CARBON STORAGE AND SEQUESTRATION
IN THE SOMERSET LEVELS, UK
Desk-based Assessment and Report
Prepared for
Somerset County Council
by
Prof. A. G. Brown BSc PhD FGS FSA
Project reference:2009/1
Version: November 2009, Final
Contact: Tony Brown, A G Brown Consulting,
North Lodge, London Minstead, Nr Lyndhurst,
Hants, SO43 7FT
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
Contents
1 Executive Summary................................................................................................ 1
2 Background............................................................................................................. 2
3 Data Sources and Methods .................................................................................... 2
4 Total Carbon Storage: The Approach & Methods ................................................... 3
5 Analysis & Results .................................................................................................. 5
6 Losses of Organic Carbon due to Peat Extraction .................................................. 7
7 Losses of Organic Carbon due to Oxidation and Climate Change........................ 10
8 Present and Potential Sequestration Rates Under Changing Land Use............... 12
9 Sensitivity of the System to Climate Change and Land Use Substitution ............. 16
10 Conclusions ........................................................................................................ 20
11 Recommendations.............................................................................................. 21
12 References ......................................................................................................... 23
Figures
Figure 1:Soil Survey of the Somerset Levels. Copyright Soil Survey of Great Britain.3
Figure 2:Part of the peat thickness map of the Somerset Levels based on survey by
Cope and Colborne (1981) ........................................................................................ 4
Figure 3: Peat extraction designations. Source Somerset County Minerals Planning
Authority..................................................................................................................... 8
Figure 4: The total carbon emission or removals across the U.K due to soil losses
calculated within administrative area. From Mobbs & Thomson 2005-2006............ 18
Tables
Table 1: Estimated total organic carbon stored in the Somerset Levels. ................... 6
Table 2: Estimated total organic carbon in the top 1m of the Somerset Levels. ........ 7
Table 3: Estimates of the organic carbon loss in tonnes ha-1 for a range of depths of
extraction. .................................................................................................................. 7
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
Table 4: Estimates of organic carbon loss using extant permissions and extraction
scenarios. .................................................................................................................. 9
Table 5: Peat wastage rates from Brown et al. (2003), Brunning (2003) and the resampling of the National Soil inventory by Bellamy et al (2005). ............................. 11
Table 6: Estimates of annual organic carbon loss using various estimates of peat
wastage (see text for explanation). .......................................................................... 12
Table 7: Flux estimates for land use conversion used in this study. Net losses of
organic carbon are in italics. .................................................................................... 14
Table 8: Estimates of the area required to offset 84,000m33 of peat extraction using
the data presented in Table 7. The extracted area is assumed to be left as a lake
with 0 flux................................................................................................................. 16
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
CARBON STORAGE AND SEQUESTRATION IN THE
SOMERSET LEVELS, UK: Desk-based Assessment and
Report
1 Executive Summary
This report is the first attempt to quantify the carbon storage and losses due
principally to peat extraction in the South Somerset Levels. The estimates are based
upon the peat survey of Cope & Colborne (1981), peat extraction figures and
planning designations from Somerset County Council Minerals Planning Authority
and analytical data from Brown et al. (2003). The analytical data is used to convert
peat volumes to organic carbon taking into account the carbon equivalence of the
peat allowing for non-organic components and peat density. The results suggest that
the total carbon storage of the Somerset Levels is approximately 10.9 M tonnes and
that carbon stored within the top 1m of peat, the most vulnerable to wastage by
erosion, amounts to approximately 3.3 M tonnes. At present about 1,400 tonnes
carbon is removed from Somerset’s peat carbon store per year due to peat
extraction, representing 0.01% of the total carbon stored. Using estimates from a
variety of sources the total carbon loss from peat extraction and peat wastage can
be estimated at 21,400 t C year-1 or approximately 0.2% of the total store per year.
