission
as proposed by DID Sarawak, 2001; Hooijer et al., 2006,
2010; Couwenberg et al., 2010), excluding forest biomass
and fire losses.
The uncertainty in the rate of carbon emission from
drained tropical peatland is caused partly by the reliance on
measurements of gaseous CO
2
emissions that are difficult to
conduct and interpret. Unless CO
2
emission studies are car-
ried out on a large scale (i.e. a large number of measure-
ments conducted over a long time period at a large num-
ber of monitoring locations) and over a range of environ-
mental conditions (in terms of water table, vegetation cover
and temperature), data uncertainty is considerable (Couwen-
berg et al., 2010; Murdiyarso et al., 2010; Jauhiainen et al.,
2012). For example, widely quoted estimates of CO
2
emis-
sion from oil palm plantations on peat have been based on
fewer than 50 observations, including replicates, at single lo-
cations (Murayama and Bakar, 1996; Melling et al., 2005).
Moreover, few studies have estimated net CO
2
emissions re-
sulting 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 peatland
in drainage water (Alkhatib et al., 2007; Baum et al., 2007;
Moore et al., 2010). Recent effortsto calculate the net change
in peat carbon stock from the difference between all esti-
mated fluxes into and out of the peat, including changes in
biomass (Herchoualc’h and Verchot, 2011), have been incon-
clusive because of the limited data available and the cumula-
tive uncertainties associated with each component.
There is a need, therefore, for a simple and reliable ap-
proach to determining net carbon losses from drained tropical
peatlands, especially in view of the urgent requirement for
land use planning policies that reduce CO
2
emissions from
SE Asian peatlands, which form a substantial part of global
emissions (Hooijer et al., 2006, 2010; Malhi, 2010). Mea-
surements 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 peat carbon stock are
integrated over time without requiring instantaneous mea-
surements, thereby providing a more accurate value for to-
tal carbon loss even if the individual loss components (CO
2
,
CH
4
, DOC and POC) can not be separated using this method
alone. The use of this approach on SE Asian peatlands has
been hampered by a scarcity of reliablelong-term subsidence
data and adequate information on bulk density and carbon
content of the peat (cf. review by Couwenberg et al., 2010).
When determining carbon loss from subsidence data, it is
necessary to know the extent to which subsidence is the re-
sult of peat oxidation compared to physical volume reduc-
tion. The following subsidence components need to be sepa-
rated:
Oxidation: decomposition of peat in the aerated zone
above the water table owing to breakdown of organic
matter, resulting in carbon loss through release of
gaseous CO
2
to the atmosphere (Neller et al., 1944;
Jauhiainen et al., 2005, 2008; Hirano et al., 2009), and
removal as DOC and POC in drainage water (Alkhatib
et al., 2007; Baum etal., 2007; Moore et al., 2010). This
process, acting alone, does not increase bulk density of
the peat and could in fact decrease it.
Compaction and shrinkage: volume reduction of peat in
the aerated zone above the water table. Compaction re-
sults from the pressure applied on the peat surface by
heavy equipment; shrinkage occurs through contraction
of organic fibres when drying. These two processes can
often not be separated in practice and they are consid-
ered together as “compaction” in this paper. Both pro-
cesses lead to an increase in peat bulk density.
Consolidation: the compression of saturated peat below
the water table owing to loss of buoyancy of the top
peat, increasing strain on the peat below. Primary con-
solidation is caused by loss of water from pores in the
peat; it occurs rapidly when groundwater is removed
fast, especially where a dense drainage system is im-
plemented in peat of high permeability. Secondary con-
solidation is a function of the resistance of the solid peat
material itself to compression; this is a slow process that
makes up only a small fraction of total consolidation
(Berry, 1983; Mesri and Aljouni, 2007). Both processes
increase peat bulk density.
Fire and erosion of peat particles by water flow both remove
peat from near the surface. Their impact on the bulk density
of the remaining peat is unknown.
Biogeosciences, 9, 1053–1071, 2012 www.biogeosciences.net/9/1053/2012/
A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands 1055
In this study, we investigated in detail the parameters in-
volved in subsidence and carbon loss from tropical peatlands,
aiming to reduce uncertainties by better quantifying the ox-
idation component. Our main objectives were to determine
(1) the rate of peat subsidence and changes over time, (2) the
contribution of oxidation to subsidence, (3) carbon loss from
peat oxidation, and (4) the relationship between carbon loss
and environmental factors especially water table depth and
land cover. We were able to carry out a much larger number
of subsidence measurements (at over 200 locations) than ear-
lier reported studies (at less than 30locations), acrossa wider
range of conditions, and compared the results with findings
on subsidence and carbon loss from previous studies of trop-
ical and sub-tropical peatlands, and with CO
2
emissions that
were measured in parallel on the same sites (Jauhiainen et
al., 2012).
2 Methods
2.1 Measurement locations, climate and land
conversion history
The monitoring sites were located on large peat domes in In-
donesia, in Acacia tree (for paper production) plantations and
adjacent natural forest in the province of Riau and in mature
oil palm plantations (for palm oil production) in the province
of Jambi. All sites experienced similar climatic conditions,
with an average long-term annual rainfall of around 2500 mm
and a mean annual air temperature just below 30
C. During
the study period (2007–2010), dry season rainfall was some-
what above the long-term average in the years 2007, 2008
and 2010, with rainfall deficits (broadly defined as rainfall
below 100mmmonth
1
; Vernimmen et al., 2012) only oc-
curring in one or two months at any location. The rainfall
regime in 2009 was about average.
All plantation sites are drained by a network of canals, 5
to 8m wide, over 3m deep and spaced 500 to 800m apart, to
lower the groundwater table to a level suitable for growth of
the plantation crops. Fire was used for land clearance prior to
planting oil palms but not in the Acacia plantations. Except
for 14 locations where monitoring started 1yr after drainage,
in 2002, monitoring in Acacia plantations started 3 to 8yr af-
ter drainage; the average period after drainage over the study
period was 6 yr. Acacia crop rotations at study locations
over the measurement period varied mostly from immature
to mature, with open conditions occurring in a minority of
locations where harvesting took place. In such cases, care
was taken not to disturb the immediate surrounding of subsi-
dence poles. Locations where disturbance was evident from
the record were excluded from analyses. Study locations in
oil palm plantation were drained for 14 to 19 yr when subsi-
dence monitoring started, with an average of 18 yr over the
study period, and nearly all locations had mature oil palm
cover.
Monitoring locations were established along 16 tran-
sects between 0.5 and 12km long, located perpendicular to
drainage canals and covering a wide range of peat thickness
and water table depths (Table 1, Fig. 1). Distances between
monitoring locations varied from 50 to 400 m depending on
site conditions. Transectsin Acacia plantation were extended
2km into adjacent peat swamp forest where this still re-
mained. Data used in this analysis were obtained from a total
of 218 monitoring locations (125 in Acacia plantation, 42 in
oil palm plantation and 51 in peat swamp forest adjacent to
Acacia plantation).
2.2 Peat surface subsidence measurements
At all study locations, the peat surface level was measured
using 5cm diameter, perforated PVC tubes as subsidence
poles, inserted vertically through the peat and anchored
firmly to at least 0.5 m in the underlying mineral substrate.
To minimize measurement error, permanent markers made
of light thin metal or wood were placed on the peat surface.
Compression of the surface peat was avoided by ensuring
field staff did not step within a 0.5m radius around pole lo-
cations, and only on planks during installation. As a further
precaution the initial period after installation, from 3 months
to more than a year(as much asthe data series length allowed
while still maintaining a full 2yr record), was excluded from
analyses. Peat subsidence was monitored at intervals of 1 to
3 months in the Acacia plantation. A full 2yr data series for
use in analyses was selected for each transect from the 3yr
data collection period of September 2007 to August 2010 on
the basis of data availability (some transects were periodi-
cally inaccessible due to external factors), with all records
overlapping by at least 1 yr. Included in the 125 study loca-
tions in Acacia plantation were the 14 subsidence poles that
had been installed one year after drainage, providing data for
a longer period (2002–2010).
For the oil palm plantation, a one-year data series (July
2009 to June 2010), monitored at 2-weekly intervals, was
available for analysis. Cumulative subsidence at all locations
in both Acacia and oil palm plantations was recalculated to
annual mean values that allowed comparison.
As no monitoring data from subsidence poles were avail-
able for the first year after drainage, we used company
records of repeated elevation surveys along transects across
Acacia plantations, before and approximately 1yr after
drainage, for this period.
2.3 Water table depth measurements
The depth of the water table below the peat surface was mon-
itored in the perforated PVC subsidence tubes at 2-weekly
to 3-monthly intervals at the same 218 locations as subsi-
dence. Average water table depths were calculated from
records over the same period as the subsidence records used
in analyses.
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1056 A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands
Table 1. Summary of measurement site characteristics. Averages are provided with standard deviations.
Acacia plantation Oil palm plantation Plantation mean Drained forest
2
6yr after drainage 18yr after drainage 6–18yr after drainage
Site location (Lat/Long) 102.334/0.595 103.601/1.566 102.334/0.595
Number of measurement locations 125 42 167 51
Peat depth m 9± 2.6 7.7± 1.4 8.4 9.9± 3.2
Peat bulk density of top 1m gcm
3
0.089± 0.018 0.087± 0.018 0.088
Peat bulk density> 1m depth
1
gcm
3
0.073± 0.015 0.078± 0.007 0.075
Water table depth m 0.7± 0.2 0.73± 0.23 0.71 0.33± 0.16
1
Measured at 1 to 2.3m depth; affected by initial consolidation.
2
Natural forest strip up to 2km from plantation boundary; affected by drainage.
39
1
FIGURES 2
3
4
5
6
7
8
9
Fig. 1. Cross section along typical study transect in Sumatra, 6 years after drainage, showing 10
variation in peat depth, average water level, land use and monitoring location density. 11
-6
-4
-2
0
2
4
6
8
10
12
14
0 2000 4000 6000 8000 10000 12000
Distance from start transect (m)
Elevation above mean Sea level (m
)
Surface elevation 2007
Monitoring points
Average water level
Bottom of peat
Mineral soil (clay loam)
Peat
ForestPlantation
(
acacia
)
Fig. 1. Cross section along typical study transect in Sumatra, 6 yr after drainage, showing variation in peat depth, average water level, land
use and monitoring location density.
2.4 Peat characteristics
Peat thickness and type (fibric, hemic or sapric) were deter-
mined at the time of pole installation using locally produced
augers and visual interpretation.
Bulk density (BD) was determined at 22 locations in the
Acacia plantation, 19 that were drained 4 to 7yr before and 3
about two years after drainage, and at 10 locations in the oil
palm plantation. Peat samples were collected from the sides
of pits excavated in peat, using sharpened steel cylinders to
avoid peat compression that may result from using a vertical
corer. This was done quickly after pit construction, to avoid
deformation or drying of the peat. To further avoid com-
pression and to ensure inclusion of smaller wood remains,
relatively large cylinders of 8 cm diameter and 8cm length
(402cm
3
) were used. All pits were at least 1 m away from
trees or palms, where the presence of tree roots was found to
be minimal. Water was pumped from deeper pits to facilitate
sampling. In the Acacia plantation, pits of 1 m diameter were
up to 1.2m deep and three replicate samples were taken at in-
tervals of 0.15 to 0.3m starting at 0.075m below the surface.
In the oil palm plantation, pits were 2 to 2.5 m deep and sam-
ples were collected at intervals of 0.1 m, commencing 0.1 m
below the surface (Fig. 2). A total of 1201 peat samples were
oven dried at 105
C for up to 96h (as long as necessary to
ensure that dry weight of samples had fully stabilized) to re-
move moisture, and weighed to calculate BD.
Large wood remains could not be collected in the cylin-
ders, so additional checks were carried out to determine if un-
dersampling of such remains would affect the BD values. A
total of 20 samples of partly decomposed woodtaken in 3 soil
pits from peat below 1 m depth in oil palm plantations, with
an average volume (± SD) of 326 ± 104 cm
3
, were dried and
weighed following the same protocol as cylinder samples. In
addition, the wood content of the peat in oil palm plantation
sites was assessed visually on 10 cleaned pit sides, through
detailed descriptions of peat surfaces of 0.1 by 0.3m at 0.1m
depth intervals, prior to sampling.
Ash content in 223 subsamples from Acacia plantation
sites was determined by loss on ignition in a muffle furnace.
Biogeosciences, 9, 1053–1071, 2012 www.biogeosciences.net/9/1053/2012/
A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands 1057
40
1
Fig. 2. Peat profile in an oil palm plantation, 18 years after drainage. The top peat is amorphous 2
without visible plant remains, indicating advanced decomposition, while the peat below 0.3 m is 3
increasingly fibrous going downwards, with abundant remains of wood and roots being evident below 4
0.5 m. 5
Fig. 2. Peat profile in an oil palm plantation, 18 yr after drainage.
The top peat is amorphous without visible plant remains, indicating
advanced decomposition, while the peat below 0.3 m is increasingly
fibric going downwards, with abundant remains of wood and roots.
2.5 Determining the compaction component of
subsidence
The contribution of compaction (including shrinkage) and
oxidation to subsidence was calculated by determining the
net increase in BD of the peat above the water table caused
by compaction, and the total amount of subsidence in that pe-
riod (e.g. Stephens and Speir, 1969; Schothorst, 1972; Ewing
and Vepraskas, 2006; Leifeld et al., 2011). We used a varia-
tion modified from Driessen and Soepraptohardjo (1974), as
follows:
V
ox
=
(
(V
1
× BD
1
) (V
rest
× BD
2
)
)
/BD
1
and:
V
comp
= V
rest
× (BD
2
BD
1
)/BD
1
and:
P
ox
= V
ox
× BD
1
/
(V
ox
× BD
1
) + (V
comp
× BD
1
)
where:
V
ox
=peat volume loss due to oxidation (cm
3
),
V
comp
=volume loss due to compaction (cm
3
),
V
rest
=peat volume after subsidence, above deepest
groundwater level (cm
3
),
V
1
=peat volume before subsidence, above deepest
groundwater level (cm
3
),
BD
1
=original bulk density above deepest groundwater
level (g cm
3
),
BD
2
=new bulk density above deepest groundwater
level, after subsidence (g cm
3
),
P
ox
=percentage of subsidence caused by oxidation.
This method may be demonstrated by taking two extreme
conditions as examples: if the height and volume of a peat
column above the water table is reduced by 50 %, and the BD
of the peat has doubled over the same period, all subsidence
can be explained by compaction alone. If on the other hand
no change in BD is observed, it may be concluded that there
has been no compaction and all subsidence is fully explained
by oxidation (assuming consolidation can be excluded). In
practice, compaction and oxidation usually both explain a
part of total subsidence.
The method ideally requires data on peat BD at the start
and end of a subsidence record of many years, at the same
site, or data from a nearby undrained reference site. How-
ever pre-drainage BD data were not available for the study
sites, and truly intact forest areas on deep peat were not ac-
cessible close to the study areas. Therefore, two alternative
approaches were followed. First, it was assessed whether
the current BD of the peat below the lowest average water
table depth, which is about 1 m for our sampling locations,
could be considered representative for the BD of the upper
peat layer at the start of drainage, i.e. for the original peat
before subsidence started. Secondly, BD profiles above the
water table in the Acacia and oil palm plantations, at 2, 4–7
and 18 yr after drainage respectively, were compared. This
approach assumed that the pre-drainage peat profiles at the
different sampling locations were sufficiently similar to al-
low such comparison, considering the very similar bulk den-
sities below the water table at the locations (Fig. 3) and their
comparable settings on deep peat domes in the same climate
region.
2.6 Determining the consolidation component of
subsidence
The rate and timing of primary consolidation in the first
year was estimated by evaluating the shape of the subsidence
curve in the first years after drainage. The contribution of
secondary consolidation to subsidence was determined from
the relation between subsidence rate and peat depth, 6yr on
average after drainage. If consolidation has occurred, the
physics of soil compression determine that the rate of sur-
face lowering it causes is proportional to the thickness of the
saturated peat layer (i.e. below the water table) that is being
compressed (Berry, 1983; Mesri and Aljouni, 2007). There
is also a relationship with the depth of the surface peat un-
saturated zone, which determines the loss of buoyancy (i.e.
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1058 A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands
the increase in strain on the saturated peat below), but in our
assessment of secondary consolidation this effect was min-
imized by only selecting locations with average water table
depths within a range of 0.5 to 1m.
2.7 Determining the oxidation component ofsubsidence
and carbon loss
The contribution of oxidation to subsidence was obtained as
the remainder after subtracting the consolidation and com-
paction components from total subsidence. Carbon loss is
calculated from the thickness of peat that is lost as a result
of oxidation by applying a BD for peat below the water ta-
ble, measured as described above, and a carbon concentration
for the original peat of 55 % by dry weight, as reported by
Suhardjo and Widjaja-Adhi (1977) for a peatland site near
the Acacia plantation. This value is nearly identical to the
value of 56% found to be representative for hemic and fibric
tropical peat in SE Asia by Page et al. (2011).
3 Results
3.1 Subsidence rates
Subsidence in the first year after drainage, along two tran-
sects in the Acacia plantation, was found to be 60 and 90 cm
(75cm on average), from repeated elevation surveys over
that period. Over the subsequent 4yr a further subsidence
of 67 ± 11 cm (average ± SD) was observed at 14 locations,
with an average in the second and third year of 19 ± 4 cm
(Fig. 4). On the basis of these two datasets, total subsi-
dence over the first 5yr after drainage was determined to
be 142cm on average. At around 6 yr after drainage, av-
erage subsidence as measured at 125 locations stabilized at
around 5± 2.2cmyr
1
(Figs. 4 and 5). In the oil palm plan-
tation the average subsidence was 5.4± 1.1cmyr
1
; 18yr
after drainage. On this basis, we conservatively applied an
annual subsidence rate of 5cmyr
1
in further calculations
of subsidence more than 5 yr after drainage. In the natu-
ral forest average subsidence was 2.4± 1.6cmyr
1
at the
forest-plantation edge, reducing to 0–1cm yr
1
at a distance
of 2km inside the forest from the plantation boundary.
For Acacia plantations, a subsidence record over 8yr
(2002–2010) was assembled from 3 sources: field elevation
surveys before and after the first year after drainage, data
from 14 subsidence poles for years 2–5, and data from 125
poles for 6yr on average after drainage. For oil palm planta-
tions, no accurate subsidence measurements were available
prior to our monitoring which commenced more than 14yr
after initial drainage. We therefore applied the Acacia plan-
tation record of 142cm subsidence over the first 5yr to the
oil palm plantation sites, on the basis that the two types of
sites were shown to be very similar in terms of peat type,
peat depth, water table depth and subsidence rate (compar-
ing rates after 6 and 18 yr on average). To this was added
an annual subsidence rate of 5cm yr
1
for the period of 6 to
18yr after drainage, resulting in a total subsidence of 212 cm
over 18 yr. This number was supported by field evidence in-
cluding the observation that the peat surface had dropped by
between 1.2 and 1.5 m relative to the overflow of a weir that
was constructed one to two years after drainage and that was
anchored in the underlying mineral subsoil (resulting in the
overflow being “dry” 16 yr after construction, and the weir
no longer operational).
3.2 Water table depths
The average water table depth in the Acacia plantation was
0.7m (Table 1), with a range of values between the 10th and
90th percentiles of 0.47 to 0.98m. In the oil palm plantation
the water table depth regime was similar with an average of
0.73m and a 10-percentile range of 0.33 to 1.03 m. In the
drainage-impacted forest the average water table depth was
0.33m decreasing to zero (i.e. at the peat surface as defined
by the bottom of hollows in the surface hummock-hollow to-
pography) at a distance of 2km awayfrom plantation perime-
ter canals, inside the forest.
3.3 Peat bulk densities and other characteristics
Bulk density profiles determined for both Acacia and oil
palm plantations (Fig. 3) provided average values around
0.15gcm
3
near the peat surface, declining to a con-
stant value of around 0.075gcm
3
at depths below 0.5m.
A small set of data collected at a Acacia plantation site
2yr after drainage showed that the near surface BD was
0.085± 0.01gcm
3
(n = 36). The average BD (± SD) of
the top metre of peat was 0.089 ± 0.018 gcm
3
(n = 228)
and 0.087± 0.018 g cm
3
(n = 330) in the Acacia planta-
tion sites (4–7yr after drainage) and oil palm plantation sites
(18yr after drainage), respectively (Table 1). At depths of
1–1.2m in the Acacia plantation and 1–2.5m in the oil palm
plantation these values were 0.073± 0.015 g cm
3
(n = 171)
and 0.078± 0.007gcm
3
(n = 436), respectively (Fig. 3).