Estimated carbon losses are given for a range of extraction depths and for the areas
of peat under planning designations and that have been worked out. Published
values and the LULUCF methodology is used to compare these rates with possible
sequestration rates from restoration and changes in land use. It is suggested that
offset could be achieved through changes from arable, grassland or open water to
commercial reed beds or wet woodland and/or the raising of water tables. This desk
study is a first approximation and the data used is incomplete. Firstly the peat survey
does not include all peat within the south Somerset Levels. Secondly average
depths and compositional figures have been used for the extraction areas, and
thirdly no analysis has been undertaken of the most likely sequestration rates for the
particular conditions of the Somerset Levels. Recommendations are made as to how
SCC could improve upon these estimates, reduce the error of estimation and provide
data suitable for informed planning policy concerning carbon budgeting for the
Somerset Levels area and its contribution to the County’s carbon budget.
1
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
2 Background
This report emanates from a need to inform planning considerations in relation to
minerals working and subsequent restoration strategy by Somerset County Council
(SCC). It is also pertinent to SCC’s carbon management strategy and its climate
change strategy. The Somerset Levels and Moors are the major soil-based carbon
store in the County and are also of national importance in relation to wildlife, cultural
heritage (including wetland archaeology; Brunning et al. 2000) and environmental
quality (English Nature 1997). It is worth noting that more than twice as much carbon
is stored in soils, as in vegetation or the atmosphere, and organic soils, such as the
Somerset peats make up the bulk of carbon soil stocks in the UK (Bradley 1997;
Bellamy et al., 2005). Given that the Somerset Levels and Moors is the second
largest area of lowland organic soils in the UK (the East Anglian Fenlands being the
largest), it follows that the Levels contain a carbon store of national importance. This
report provides the first estimate of the magnitude of this store and estimated carbon
flux rates under certain specified conditions.
3 Data Sources and Methods
The data used in this report comes from three sources. Firstly the most accurate
estimate available of the area and depth of peat. There are five peat maps available
for the Somerset Levels; the geological survey 1:50,000 Glastonbury map (Sheet
312), the 1:100 K Soil Survey Soil Map of the Somerset Levels (Figure 1) and three
peat maps based upon the Peat Depth Survey by Cope and Colborne (1981). This
survey was previously used to generate a peat depth map (Figure 2), a clay
thickness map and a peat type map. The data used here were the direct
measurements of peat thickness from this survey as supplied to the author by
Somerset County Council. This is supplemented by figures for peat extraction and
mapped areas of extraction designation as supplied to the author by Somerset
County Council Minerals Planning Authority. The second source of data is the survey
of the Somerset Levels undertaken by Brown et al. in May of 2003 for Somerset
County Council with funding from the Environment Agency (Brown et al., 2003). This
report provided data on peat stratigraphy, peat humification, reduction-oxidation
potential (Eh), bulk density (BD), loss on ignition (LOI), ultra-violet (UV) peat
reflectance and age-depth profiles for 8 of the major Levels of the Brue and Parrett
Valleys.
2
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
Figure 1:Soil Survey of the Somerset Levels. Copyright Soil Survey of Great
Britain.
This report has been used to estimate the carbon content from the peat survey
allowing the calculation of organic carbon (OC) storage. The third source of data is
published data on carbon sequestration rates for different land uses and land use
conversions from published literature.
4 Total Carbon Storage: The Approach & Methods
This study has used estimated organic carbon from the loss-on-ignition (LOI) data
and bulk-density (BD) data sets provided by Brown et al. (2003). The justifications
for this choice are listed below;
3
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
A) LOI is a robust and accurate measure of organic carbon from organic matter (OM)
for highly organic soils and peats (see further discussion below).
B) This is the largest dataset available for the Somerset Levels and has a good
geographical coverage.
C) The dataset includes both LOI and BD on the same cores allowing an accurate
adjustment for depth and tillage-related compaction and oxidation.
D) This methodology allows easy comparison with other studies of carbon storage
which have also used OC derived from OM.
Although this methodology will only estimate organic carbon storage this is
acceptable in this case for two reasons. Firstly the Somerset Levels contain very
little inorganic carbon in their sediments, due to the low occurrence of Mg and Ca
carbonates, as a result of both a lack of precipitation and due to leaching (Brown et
al, 2003). Secondly it is the organic component of the total carbon store that is
sequestered (net flux to storage) from the atmosphere and that can be lost in
oxidation or peat extraction.