In the oil palm plantation, the average dry BD (± SD)
of larger wood remains in peat at depths below 1m was
0.083± 0.036gcm
3
(n = 20), only 0.005gcm
3
above
that of the non-woody peat matrix and far below the BD of
fresh wood which is generally above 0.4gcm
3
(confirm-
ing that the wood remains in this peat are in fact largely de-
composed even if they may look quite fresh). The content
of larger wood remains in peat below 1 m depth was visu-
ally estimated to be 15% on average, compared to 5% over
the top metre of peat (10 % at 0.6–1m depth and 0% at 0–
0.5m). Given the relatively small difference between BD of
the wood remains and the peat matrix, and the relatively lim-
ited occurrence of large wood remains by volume, there was
no need to correct BDs as measured in cylinder samples for
the undersampling of larger wood remains.
Biogeosciences, 9, 1053–1071, 2012 www.biogeosciences.net/9/1053/2012/
A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands 1059
Dear Janet,
thanks for the response. Attached is a Word file with the clarification of the remaining
text change, that you request, and the corrected figures 3 and 7.
Thanks a lot for processing this quickly.
Cheers, Al Hooijer
We requested:
Page 7, Figure 3. There are three inconsistencies in the legend (within the figure). (1)
both ‘yr’ and ‘yrs’ – replace all by ‘yr’?. (2) ‘Bulk Density (g/cm3)’ – replace by
‘Bulk Density (g cm-3)’? (3) Literature references are without comma: replace “Kool
2006” with “Kool, 2006” and make the same change in “Anshari 2010”? We could
send a new figure, but maybe better if this is fixed at the BG end?
Herewith the corrected Figure 3, as you requested:
-3
-2.5
-2
-1.5
-1
-0.5
0
0.00
0.05
0.10
0.15
Bulk Density (g cm
-3
)
Depth below peat surface (m
Profiles; 3-5 replicates
Average of 10 profiles
-3
-2.5
-2
-1.5
-1
-0.5
0
0.00
0.05
0.10
0.15
Bulk Density (g cm
-3
)
Depth below peat surface (m
2 yr after drain. (3 profiles)
5 yr after drain. (19 pr.)
18 yr after drain. (10 pr.)
-3
-2.5
-2
-1.5
-1
-0.5
0
0.00
0.05
0.10
0.15
Bulk Density (g cm
-3
)
Depth below peat surface (m
Secondary (Kool, 2006)
Primary (Anshari, 2010)
Secondary (Anshari, 2010)
Fig. 3. Vertical profiles of bulk density in SE Asian peatlands. Left: individual profiles (with 3 to 5 replicates at each depth) in Sumatra oil
palm plantations (this study), 18yr after drainage. Middle: average profiles for Sumatra Acacia and oil palm plantations at 2, 5 and 18yr
after drainage respectively; each location represents the average of 3 to 19 profiles that all have 3 to 5 replicate samples at each depth (this
study). Right: average profiles in undrained secondary natural peatland forest in Kalimantan (Indonesia) as reported by Kool et al. (2006;
average of 9 profiles with peat depths over 4m) and in primary and secondary forest as provided by Gusti Anshari (averages of 4 and 6
profiles per site). The latter data are also reported, in summary, by Anshari et al. (2010).
The average ash content of 223 subsamples was 1.2% by
weight, with a standard deviation of 1.13%. No relationship
between ash content and sampling depth was apparent.
3.4 Peat thickness and type
Valuesfor peat thickness in the Acacia plantation varied from
3.8 to 16.9m, with an average (± SD) of 9± 2.6m (Table 1).
The range in oil palm plantation was 5.6 to 10.7m, with an
average of 7.7 ± 1.4 m. The overall average peat thickness
for both plantation types was 8.4m. The top 0.3m to 0.5m
of peat in oil palm plantation sites was generally hemic (with
limited plant remains visible; Fig. 2), with some fibric peat
(abundant plant remains visible) and sapric peat (no plant re-
mains visible). In Acacia plantation sites, that were drained
more recently, the top layer of peat was mostly described as
more fibric, but going towards hemic. Peat at greater depth
was nearly always fibric, and often woody, except the lowest
few metres where peat was often hemic or sapric and some-
times described as “muddy”, indicating higher mineral con-
tent.
3.5 The effect of consolidation on peat bulk density
On the assumption that all or nearly all primary peat con-
solidation occurs in the first year after drainage (Andriesse,
1988), we estimated the consolidation amount by subtracting
a subsidence value of 0.19 m (25 %), obtained for the sec-
ond and third year, from a total subsidence of 0.75 m in the
first year, providing a consolidation component of 0.56 m.
This amounts to 7% of the average peat thickness of 8.4m
at our study sites, implying that the BD of 0.075gcm
3
that
is now found below the lowest water table at the study sites,
results from the consolidation of a peat column with a “pre-
drainage” BD of around 0.07g cm
3
.
No statistically significant relation between subsidence
rate and peat thickness (R
2
= 0.002, with subsidence being
around 5 cm yr
1
for all peat thickness values) was evident
6yr after drainage, confirming that secondary consolidation
is negligible in these peatlands.
3.6 The oxidation component of subsidence, and
resulting carbon loss
Accounting for consolidation, a total of 0.86m of the total
subsidence of 1.42m at the Acacia plantation sites, over the
first five years, was caused by a combination of compaction
and oxidation. Following the method described in Sect. 2.5,
the BD profile indicates an oxidative peat loss that is equiva-
lent to an average CO
2
emission of 178t ha
1
yr
1
(Table 2,
Fig. 6), which explains 75% of cumulative subsidence at the
Acacia plantation monitoring sites over this period. Apply-
ing the same method to the oil palm plantation data, assum-
ing the same subsidence of 1.42m over the first 5yr and a
subsidence of 5cmyr
1
in the subsequent 13yr, an equiva-
lent average peat oxidative CO
2
emission of 119t ha
1
yr
1
is obtained, which explains 92% of subsidence over 18yr.
www.biogeosciences.net/9/1053/2012/ Biogeosciences, 9, 1053–1071, 2012
1060 A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Jan-02 Jan-03 Jan-04 Jan-05 Jan-06 Jan-07 Jan-08 Jan-09 Jan-10
Cumulative subsidence (m)
Forest cleared;
Drainage starts;
A
cacia planted
Start precise
monitoring
First Acacia harvest,
followed by water
management improvements
-5
-4
-3
-2
-1
0
0 1020304050
Years since start drainage
Cumulative subsidence (m)
Sumatra, measured
Malaysia, measured
California, measured
Florida, measured
Calculated, 0.7m, 30C
SE Asia, calculated
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Jan-02 Jan-03 Jan-04 Jan-05 Jan-06 Jan-07 Jan-08 Jan-09 Jan-10
Cumulative subsidence (m)
Forest cleared;
Drainage starts;
A
cacia planted
Start precise
monitoring
First Acacia harvest,
followed by water
management improvements
-5
-4
-3
-2
-1
0
0 1020304050
Years since start drainage
Cumulative subsidence (m)
Sumatra, measured
Malaysia, measured
California, measured
Florida, measured
Calculated, 0.7m, 30C
SE Asia, calculated
Fig.4. Top: average subsidence rates as measured at 14 locations inAcacia plantations, over the first 9yr after drainage. Bottom: as measured
at a larger number of drained peatland locations in Sumatra (this study), Malaysia (from W
¨
osten 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 22.5cm yr
1
for those years, which is the average subsidence
rate over 1914 and 1915 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.
Applying 92% oxidation to the average subsidence rate
measured in the Acacia plantation, 6yr on average after
drainage, the resulting carbon loss is 68t CO
2eq
ha
1
yr
1
(CO
2
equivalents, i.e. assuming in this calculation that no
carbon is lost as CH
4
, DOC or TOC). For the oil palm site,
18yr after drainage, this value is 78 t CO
2eq
ha
1
yr
1
. For
these plantations in general, 6yr or more after drainage, an
average minimum loss of 73 t ha
1
yr
1
may be accepted at
an average water table depth of 0.71 m (Table 2).
When calculating total cumulative carbon loss from plan-
tations, both the very high loss in the first 5 yr and the
lower loss in the subsequent period must be accounted
for. Over a 25yr period, the average annual carbon loss
thus becomes 90 t CO
2eq
ha
1
yr
1
for Acacia plantation and
109t ha
1
yr
1
for oil palm plantation, with an average of
100t ha
1
yr
1
for all plantations. Over a 50 yr period,
these values become 79t ha
1
yr
1
and 94t ha
1
yr
1
re-
spectively, with an average of 86t ha
1
yr
1
.
3.7 Relationships between subsidence rate and water
table depth
Linear correlation regressions between subsidence rate and
average water table depth were determined separately for
Acacia plantation and drained natural forest. The relation-
ship between water table depth and subsidence in Acacia
plantation, 6 or more years after drainage, is as follows (see
Fig. 5):
S = 1.5 4.98× WD
where:
Regression: N =125, F =33.38, p < 0.001, R
2
= 0.21,
Intercept=1.50, SE=0.63, p = 0.02,
Slope=4.98, SE=0.86, p < 0.001,
S =annual subsidence of the peat surface (cmyr
1
)
WD=average water table depth below the peat surface
(m; negative).
Biogeosciences, 9, 1053–1071, 2012 www.biogeosciences.net/9/1053/2012/
A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands 1061
43
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Fig. 5. Subsidence rates in relation to water table depths as measured in Acacia plantations 6 years 15
after drainage, oil palm plantations 18 years after drainage, and adjacent forest in Sumatra, 16
Indonesia. Top: data for individual monitoring locations. Measurements in Malaysia oil palm 17
plantations are shown for comparison (from DID Malaysia, 1996). The linear relations shown are for 18
Acacia plantations (excluding oil palm plantations) and forest. Bottom: averages for the Sumatra 19
plantation data, grouped by (sub-) transects of 5 to 9 adjacent monitoring locations. Linear relations 20
for Florida Everglades are also shown (adapted from Stephens et al., 1984). 21
22
0
2
4
6
8
10
12
-1.25-1.00-0.75-0.50-0.250.00
Average water table depth (m)
Subsidence (cm y
-1
)
Sumatra acacia plantation
Sumatra oil palm plantation
Malaysia oil palm plantation
Drained forest
Linear (acacia plantation)
Linear (forest)
0
5
10
-1.25-1.00-0.75-0.50-0.250.00
Average water table depth (m)
Subsidence (cm y
-1
)
Sumatra acacia plantation
Linear (acacia plantation)
95% confidence (ac. plant.)
Florida measured (T=25 C)
Florida calculated (T=30 C)
43
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Fig. 5. Subsidence rates in relation to water table depths as measured in Acacia plantations 6 years 15
after drainage, oil palm plantations 18 years after drainage, and adjacent forest in Sumatra, 16
Indonesia. Top: data for individual monitoring locations. Measurements in Malaysia oil palm 17
plantations are shown for comparison (from DID Malaysia, 1996). The linear relations shown are for 18
Acacia plantations (excluding oil palm plantations) and forest. Bottom: averages for the Sumatra 19
plantation data, grouped by (sub-) transects of 5 to 9 adjacent monitoring locations. Linear relations 20
for Florida Everglades are also shown (adapted from Stephens et al., 1984). 21
22
0
2
4
6
8
10
12
-1.25-1.00-0.75-0.50-0.250.00
Average water table depth (m)
Subsidence (cm y
-1
)
Sumatra acacia plantation
Sumatra oil palm plantation
Malaysia oil palm plantation
Drained forest
Linear (acacia plantation)
Linear (forest)
0
5
10
-1.25-1.00-0.75-0.50-0.250.00
Average water table depth (m)
Subsidence (cm y
-1
)
Sumatra acacia plantation
Linear (acacia plantation)
95% confidence (ac. plant.)
Florida measured (T=25 C)
Florida calculated (T=30 C)
Fig. 5. Subsidence rates and water table depths as measured in Acacia plantations, oil palm plantations and adjacent forest in Sumatra,
Indonesia. Top: data for individual monitoring locations. Measurements in Malaysian oil palm plantations are shown for comparison (from
DID Malaysia, 1996). The linear relations shown are for Acacia plantations (excluding oil palm oil plantations) and forest. Bottom: averages
for the Sumatra plantation data, grouped by (sub-) transects of 5 to 9 adjacent monitoring locations. Linear relations for Florida Everglades
are also shown (adapted from Stephens et al., 1984).
Table 2. Subsidence rates and carbon loss over different time periods, as determined from subsidence and bulk density data.
Acacia plantation sites Oil palm plantation sites Plantation average
Total subsidence in first 5 yr (m) 1.42
after drainage
Average subsidence and SD, >5yr cmyr
1
5± 2.2 5.4± 1.1 5.2
after drainage
Carbon loss 0–5yr t CO
2eq
ha
1
yr
1
178
after drainage (measured)
Carbon loss 0–18yr t CO
2eq
ha
1
yr
1
119
after drainage (measured)
Carbon loss 5–8yr t CO
2eq
ha
1
yr
1
68
after drainage (measured)
Carbon loss 18yr t CO
2eq
ha
1
yr
1
78
after drainage (measured)
Carbon loss 0–25yr t CO
2eq
ha
1
yr
1
90 109 100
after drainage (calculated)
Carbon loss 0–50yr t CO
2eq
ha
1
yr
1
79 94 86
after drainage (calculated)
www.biogeosciences.net/9/1053/2012/ Biogeosciences, 9, 1053–1071, 2012
1062 A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands
44
1
0
50
100
150
200
0 1020304050
Years since start drainage
Carbon loss (CO
2eq
t ha
-1
y
-1
)
Determined over 0-5 years (acacia)
Determined over 0-18 years (oil palm)
Calculated over 5-50 years (all plantations)
Average over 0-50 years (all plantations)
Average over 0-25 years (all plantations)
2
Fig. 6. Development in carbon loss over time in the Acacia and oil palm plantations studied. Presented 3
are the values over 5 years and 18 years as determined for Acacia and oil palm plantations 4
respectively, as well as long-term averages. 5
Fig. 6. Development in carbon loss over time in the Acacia and oil palm plantations studied.
The analysis of variance confirms that the linear relationship
between peat subsidence rate and peat water table depth is
statistically valid at the <0.05 significance level. Based on
the t-test on coefficients, the intercept is found to differ sig-
nificantly (p = 0.02) from zero.
The relationship for drained forest, at water table depths
of 0–0.7m, is:
S = 0.41 6.04× WD
where:
Regression: N =51, F =26.43, p < 0.001, R
2
= 0.35,
Intercept=0.41, SE=0.43, p = 0.34,
Slope=6.04, SE=1.17, p < 0.001.
This analysis, too, confirms that there is a statistically valid
linear relationship between the peat subsidence rate and peat
water table depth. The t-test on coefficients suggests that the
intercept does not differ significantly (p = 0.34) from zero.
Therefore, we may recommend a somewhat modified rela-
tion with an intercept of zero for use in applications where a
water table depth at the peat surface (i.e. of zero) is assumed
in emission calculations for intact natural peatland forest:
S = 7.06× WD
where:
Regression: N =51, F =197.12, p < 0.001, R
2
= 0.80,
Slope=7.06, SE=0.50, p < 0.001.
The correlations found for these regression relationships in-
cluding intercept are not high, partly due to data limitations
and partly because factors other than water table depth also
influence subsidence (see Discussion), but also as a conse-
quence of splitting the dataset into Acacia plantation” and
“forest” subsamples with limited water table depth ranges. If
a relation is fitted through the combined “Acacia plantation”
and “forest” data set, resulting in an “intermediate” relation
that may be applied where peatland land cover conditions are
not clear, a stronger relationship is obtained:
S = 0.69 5.98× WD
where:
Regression: N = 176, F = 128.42, p < 0.001, R
2
= 0.43,
Intercept=0.69, SE=0.34, p < 0.05,
Slope=5.98, SE=0.53, p < 0.001,
This analysis again confirms a valid linear relationship be-
tween the peat subsidence rate and peat water table depth.
Based on the t-test on coefficients, the intercept is found to
differ significantly (p < 0.05) from zero.
3.8 Relationships between carbon loss and water table
depth
The CO
2
emissions equivalent to peat subsidence losses
caused by drainage in Acacia plantation and natural forest
(Fig. 7) were determined by applying an oxidation percent-
age of 92%, a carbon content of 55% and a bulk density of
0.075gcm
3
. For plantations, the resulting relationship with
average water table depth is:
CL= 21 69× WD.
For drained natural forest (using the subsidence relation with
an intercept through zero):
CL= 98× WD.
Combined (for deforested unproductive peatlands):
CL= 9 84× WD
where:
CL= carbon loss, intCO
2eq
ha
1
yr
1
.
Biogeosciences, 9, 1053–1071, 2012 www.biogeosciences.net/9/1053/2012/
A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands 1063
We requested:
Page 11, Figure 7, the references in this figure are missing a comma, e.g.
“Jauhiainen et al. 2011” should be “Jauhiainen et al., 2011” Also, this particular
reference should be “2012” (see note under REFERENCES, below).
Herewith the corrected Figure7, as you requested:
0
25
50
75
100
-1-0.75-0.5-0.250
Average water table depth (m)
Carbon loss ( CO
2eq
t ha
-1
y
-1
)
Florida Everglades peatland (T=25C)
Wösten and Ritzema, 2001; all peatland
Hooijer et al., 2006, 2010; deforested
Couwenberg et al., 2010; all peatland
Jauhiainen et al., 2012; plantation
This study; plantation
This study; drained natural forest
Fig. 7. Comparison of the relation between carbon loss (CO
2eq
) and water table depth in tropical peatlands, more than 5 yr after drainage, as
determined in this and other studies. The Florida Everglades relation is calculated from data in Stephens and Speir (1969), that may also have
been used to calculate the relation in W
¨
osten and Ritzema (2001). The relations by Hooijer et al. (2006, 2010) and Couwenberg et al. (2010)
were based on partly different sets of literature sources. The relation by Jauhiainen et al. (2012) is based on daytime CO
2
flux measurements
in the same Acacia plantation as the current study, excluding root respiration and corrected for diurnal temperature fluctuation.
4 Discussion
4.1 Comparison with other published tropical and
sub-tropical peat subsidence rates
The average subsidence rate of 5cmyr
1
found for Acacia
and oil palm plantations, more than 5 yr after initial drainage,
is close to most literature values. Subsidence rates reported
for a Johor (Malaysia) oil palm plantation, between 14 and
28yr after drainage, were 4.6cm yr
1
on average at 17 lo-
cations for which water table depth data are not available
(W
¨
osten et al., 1997), and 3.7cmyr
1
at 11 other locations
with an average water table depth of 0.5m (DID Malaysia,
1996). Mohammed et al. (2009) report, on the basis of
field monitoring in oil palm plantations on peat of 3 to 4 m
in thickness, that subsidence stabilizes at 4.3cm yr
1
after
15years under best practice management with average wa-
ter depths of 0.4m. DID Sarawak (2001) reported a con-
stant average subsidence rate of 5cmyr
1
in Sarawak af-
ter the initial two years following drainage, at a water ta-
ble depth of 0.6 m, but also proposed that the rate of an-
nual subsidence increased by 1 cm for every 10 cm lower-
ing of the water table, which would result in 7 cm yr
1
sub-
sidence at a water table depth of 0.7m. Andriesse (1988)
suggested a stabilization of subsidence at long-term rates of
up to 6cm yr
1
, based on observations in a number of loca-
tions in SE Asia. In the Everglades, USA, an average sub-
sidence rate of 3cm yr
1
was reported over more than 50 yr
after the initial year (Stephens and Speir, 1969; Fig. 4), but
this was for a different peat type in a sub-tropical region with
a lower surface peat temperatureof 25
C, compared to 30
C
in plantation sites in Indonesia (Jauhiainen et al., 2012), and
for higher water table levels. Applying an equation that re-
lates subsidence to temperature and water table depth, based
on long-term controlled field plot experiments, Stephens et
al. (1984) calculated that the Everglades peat would have
subsided by 8cmyr
1
in the long term had it been in fully
tropical conditions with a peat surface temperature of 30
C.
In peatland with an initial organic content of around 80% in
the Sacramento Delta, California, subsidence after the initial
5yr proceeded at a constant rate of 7.5cm yr
1
for over 50yr
(Deverel and Leighton, 2010; Fig. 4).
These studies support our finding that subsidence rates
stabilize between 4 cm yr
1
and 5.5cm yr
1
in drained SE
Asian peatlands, at average water table depths around 0.7m,
after an initial phase of more rapid subsidence. The vari-
ation in reported subsidence rates in tropical peat seems to
be small compared to temperate peats (Couwenberg et al.,
2010). This may be related to the effect of soil temperature,
which is more constant in time and space in the tropics (at
around 30
C) compared to temperate climates, on peat oxi-
dation.