Figure 2:Part of the peat thickness map of the Somerset Levels based on survey
by Cope and Colborne (1981)
4
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
4.1
Methods
Loss On Ignition: was measured using a standard procedure of ignition in a muffle
furnace for 4 hours at 550oC as recommended by Dean (1974). It is known that in
theory this can lead to inaccuracies through the loss of interstitial water from clays
above 400°C (Ball, 1964, Dean, 1974) and the decomposition of weak carbonates
(inorganic carbon in carbonate sediments). However, it has been shown to be
reliable for a wide range of organic soils and especially peats (Beaudoin, 2003) and
given the lack of clays and carbonates in most of the Somerset Levels peats it can
be regarded as just as reliable as wet oxidation using the modified Walkley-Black
method (Beaudoin, 2003) or the use of a carbon analyser.
Bulk Density: Bulk density was calculated in 1cm increments down the cores by
sampling a known volume (0.5 cm3) weighing it, drying it at 50 OC, re-weighing and
converting it to dry weight per cm3.
Conversion of Organic Matter to Organic Carbon: LOI has also been shown to be
closely related to OC with conversion factors ranging from 1.7 to 2.2 (Sutherland,
1998). Given the highly organic-rich nature of the Somerset Levels peats (Brown et
al., 2003) the commonly used ‘Von Bemmelen’ conversion factor was used of 1.72
which is equivalent to an assumption that 58% of the OM is OC (Broadbent, 1953;
Allison, 1965; Nelson and Sommers, 1996). This approach has been criticised by
Howard & Howard (1990) but even their study shows that the relationship between
topsoil OM and OC is linear and has an R2 of 0.98 and a slope (conversion factor) of
1.679. The highly organic nature of these peats, with low clay contents and no
carbonates makes this approach reliable in this case. It is quite likely that in many
parts of the levels the peat has a higher OC value than this conversion factor
assumes and so this can be regarded as a conservative estimate.
5 Analysis & Results
Organic-rich sediments have a high sensitivity to compaction and humification and
so mass accumulation rates have to be adjusted for these effects by using measured
bulk density as described above (Macaire et al 2005; 2006). The OM is then
converted into a volume of stored OC in metric tonnes using Eqn. 1.
OCs = (%LOI.BD.PV)/Cf
Eqn. 1
where BD is the bulk density (kg m3) PV is peat volume (m3) and Cf is the OM-OC
conversion factor. Both OC and BD have been derived from Brown et al. (2003) and
5
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
PV has been derived from the Peat Depth Survey by Cope and Colborne (1981).
This survey did not cover all of the area as it terminated at the constriction of most of
the tributary valleys. However, as the map shows the depths of peat in these valleys
was generally low, and is probably slightly lower in OM content due to clastic
sediments emanating from the valley catchments. However, there are significant
deposits, including deep peats upstream of Langport (South Moor; Aalbersberg
2000) and the middle reaches of the Brue Valley (Brunning pers. com.) which were
not included in the survey and therefore the PV used is an underestimate of the total
peat volume of the Somerset Levels. This is considered the largest source of error in
the analysis presented here and forms part of the recommendations for further work.
5.1
Total Organic Carbon Storage
For the total storage in the Somerset Levels (Table 1) an average of all the data
(LOI, BD) from all the profiles measured by Brown et al (2003) has been used. This
allows for the universally observed variation in bulk density within the top 0.4m
caused by oxidation and cultivation. The same procedure was followed for LOI which
is typically lower in the upper 15-20cm. The very high LOI below this depth (95-99%)
and the small random fluctuations of bulk density suggests that for the 1.5-2m these
estimates should be accurate and within the error estimation of the BD and LOI
measurements.
Av.
peat
depth
Av bulk
density
(n=506)
Av LOI
(n=506 )
Total area
of survey
g cm-3
%
km
(m)
3.28
0.0317
89.47
202.32
2
Est.
peat
vol.
M m3
663.61
Est. C
M metric
tonnes
10.94
Est.
error
of %C
± 5%
Table 1: Estimated total organic carbon stored in the Somerset Levels.
5.2
Total Carbon Storage in Top 1m.