The studies in SE Asia mentioned above apply to deep
deposits of fibric peat with a pre-drainage BD of around
0.07 to 0.1g cm
3
and low mineral content. Where lower
subsidence rates are reported, this is usually for shallower
peat with higher BD and mineral content. Murayama and
Bakar (1996) reported that subsidence rates in drained peat-
lands in Peninsular Malaysia, with bulk densities between 0.1
and 0.35 gcm
3
, were 2 to 4cm yr
1
after the initial year,
and that subsidence decreased as BD increased. Dradjad et
al. (2003) reported a subsidence range of 2.4 to 5.3cmyr
1
over a 14yr period, in peaty swamp soil around 2 m thick
with a mineral content as high as 73 to 86% (i.e. not re-
ally peat). Deverel and Leighton (2010) also found a strong
relationship between soil organic content and subsidence rate
in the Sacramento Delta, with subsidence rates some 100yr
after initial drainage having declined to less than 1 cm yr
1
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1064 A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands
in areas where the organic content of the top soil was be-
low 10 %, but still as high as 3.5 cm yr
1
where the organic
content remained 60%, corresponding to an original organic
content of around 80%.
4.2 Subsidence rates in relation to water table depth
and other environmental variables
The relationships between subsidence and water table depth
found in this study for peatlands that have been drained
for 6 yr on average (Sect. 3.7) are not very different from
the relations reported for the drained subtropical Everglades
peatlands in Florida on the basis of long-term field experi-
ments (Stephens et al., 1984; Fig. 5), or by Couwenberg et
al. (2010) in a review of limited data for SE Asia. While it is
possible that non-linear relationships apply, our data do not
suggest that these would be more appropriate because they do
not yield higher goodness of fit (R
2
) values. Non-linear rela-
tions are not apparent either from data presented by Stephens
et al. (1984), so we conclude that applying a linear regression
is appropriate for the current purpose.
It has been suggested that no further increase in peat sub-
sidence and CO
2
emission rates occurs when the water ta-
ble falls below a threshold depth (Couwenberg et al., 2010).
Our data do not indicate such a threshold within the range
of water table depths observed in this study (0 to 1.2 m) and
we suggest that the linear relationship applies to water table
depths up to at least 1 m, while acknowledging that the rela-
tionship for greater water table depths may be different.
In contrast with relationships published to date, the regres-
sion in this study for Acacia plantation does not intercept
with the origin, indicating that substantial subsidence would
occur in these drained peatlands even if average water tables
could be maintained near to the peat surface. It is probable
that the higher temperatures and increased peat surface aer-
ation, resulting from reduced vegetation cover and increased
peat soil disturbance in plantations, enhance peat oxidation
regardless of the position of the water table. Only part of
the variation in subsidence and carbon loss can therefore be
explained by the relationship with water table depth, and part
of it must be attributed to other factors.
In the oil palm plantation, subsidence is virtually con-
stant at all locations (Table 2, Fig. 5), with no relation-
ship with water table depth. This difference with Acacia
plantation may be because the oil palm sites were contin-
uously fertilized with large amounts of nitrogenous fertil-
izer (600 kg ha
1
yr
1
urea), whereas the Acacia received
a small amount of fertilizer during the planting phase only.
Inorganic nitrogen increases the rate of microbial breakdown
of organic matter (Berg, 2000; Berg and Laskowski, 2006).
Therefore, the relationship obtained for Acacia plantations
can provide a tentative minimum estimate for oil palm plan-
tations until further studies enable determination of a more
specific relationship for the latter.
The intercept of the relationship for drained forest, at wa-
ter table depths of 0–0.7m, does not differ significantly from
zero, confirming that in natural forest the peat carbon store is
stable if the water table is close to the peat surface. At 0.7m
water table depth, the Acacia plantation” and the “forest”
subsidence rates are the same (Fig. 5) and the Acacia planta-
tion” relationship may be applied to drained natural peatland
forest when average water table depths are below 0.7m.
The difference between the Acacia plantation” and “for-
est” relationships, especially at high water levels, demon-
strates that the presence of an intact natural forest cover re-
duces oxidation, even if the water table is lowered. Since
plantations and natural forest are at opposite ends of the spec-
trum of land use impacts on peatlands, in terms of vegeta-
tion cover and soil disturbance, the two relationships may
be assumed to represent extremes of a range from relatively
undisturbed to highly disturbed peatlands. The “intermedi-
ate” relation may therefore be tentatively applied to degraded
natural forest and deforested unproductive peatlands, that ex-
perience intermediate levels of disturbance.
4.3 Changes in subsidence rate over time and the role of
consolidation
Several studies of SE Asian peatlands report a rapid and ma-
jor drop by around 100cm in the peat surface in the first one
to two years after drainage, followed by stabilization at a
lower, constant rate (Andriesse, 1988; DID Sarawak, 2001),
similar to that found in the current study (Fig. 4). Values of
this order also occur in other regions of the world, e.g. a pri-
mary subsidence of 60 to over 100cm (Drexler et al., 2009;
Deverel and Leighton, 2010) was reported immediately af-
ter drainage of the Sacramento Delta peatlands. All studies
agree that this initial drop can be attributed mostly to primary
consolidation of the peat, while it is generally concluded that
secondary consolidation after this initial phase is negligible
(Stephens and Speir, 1969; Andriesse, 1988), as found in the
current study. Indeed all previous studies applying subsi-
dence measurements to estimate peat oxidation and carbon
loss (Schothorst, 1977; Stephens et al., 1984; W
¨
osten et al.,
1997; Couwenberg et al., 2010; Deverel and Leighton, 2010;
Leifeld et al., 2011) have explicitly or implicitly assumed the
effect of consolidation, after an initial “dewatering” phase, to
be negligible.
The near absence of consolidation after the first year of
plantation drainage can be explained by the high hydraulic
conductivity of fibrous tropical peat (10 to over 100 m d
1
;
DID Sarawak, 2001; Hooijer et al., 2009) that allows the pri-
mary consolidation process in intensively drained areas to be
completed rapidly as water is removed easily from the peat,
and by the small amount of secondary consolidation that oc-
curs in fibrous peat in general (6% of total consolidation;
Mesri and Aljouni, 2007).
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A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands 1065
For the Florida Everglades, Stephens and Speir (1969)
concluded that subsidence continued at a constant rate for
over 50yr after the initial phase. Constant subsidence is also
observed in the Sacramento Delta (Deverel and Leighton,
2010; Fig. 4). Whilst a gradual reduction to a subsidence rate
of 2cm yr
1
, beyond 28 yr after drainage, was suggested by
W
¨
osten et al. (1997), it should be noted that this value was
based on an estimated projection rather than only on mea-
surements. Subsidence rates presented in this study, as well
as those presented by W
¨
osten et al. (1997), are all around
5cmyr
1
after 6, 18 and 14–28 yr respectively, suggesting a
constant subsidence rate rather than a clear gradual decrease.
On the basis of the evidence available we conclude that
subsidence rates in Acacia and oil palm plantations in SE
Asia, more than 5yr after drainage at constant water table
depths, are likely to remain constant or nearly constant at
around 5 cm yr
1
as long as there is “fresh” peat available
for oxidation (Fig. 4). When the peat deposit is nearly de-
pleted and all remaining peat is compacted and drained, the
subsidence rate would be expected to decline. This also ap-
plies when, as oxidation has removed the upper layers, lower
peat layers are being accessed that may have higher BD and
mineral content. We therefore emphasize that the current as-
sessment applies to fibric to hemic peat with very low min-
eral content and low BD.
4.4 Unexplained variation in subsidence rates and
water table depths
Subsidence rates determined at individual monitoring loca-
tions show considerable variation, with values varying from
1.2 to 11.2cm yr
1
(Fig. 5). However, when values are aver-
aged over subgroups of 5 to 9 adjacent monitoring locations
over relatively short distances along transects, this range nar-
rows to 2.9–7.4cmyr
1
(Table 3, Fig. 5). The standard de-
viation in subsidence rates, expressed as a percentage of the
mean value, is still considerable at 42% on average at the
subgroup level (Table 3). This variation in subsidence is
not matched by variation in water table depth or peat thick-
ness, with standard deviations of 21 % and 9% of the mean,
respectively, over the same subgroups. This suggests that the
variation within these groups, between individual measure-
ments, is related partly to highly localized variations in phys-
ical conditions, including heterogeneity in the near-surface
peat and in canopy cover, with the latter affecting peat sur-
face temperature.
It follows that accurate subsidence and water table depth
measurement requires a large number of locations monitored
over long periods to cover not only the obvious variations
in land cover and water management, but also the unknown
random heterogeneities. In the end, however, substantial un-
explained variation in measured subsidence is likely to al-
ways remain as some physical conditions that may affect car-
bon loss and subsidence can not or hardly be measured. For
example, it is not possible to measure the variation in peat
characteristics (BD, wood content) at the micro-scale with-
out sampling the peat in a pit, which destroys the monitoring
location.
The nature of water table depth measurements may also
explain part of the variation. The 2-year subsidence records
used in this study did not all cover the same period, although
all overlap by at least 1yr, introducing differences in rainfall
and soil moisture regime experienced by the different loca-
tions; moreover data gaps occurred on some of the records,
which affected the average water table depth numbers but not
the subsidence numbers.
4.5 Study site bulk density compared to literature
values
The average “original” BD value of 0.075gcm
3
found in
peat below 1m depth in this study, translating to a value of
0.07gcm
3
prior to consolidation, is at the low end of val-
ues for SE Asian peats presented by Page et al. (2011), who
find that average values reported by 15 individual studies, in
both intact and deforested peatlands, are between 0.08 and
0.13gcm
3
with an average of 0.09gcm
3
. Lower-range
values reported by individual studies are below 0.06g cm
3
in only 3 out of 15 studies, and all 15 report average val-
ues above 0.07g cm
3
. In deforested peatlands, Page et
al. (2011) find BD values of near-surface peat to be higher
than at greater depth, but in forested peatlands no consistent
increase or decrease with depth was identified. This lack of
a clear trend with depth, at depths between 0 and 4m, is also
confirmed by peat profiles in primary and secondary forest
in Kalimantan (Indonesia) (Kool et al., 2006; Anshari et al.,
2010) (Fig. 3).