The methods used in the calculation of the OC in the top 1m of the Levels (Table 2)
are identical to those in section 4 except that the data was averaged for the top 1m
of the cores analysed by Brown et al. (2003). This has allowed for the higher BD of
the top 1m of peat.
6
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
Peat
depth
(m)
1.0
Av LOI
Av bulk
density
(n=300)
g cm-3
0.0368
Total area
(n=300 ) of survey
km2
%
Est.
peat
vol.
85.75
202.32
202.32
M m3
Est. OC
M metric
tonnes
Est.
error of
%OC
3.71
± 5%
Table 2: Estimated total organic carbon in the top 1m of the Somerset Levels.
As can be seen from a comparison of Tables 1 and 2 approximately 30% of the total
organic carbon in the Somerset Levels is stored in the top 1m of peat. This figure is
important because the top 1m is most vulnerable to erosion and wastage. This figure
would be expected to remain approximately constant (as a ratio) even with any
revised estimate of the total area of peat as long as the average peat thickness is
around 3.3m.
6 Losses of Organic Carbon due to Peat Extraction
From the above estimates it is possible to calculate the organic carbon that would be
lost by removing peat from 1 ha depending on the thickness of peat available for
extraction (Table 3). The full thickness of peat soils are generally extracted but the
thickness of the peat soils is highly variable. Some sites have been worked for a
thickness of 0.75m and others have exposed up to 4m of peat. Typically peat sites
being worked today are 1 to 2m thick. Due to the uncertainties about variations in the
peat characteristic (LOI, BD etc.) in the areas which have been or could be extracted
and the relative uniformity of the OC content of the peat a standard conversion factor
based upon the above calculations has been used which is 1m3 peat in situ =
0.01646 tonnes of OC.
1m
2m
3.3m
4m
165
329
543
658
Table 3: Estimates of the organic carbon loss in tonnes ha-1 for a range of depths
of extraction.
Somerset Peat is currently extracted at a rate of around 84,000 m3 per year
(Minerals Extraction in Great Britain, Business Monitor PA1007, Office National
Statistics). This converts to 1,400 tonnes carbon per year, equivalent to an annual
loss of 0.01% of the total organic carbon store.
7
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
Peat Production Zone (PPZ)
Area of Search identified in Minerals Local Plan
Extraction unlikely/already completed – in ownership of conservation body,
eg. RSPB, Natural England
Future extraction possible – valid planning permission
Extraction current
Permission lapsed, expired – future extraction unlikely/ extraction already
completed
Extraction unlikely
Extraction unlikely – within Area of Search but also environmental
designated land
Figure 3: Peat extraction designations. Source Somerset County Minerals Planning
Authority.
Maps supplied by SCC in MapInfo format were used to estimate the areas of the
Peat Production Zone, current working areas with planning permission, areas with
planning permissions not worked currently but likely to be worked in the future, areas
worked out and completed, and areas now owned by conservation organisations
8
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
such as Natural England and RSPB which have largely been worked out. These
areas are all shown in Figure 3 above.
Planning permissions currently being worked (green) cover an area of approximately
275ha which includes land both inside and outside of the PPZ boundary. A further
30ha are covered by a valid planning permission (orange) and may therefore be
worked in the future.
Extraction scenario
or policy control
Estimated OC
equivalent
Peat volume
m3
metric tonnes
Estimated
fraction of the
total C reserves
of the Somerset
Levels
0.32%
Extant permissions
Total (October 1999)
2,150,000
35,000
Approximate volume of
peat extracted 1999 2008
900,000
15,000
Estimated extant
permissions end of
2008
1,200,000
19,000
Cradlebridge
application
328,000
5400
0.005%
Current extraction rate
85,000 yr-1
1,400 yr-1
0.01%
Extraction rate
scenario
25,000 yr-1
411 yr-1
0.004%
Table 4: Estimates of organic carbon loss using extant permissions and
extraction scenarios.
From figures and the maps supplied by SCC and extraction volumes provided by the
ONS Business Monitor Report PA1007 it is possible to estimate the OC storage
contained in the total extant permissions and the annual rate of loss of carbon
attributed to peat extraction at the current rate. An extraction rate scenario of
25,000m3 peat per year is also provided which is the volume of supply if the UK
Biodiversity Action Plan target of 90% total growing products market to be non-peat
9
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
by 20101 assuming the growing products market is around 6.8Mm3, around 40% of
the extracted peat is UK sourced and Somerset continues to provide 9% of the UK
portion of peat (Table 4).