The value of 0.061gcm
3
over 1–2m depth derived from
data used by Anshari et al. (2010) is the lowest average re-
ported for any group of peat profiles in undrained peatland
forest in SE Asia. By contrast, data for intact forest presented
by Kool et al. (2006) yield an average of 0.074gcm
3
over
1–4m, which is higher than the pre-consolidation value of
0.07gcm
3
applied in the current study.
On the basis of the above assessment of literature values,
and noting that the BD values below 1m at the different lo-
cations in the current study are all very similar, we conclude
that our BD values below 1 m are indeed representative of
the pre-drainage conditions, somewhat increased by primary
consolidation. Therefore, total compaction since the start of
drainage, after the initial 1yr consolidation phase, may be
estimated by comparing the current BD of peat below 1m
depth with that above it as explained in Sects. 2.5 and 3.6.
4.6 Determining the carbon loss from oxidation, as a
percentage of total subsidence
Very few studies have separated the oxidation and com-
paction components of subsidence in tropical and sub-
tropical peatlands using BD profiles. Stephens and
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1066 A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands
Table 3. Summary statistics of water depth, peat thickness and subsidence rate along all plantation transects, as calculated over groups of 5
to 9 adjacent measurement locations each. Mean, maximum and minimum values are calculated from average values for individual locations.
The “Riau” locations are in Acacia plantation, the “Jambi” locations in oil palm.
(Sub-) transect No of mon. Water table depth Peat thickness Subsidence
code points
Mean Min Max SD SD Mean Min Max SD SD Mean Min Max SD SD
m m m m % mean m m m m % mean cmyr
1
cmyr
1
cmyr
1
cmyr
1
% mean
A Riau 6 0.56 0.72 0.41 0.11 20 5.3 5.7 4.6 0.4 8 5.9 4.2 9.5 1.9 32
B Riau 6 0.63 0.77 0.47 0.10 16 6.7 7.2 6.2 0.5 7 5.2 3.4 7.7 2.1 40
C Riau 6 0.54 0.80 0.29 0.11 21 7.8 10.2 7.1 0.6 8 4.5 2.2 7.8 2.1 46
D Riau 5 0.72 0.91 0.65 0.10 14 7.8 8.6 6.9 0.5 7 5.7 3.1 7.4 2.4 42
E Riau 6 0.84 1.05 0.56 0.10 11 8.1 8.4 7.8 0.5 7 5.6 2.3 10.4 2.1 38
F Riau 6 0.56 0.69 0.43 0.11 19 8.6 9.1 8.1 0.6 7 4.0 1.5 6.3 1.9 48
G Riau 5 0.43 0.56 0.28 0.11 26 11.6 11.8 11.5 0.7 6 3.4 1.6 5.1 2.0 59
H Riau 5 0.42 0.50 0.35 0.11 25 11.8 11.9 11.6 0.5 4 2.9 1.5 4.6 1.5 51
I Riau 5 0.67 0.97 0.47 0.11 17 6.0 7.7 5.1 0.4 6 3.8 1.3 6.2 1.4 37
K Riau 5 0.74 0.83 0.62 0.11 15 8.3 9.3 7.7 0.3 3 4.7 1.6 11.0 1.7 36
L Riau 5 0.61 0.71 0.55 0.12 19 12.2 13.1 11.6 0.2 2 3.1 1.2 4.1 1.7 57
M Riau 5 0.52 0.60 0.46 0.13 24 14.6 16.9 13.3 0.1 1 3.5 2.3 4.5 1.9 53
N Riau 5 0.74 0.80 0.66 0.10 14 12.8 14.7 10.2 0.1 1 5.4 4.5 6.2 1.8 34
O Riau 9 0.71 0.99 0.49 0.12 17 12.7 14.4 10.5 0.3 3 5.8 4.3 8.5 1.9 32
P Riau 5 0.88 1.07 0.78 0.13 15 8.7 9.3 8.4 0.3 4 5.3 3.0 7.9 2.0 38
Q Riau 5 0.73 0.81 0.62 0.13 18 8.8 9.4 8.4 0.4 4 3.3 2.3 4.5 2.3 69
R Riau 6 0.69 0.81 0.58 0.21 31 9.0 10.0 8.5 1.4 15 4.0 1.4 5.7 2.4 61
S Riau 5 0.73 0.81 0.66 0.26 35 4.6 5.9 3.8 1.5 33 7.4 3.1 11.2 2.4 32
T Riau 7 0.75 0.97 0.55 0.26 34 8.3 10.1 7.5 1.5 18 7.3 3.7 10.5 2.4 33
U Riau 7 0.93 1.26 0.65 0.21 23 8.6 9.0 8.0 1.3 15 6.4 4.7 8.4 2.2 34
V Riau 6 1.08 1.19 0.97 0.22 20 8.0 8.0 8.0 1.1 13 5.9 4.3 9.8 2.2 37
1 Jambi 9 0.75 0.84 0.66 0.19 25 6.4 7.2 6.0 1.0 16 5.3 4.5 6.0 2.0 38
2 Jambi 8 1.06 1.14 1.00 0.19 18 6.5 8.1 6.0 1.0 16 4.9 3.5 6.5 2.0 41
3 Jambi 9 0.73 0.90 0.51 0.22 31 6.9 8.5 5.6 1.2 18 4.8 3.5 6.0 1.9 40
4 Jambi 9 0.73 0.77 0.69 0.12 16 9.2 10.7 8.8 0.7 7 6.1 5.0 8.0 1.9 31
5 Jambi 7 0.32 0.34 0.30 0.11 33 9.2 9.9 8.7 0.6 6 5.9 4.0 8.0 1.8 31
All Riau 125 0.70 0.86 0.56 0.14 20 8.89 9.81 8.17 0.62 8 4.92 2.76 7.51 1.99 43
All Jambi 42 0.72 0.80 0.63 0.17 25 7.64 8.87 7.01 0.91 13 5.40 4.10 6.90 1.93 36
All 167 0.71 0.85 0.57 0.14 21 8.66 9.63 7.95 0.67 9 5.01 3.01 7.40 1.98 42
Speir (1969) calculated that oxidation accounted for 78 %
of subsidence in the Everglades peatlands over a period of
more than 50 yr since drainage; this was confirmed by CO
2
flux measurements at the same sites (Neller, 1944) and under
laboratory conditions (Volk, 1973). In a reassessment of the
same data, Stephens et al. (1984) estimated that the increase
in BD in field plots reported by Neller (1944) explained only
10 to 15 % of subsidence, implying that 85 to 90% could be
attributed to carbon loss. Deverel and Rojstaczer (1996) and
Deverel and Leighton (2010) found that oxidation accounted
for 68% of subsidence in Californian peatlands more than
70yr after drainage, based on CO
2
flux measurements and a
carbon balance model. This value applies, however, to peat
with a mineral content that is 20% higher than in the Ev-
erglades or SE Asia, which is expected to reduce the rela-
tive contribution of oxidation to subsidence. Based on CO
2
flux measurements, Murayama and Bakar (1996) concluded
that oxidation caused 50 % to 70% of subsidence at sites in
Malaysia; however this was also for shallow peat with high
mineral content. In peatland plantations in Johor, Malaysia a
cumulative value of 61% was reported (DID Malaysia, 1996;
W
¨
osten et al., 1997); however, the authors do not explain
over what period after drainage this value applies. Moreover,
the same study also reports complete loss of the peat layer,
i.e. 100% oxidation, at up to one third of the subsidence
monitoring locations where peat was thin at the startof moni-
toring. Couwenberg et al. (2010) assumed a minimum oxida-
tion percentage of 40%, but this was based mostly on studies
in temperate climates whereas others (Volk, 1973; Stephens
et al., 1984; Brady, 1997) have shown that peat oxidation
increases at higher temperatures, causing a doubling of sub-
sidence rate for every 10
C increase. Assuming an average
peat surface temperature of 10
C in temperate peatlands and
30
C in the tropics, the oxidation rate would be expected to
be four times higher in the latter and make up a far larger
proportion of subsidence. Peat temperature and its potential
impact on tropical peat oxidation is discussed in more detail
in Jauhiainen et al. (2012).
While the oxidation contribution to peat subsidence in-
creases over the first few years after drainage as primary
consolidation and compaction diminish, the net carbon loss
in fact decreases over this period, before stabilizing. It
may be that a finite pool of the most labile carbon com-
pounds decomposes rapidly, leaving only recalcitrant car-
bon compounds that are more resistant to decomposition
(Berg, 2000). In addition, a lower water table in this ini-
tial “dewatering phase”, applied to rapidly consolidate the
peat surface before planting, may further increase oxida-
tion. A similar finding of an initial “spike” in carbon
loss was reported for the Sacramento Delta by Deverel and
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A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands 1067
Leighton (2010), who calculated that emissions reduced
from 154t CO
2eq
ha
1
yr
1
a few years after drainage to
55t ha
1
yr
1
80yr later, at an average water table depth of
1m (Drexler et al., 2009).
The fact that the BD profiles in our study sites at 3–7 and
18yr after drainage are very similar (Fig. 3) indicates that
not only primary consolidation but also compaction may in
fact have become negligible after the first 5yr, at which point
nearly 100% of subsidence appears to be caused by oxida-
tion. After the initial year of subsidence, rates of compaction
and oxidation may achieve equilibrium. Compaction con-
tinues as uncompacted peat from the saturated zone enters
the unsaturated, oxidative zone. However this compaction
appears to be balanced by oxidation in an unsaturated peat
profile of constant thicknessand bulk density that moves pro-
gressively downwards over time as the water table is lowered
to match surface subsidence. The combined result of the two
processes is a peat bulk density profile that is stable in time.
The long-term oxidation contribution to subsidence of
92% in oil palm plantations, 18yr after drainage, is at the
high end of earlier estimates. Our study confirms, how-
ever, that the contribution of oxidation to peat subsidence
increases in time while consolidation and compaction are
major contributors only in the initial period. It should be
noted that most published percentage oxidation values, in-
cluding the 61% reported by DID Malaysia and W
¨
osten et
al. (1996, 1997) and the 85–90% suggested by Stephens et
al. (1984), are averages of the cumulative oxidation since the
start of drainage, and therefore systematically underestimate
the percentage oxidation after the initial period. The longer
the period after drainage that is considered, the greater the
cumulative contribution of oxidation to subsidence that will
be found. For calculations of long-term carbon emissions we
therefore recommend use of the figure of 92 % oxidation that
we find for oil palm plantation 18yr after drainage, rather
than the 75% that we found for Acacia plantations 6yr after
drainage.
4.7 Sensitivity assessment
The calculation of the percentage oxidation contribution to
subsidence, and the resulting carbon loss, is sensitive to
the value used for the original pre-drainage BD, corrected
for consolidation immediately after drainage. In Sect. 4.5
we have shown that the pre-drainage BD values around
0.075gcm
3
used in our analysis, resulting from a value of
0.07gcm
3
allowing for primary consolidation immediately
after drainage, are at the low end of published values. We
have also shown that the lowest average pre-drainage value
reported in any study is 0.061 g cm
3
(using data of Anshari
et al., 2010). This would increase to around 0.065gcm
3
allowing for consolidation. Using the latter value instead of
the 0.078 g cm
3
on average applied to the oil palm plan-
tation in this study would have yielded an oxidation per-
centage of 77 % at 18yr after drainage instead of 92 %,
suggesting that carbon loss more than 5 yr after drainage
could be at the very most 20% lower than the average value
of 73t CO
2eq
ha
1
yr
1
proposed in this paper, i.e. around
60t CO
2eq
ha
1
yr
1
. However, this low-end figure is un-
likely to apply, as such low BD values appear to be excep-
tional.
For assessing the spike in carbon loss in the first year af-
ter drainage, the estimate of primary consolidation in the first
year is also a sensitive parameter. If we would assume all the
75cm of subsidence in the first year is caused by primary
consolidation only, rather than allowing for 19 cm being
caused by oxidation and compaction (Sect. 3.5), the average
carbon loss over the first 5yr becomes 132 t CO
2eq
ha
1
yr
1
rather than 178t CO
2eq
ha
1
yr
1
. This would also decrease
the percentage oxidation that is calculated over the period,
from the 75 % now calculated for Acacia plantation over 5 yr
after drainage (Sect. 3.6) to 69 %, and from 92% for oil palm
plantation over 18yr to 90 %. While we deem the underly-
ing assumption of an absence of oxidation in the firstyr to be
unrealistic, these values of 132 t ha
1
yr
1
emission over the
first 5yr and a long-term oxidation percentage of 90 % may
be seen as the lowest possible estimates on the basis of the
evidence available.
The value for carbon content usedhas a proportional effect
on the carbon loss calculated from subsidence. Assuming
carbon content of 50% or 60% instead of 55 %, which covers
the range reported in literature for fibric and hemic peat with
low mineral content, would reduce or increase carbon loss by
10%.
4.8 Comparison of carbon loss in subsidence with CO
2
emission measurements
Gaseous CO
2
emissions at the peat surface in the same
Acacia plantation landscape have been measured using the
closed chamber technique at 144 locations (Jauhiainen et al.,
2012). Measures were taken to exclude root respiration so
the results only represent CO
2
emissions from peat oxida-
tion. After correction for diurnal temperature fluctuations,
these measurements from the same peatland yield a value
of 80t ha
1
yr
1
at an average water table depth of 0.8 m,
which is very close to the 76 t CO
2eq
ha
1
yr
1
yielded by
the subsidence method for the same water table depth. More-
over, the slope of the relationship between water table depth
and CO
2
emission presented by Jauhiainen et al. (2012) is
nearly identical to that using the subsidence method pre-
sented in this paper (Fig. 7). We conclude that the results
of the two independent approaches are mutually supportive.
4.9 Comparison with other published CO
2
emissions
from tropical peatland
At water table depths between 0.5 and 1m, that are most
common in plantations, the emission relations found for
Acacia plantations and drained forest are similar to the
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1068 A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands
46
-2.0
-1.5
-1.0
-0.5
0.0
Jan-08 Jul-08 Jan-09 Jul-09 Jan-10 Jul-10
Water table depth (m)
Natural
Acacia
Oil palm
1
Fig. 8. Time series of water table depth as measured at some selected individual locations in the 2
studied Acacia and oil palm plantations, and in nearby natural forest at 2 km from the Acacia 3
plantation. In both plantation types, the records nearest the lower and upper 10-percentile average 4
water levels are shown. 5
Fig. 8. Time series of water table depth as measured at individual locations in the studied Acacia and oil palm plantations, and in nearby
natural forest at 2km from the Acacia plantation, over a 3-years period. In plantations, the records nearest the lower and upper 10-percentile
average water levels were selected.
linear relationship reported by Hooijer et al. (2006, 2010)
and Couwenberg et al. (2010), that were based on meta-
data assessments of studies carried out in deforested trop-
ical peatlands (Fig. 7). A similar CO
2eq
emission value
was also obtained by DID Sarawak (2001) and W
¨
osten and
Ritzema (2001) who proposed that every 1.0cm of subsi-
dence results in a CO
2eq
emission of 13.3t ha
1
yr
1
, equat-
ing to a total CO
2eq
emission of 66 t ha
1
yr
1
at the subsi-
dence rate of 5cm yr
1
reported in the same publications.
We conclude that the carbon losses found in this study,
more than 5yr after drainage, are in agreement with most
earlier studies, for water table depths that are common in
plantations and for the period beyond the initial years after
drainage. At lesser water table depths, the difference with
existing relationships increases, suggesting higher emissions
from drained peatlands than have been assumed to date and
a stronger relationship with vegetation cover, and perhaps
with fertilization and peat disturbance as well. Moreover,
we found that carbon loss in the initial year is higher than in
subsequent years, resulting in considerably higher long-term
average emissions than have been reported to date.
4.10 Predicting subsidence and carbon loss under
different water management regimes
The average water table depths encountered in this study are
similar in both Acacia and oil palm plantations, at 0.7 and
0.73m respectively, which is less than thosereported in some
earlier studies (e.g. 0.95 m in Hooijer et al., 2006, 2010) and
close to the target of 0.7 m specified for the Acacia planta-
tions studied (Hooijer et al., 2009) and of 0.6m for oil palm
plantations in general (DID Sarawak, 2001). However, this
does not suggest that high and well-controlled water levels
are the norm in such plantations. The limited options for ef-
fective water level control are illustrated by the wide range of
levels encountered in the study, with 10-percentile values for
annual averages ranging between 0.33 and 1.03m and for in-
dividual measurements between 0 and 1.6m (Fig. 8). Water
table depth can vary by up to a metre over a few kilometres in
each type of plantation, and also over time within the dry and
wet seasons. It should also be noted that our measurements
were obtained in a relatively “wet” year with high rainfall
even in the dry season, and in plantations that are relatively
well managed compared to others in the region. Water ta-
ble depth variations in normal years, and in other areas, are
likely to be greater.
The implication of the relatively low slope of the regres-
sion between water table depth and subsidence found in this
study, is that the benefit of raising water tables to reduce car-
bon emissions in plantations may be smaller than earlier as-
sumed. Even if an average water table depth of 0.6 m could
be achieved in the plantations now studied, which is not guar-
anteed, subsidence would still be 4.5cmyr
1
over the long
term, and the carbon loss 63t CO
2eq
ha
1
yr
1
more than
5yr after drainage. This would be a reduction of no more
than 20% relative to the average emissions currently occur-
ring. Moreover, it should be noted that this reduction in an-
nual subsidence and emission merely means they are post-
poned to a later date, unless natural conditions could be re-
stored in these plantations.
5 Conclusions
We show, for a much larger number of locations than all pre-
vious studies in SE Asia on this subject combined, that mea-
surements of subsidence and bulk density can yield accurate
soil carbon loss values for tropical peatlands, if the contribu-
tions from the different processes of oxidation, compaction
and consolidation contributing to subsidence are accounted
for. This reduces the uncertainty of carbon loss estimates
compared to earlier peat subsidence and gaseous emission
studies. This study is also the first to determine carbon loss
from tropical peatland from subsidence in parallelwith direct
Biogeosciences, 9, 1053–1071, 2012 www.biogeosciences.net/9/1053/2012/
A. Hooijer et al.: Subsidence and carbon loss in drained tropical peatlands 1069
CO
2
gas emission measurements at the same locations (see
Jauhiainen et al., 2012); the close agreement between the
results of the two independent approaches further confirms
their validity. We recommend that this subsidence approach
is adopted in future studies of carbon loss from tropical peat-
lands, alone or in combination with direct CO
2
gas emission
measurements.
The subsidence rate of 142cm over the first 5 yr, and
5cmyr
1
over subsequent decades at water table depths
around 0.7m, is supported by findings of most smaller-scale
studies published earlier. However the accompanying car-
bon loss, of 100 t CO
2eq
ha
1
yr
1
on average over 25yr af-
ter drainage, is higher than other published estimates. This
is largely because earlier studies have assumed that peat
oxidation and carbon loss are constant from the start of
plantation development, whereas we confirmed substantially
higher loss rates in the first few years after drainage. Another
reason for the underestimation of CO
2
losses from tropical
peatland is the use of oxidation percentages from temperate
and boreal peatland studies that do not take into account the
temperature dependence of the decomposition processes in-
volved.
The average emission in the plantations studied more than
5yr after drainage, for Acacia and oil palm combined, is
found to be 73 t CO
2eq
ha
1
yr
1
at water depths around
0.7m. This is at the high end of earlier estimates. Our sen-
sitivity analysis shows that for emissions to be 20 % lower,
we would have to apply the lowest pre-drainage BD reported
for SE Asian peatlands. We therefore believe our carbon
loss calculations to be conservative. The relatively small
difference for carbon loss from Acacia plantations 6yr after
drainage and oil palm plantations 18yr after drainage, of 68
and 78t CO
2eq
ha
1
yr
1
respectively, suggeststhat the over-
all range in long-term carbon emissions from these types of
plantations in this region may be limited.
The carbon emission estimates and relations presented in
this paper, as well as the improved ability to project long-
term emissions, may help REDD projects (Reduced Emis-
sions from Deforestation and Degradation) to better quantify
the actual carbon emission reduction that can be achieved
through rehabilitation and conservation of tropical forested
peatlands, compared to a “Business as Usual” scenario of de-
forestation and drainage that is now evident in most tropical
peatlands. Reduced uncertainty in these calculations is es-
sential for REDD projects to be economically viable and ver-
ifiable (Murdiyarso et al., 2010; Asner, 2011). Our findings
support a conservative approach as they suggest that bringing
up water tables to near-natural levels alone, without restoring
a near-natural vegetation cover which may take a long time,
will still allow substantial carbon emissions.
With a subsidence rate of around 1.5 m over the first five
years after drainage, followed by a nearly constant rate of
5cmyr
1
in subsequent years, subsidence typically amounts
to 2.5 m in 25 years and could well be over 5 m within
100years if peat thickness allows it, even excluding the ef-
fect of fires. Such substantial lowering of the land surface in
coastal lowlands will affect drainability, as has been seen in
other parts of the world, and may cause loss of agricultural
production in many areas of deep peat in SE Asia unless an
alternative to gravity drainage would be technically and eco-
nomically possible. This issue has received little attention to
date, as the focus in the discussion on the future of peatlands
in SE Asia has so far been on carbon emissions and biodi-
versity loss. We recommend that the effect of subsidence on
drainability and agricultural production should be quantified
and considered in land use planning, as it is likely to affect
the longer-term economic viability of establishing agricul-
ture on peatlands.
Where plantations on tropical peatland are inevitable, in-
vestment in water management is needed to achieve water
levels that reduce subsidence and reduce carbon loss, while
at the same time protecting forest and peat resources in the
surrounding peatland. However the finding that considerable
carbon loss and subsidence occur even at the highest possi-
ble water levels in and around peatland plantations, leads to
the conclusion that carbon emission and land subsidence are
an inevitable consequence whenever tropical peatlands are
drained, regardless of management regime.
Acknowledgements. We thank the plantation companies, as well
SDWA (Singapore Delft Water Alliance) and the Deltares R&D
programme, for supporting different parts of the research. The
University of Jambi field team is thanked for data collection.
Edited by: D. Zona
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... Mid-and high-latitude peatlands comprise a mix of ombrotrophic and minerotrophic peatlands (bogs and fens) lowering of the water level and peat oxidation above the water level, which stimulates microbial decomposition and the release of C (as carbon dioxide (CO 2 )) to the atmosphere 49,50 . Initial subsidence can be rapid, accounting for surface lowering of up to 1 m in the first 2 years 50 . ...
... Initial subsidence can be rapid, accounting for surface lowering of up to 1 m in the first 2 years 50 . Subsequently, when decomposition is the dominant process, rates are in the range 3-5 cm yr -1 for plantation landscapes in Southeast Asia [50][51][52][53] , with a mean value of 2.2 cm yr -1 across all degraded and converted land covers 54 . Over time, drainage-driven subsidence changes peat dome morphology and reduces C storage capacity 55 . ...
... The reduction in the mean slope of the dome reduces water flow velocity in the drainage network and increases surface inundation risk during periods of high rainfall [56][57][58] . Over time, subsidence will bring the peat surface within reach of river flood levels or coastal high tides, increasing flooding duration and extent and the threat of saline intrusion 50,53,59 . Unlike in northern peatlands, pumped drainage to maintain low water levels for agricultural production will probably not be feasible in the tropics owing to economic and practical difficulties associated with installing and maintaining infrastructure capable of operating under high rainfall rates and flashy flows. ...
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Tropical peatlands store around one-sixth of the global peatland carbon pool (105 gigatonnes), equivalent to 30% of the carbon held in rainforest vegetation. Deforestation, drainage, fire and conversion to agricultural land threaten these ecosystems and their role in carbon sequestration. In this Review, we discuss the biogeochemistry of tropical peatlands and the impacts of ongoing anthropogenic modifications. Extensive peatlands are found in Southeast Asia, the Congo Basin and Amazonia, but their total global area remains unknown owing to inadequate data. Anthropogenic transformations result in high carbon loss and reduced carbon storage, increased greenhouse gas emissions, loss of hydrological integrity and peat subsidence accompanied by an enhanced risk of flooding. Moreover, the resulting nutrient storage and cycling changes necessitate fertilizer inputs to sustain crop production, further disturbing the ecosystem and increasing greenhouse gas emissions. Under a warming climate, these impacts are likely to intensify, with both disturbed and intact peat swamps at risk of losing 20% of current carbon stocks by 2100. Improved measurement and observation of carbon pools and fluxes, along with process-based biogeochemical knowledge, is needed to support management strategies, protect tropical peatland carbon stocks and mitigate greenhouse gas emissions. Tropical peatlands hold around 105 gigatonnes of carbon but are increasingly affected by anthropogenic activities. This Review describes the biogeochemistry of these systems and how deforestation, fire, drainage and agriculture are disturbing them. Tropical peatlands are important in terms of the global carbon cycle and in efforts to combat climate change, with a growing recognition of their potential role in natural climate solutions.Tropical peatlands occupy approximately 440,000 km2 across Southeast Asia, Central Africa and South and Central America, and are mostly forested. They are among the world’s most carbon-dense ecosystems with a belowground carbon stock of about 105 gigatonnes (Gt).Although tropical peatlands in Africa and in South and Central America remain largely intact, those in Southeast Asia have undergone widespread transformations owing to deforestation, drainage and agricultural conversion.Land-use changes result in rapid peat carbon loss, high greenhouse gas emissions, land subsidence, changes in hydrology and nutrient cycling, and an increased risk of fire.Management priorities include protection of the carbon sink function of intact forested peatlands; restoration of degraded, forested peatlands; and improved management of agricultural peatlands by raising water levels to mitigate carbon losses and greenhouse gas emissions.The response of tropical peatlands and their carbon stocks to anthropogenic warming and associated changes in hydroclimate remain an area of uncertainty. Tropical peatlands are important in terms of the global carbon cycle and in efforts to combat climate change, with a growing recognition of their potential role in natural climate solutions. Tropical peatlands occupy approximately 440,000 km2 across Southeast Asia, Central Africa and South and Central America, and are mostly forested. They are among the world’s most carbon-dense ecosystems with a belowground carbon stock of about 105 gigatonnes (Gt). Although tropical peatlands in Africa and in South and Central America remain largely intact, those in Southeast Asia have undergone widespread transformations owing to deforestation, drainage and agricultural conversion. Land-use changes result in rapid peat carbon loss, high greenhouse gas emissions, land subsidence, changes in hydrology and nutrient cycling, and an increased risk of fire. Management priorities include protection of the carbon sink function of intact forested peatlands; restoration of degraded, forested peatlands; and improved management of agricultural peatlands by raising water levels to mitigate carbon losses and greenhouse gas emissions. The response of tropical peatlands and their carbon stocks to anthropogenic warming and associated changes in hydroclimate remain an area of uncertainty.
... From the environmental point of view, controversies were triggered due to the large-scale land clearing in peat. Oil palm is not the only crop planted in peat, paper and pulp industry too cleared peatland for planting Acacia (Hooijer et al., 2012a;Paramananthan, 2016b). After each environmental calamity, especially the fire that led to haze during the El Niño years, pressure has been built on the oil palm plantations and now are under constant scrutiny by governmental and non-governmental organisations. ...
... Thereafter, the subsidence rate was further reduced to 2 cm to 4 cm per year due to proper water management practices. Hooijer et al. (2012a) in their study stated that the conversion of tropical peatlands to crop cultivation results in land subsidence and was considered as substitute measurement for the carbon dioxide emissions to the atmosphere. Their studies in Acacia and oil palm plantations in Indonesia mentioned that in the first year of draining water in peat resulted in 75 cm of subsidence and in the subsequent 4 years average subsidence rate was 16.75 cm per year. ...
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... Carbon emissions decreased with increasing VWC in both soil types; therefore, hydrologic restoration would be effective in reducing carbon loss from these soils. Several studies across myriad peatland types have reported that high water tables can limit peat oxidation (Holden, 2005;Hooijer et al., 2012;Moore and Dalva, 1993). In a study using similar apparatuses and procedures to the current experiment, while working in the Great Dismal Swamp approximately 8 km west of the study sites, Weiser (2014) found that VWC was inversely related to carbon emissions. ...
... The lower bulk density (0.44 g cm -3 ), higher organic matter content (96%), and deeper histic layer of the Histosol are all indicative of persistently wetter conditions in the Histosol site. Furthermore, drainage of the peatlands can induce subsidence that reduces peat thickness and increases bulk density (Hillman, 1997;Hooijer et al., 2012;Verry, 1997), as detected in the Ultisol site. Deep peat soils in extensively drained portions of the adjacent Great Dismal Swamp exhibited similar organic matter content (93%) and bulk density (0.16 g cm -3 , Thompson, Belcher, and Atkinson, 2003). ...
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Johnson, A.M.; Atkinson, R.B.; Steven, J.C.; Whiting, G.; Napora, K.; Sharrett, L., and Mirda, C., 2022. Determining carbon dioxide emission response in soil microcosms from a shallow mid-Atlantic peatland: The influence of water table restoration. Journal of Coastal Research, 38(2), 261268. Coconut Creek (Florida), ISSN 0749-0208. Determining carbon dioxide emission response in shallow microcosms from a shallow mid-Atlantic peatland: The influence of water table restoration. Peatlands provide ecosystem services such as pollution filtration, flood control, and carbon storage as these ecosystems contain 33% to 50% of the global soil carbon pool. Reestablishing soil saturation in degraded peatlands can limit microbial oxidation of organic matter; however, drainage history may alter soil saturation patterns and challenge restoration efforts. The purpose of this study was to investigate carbon emission response to water table restoration for two shallow peatlands including a Histosol and an Ultisol with divergent drainage histories. Soil cores were gathered from the top 22.5 cm of organic-rich surface horizons within Cavalier Wildlife Management Area in Chesapeake, Virginia. Cores were randomly assigned to four water level treatments that simulated restoration conditions, and carbon dioxide emissions were measured using an LI-6400XT portable open-flow photosynthesis system twice a week for 4 weeks. Despite higher organic matter content in the long-drained site, emissions (2.1 to 3.9 mol CO2 m2 s1) were higher than at the historically less drained site (1.5 to 2.3 mol CO2 m2 s1). Volumetric water content trends revealed that longer-drained soils retained lower water content within restoration treatments and were associated with higher carbon emissions. Restoration of long-drained peats in the mid-Atlantic region may require higher water tables than otherwise predicted to overcome a resistance to rewetting where limiting microbial soil respiration is a management objective.
... At the same time, high water contents facilitate leaching and diffusive transport of C from POM towards mineral sorption sites 14,15 . At the soil profile to landscape-scale, the strong influence of moisture on C storage manifests itself by the slow organic matter decomposition in permanently wet soils 16,17 and the fast C mineralization with increasing O 2 availability [18][19][20] . Laboratory incubation experiments underpinned the impact of water saturation and oxygen availability on C mineralization [21][22][23] . ...
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