A recent application for peat working at a site called Cradlebridge anticipated
extracting a volume of 328,000m3 peat within an area of around 27ha. This
represents an average peat thickness of 1.2m of peat available for extraction over
the full area of the proposed site or around 12,000m3 peat per hectare.
Based on the extant permissions in October 1999 and subsequent sales, around 1,2
Mm3 peat permitted for extraction remained at end of 2008, equivalent to a carbon
store of approximately 20,000 tonnes. These figures could be used to estimate the
amount of carbon sequestration required to offset future peat extraction.
7 Losses of Organic Carbon due to Oxidation and Climate Change
Organic-rich soils can lose carbon through both aerobic decomposition and
oxidation, both of which are major components of peat wastage, and through
leaching. Rates of peat wastage have been calculated for the Somerset Levels using
age/depth estimated truncation rates by Brown et al. (2003) and these are given in
Table 5. The figures used here are based on the 1775 assumed enclosure date and
exclude West Sedgemoor as this was the only site not to have shown significant
wastage since enclosure. The reason for taking the earliest date for enclosure is that
this assumes that wastage has occurred over a longer rather than shorter period and
is likely to produce a minimum estimate of the true wastage rate.
1
UK Habitat Actions – Lowland Raised Bog, “Undertake and promote research and
development of sustainable alternatives to peat to speed up reduction of peat used in both
amateur and professional markets. Aim for a minimum of 40% of total market requirements
to be peat-free by 2005 and 90% by 2010.”
10
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
Author
Method
Mean/median
Wastage Rate
Wastage rate
Range
mm yr-1
mm yr-1
Brown et al.
(2003)
age/depth rate (with
enclosure
assumption) based
truncation method
4.74 (mean)
0.87-8.70
Brunning (2003)
peat wastage pins
6.15 (median)
4.4-7.9
Bellamy et al.
(2005)
OM & OC
measurements over
25 years
> 12
-
Table 5: Peat wastage rates from Brown et al. (2003), Brunning (2003) and the
re-sampling of the National Soil inventory by Bellamy et al (2005).
This can be compared with the rates of carbon loss that have been estimated for
England and Wales derived using National Soil Inventory of England and Wales
(Cannell et al. 1999) as re-sampled by Bellamy et al. (2005) using their rates for
organic soils (>300 g kg-1). Their results strongly suggest that the rise in
temperatures over the last 25 years is the main cause of carbon loss and that the
losses have been greatest for organic-rich soils and peats. Using their figures an
estimated loss for the Somerset levels can be calculated (Table 6). However, there
are several reasons for believing that this is probably a significant overestimate.
Carbon content is exponentially related to depth in most soils, but not in peats and
so a more accurate calculation has been made by applying their estimated rates of
change by area rather than by volume and then expressing it as a percentage of the
total carbon volume. This may still be an over-estimate as there are strong reasons
to believe that carbon loss is largely due to increasing decomposition in the aerobic
horizons of the profile and that if soils are saturated then losses are greatly reduced
(Kechavarzi et al. 2007), although raising water levels can increase methane
production this does not decrease soil carbon significantly (Lloyd pers. com.). This
dependency on the depth to water table is probably the reason that Bellamy et al.
(2005) found no further increase in the rate of loss from soils with over 300g kg-1
carbon content. It follows that the maintenance of high water-levels in the moors
and levels will reduce, or at least minimise, losses due to future climate change and
this has been confirmed by the experimental data from West Sedgemoor reported in
Kechavarzi et al. (2007) and Tadham Moor reported by Lloyd (2006, 2009). This is
11
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
also an assumption made by the Intergovernmental Panel on Climate Change
(IPCC) in their assessments using LULECFS (see below).
Average adjusted
loss rate (from
Brown et al 2003)
Peat wastage rate
estimated by
Brunning (2003)
x 1000 tonnes yr-1
x 1000 tonnes yr-1
1.56
14.6-26.3
Using estimate of OC loss
from National Soil resampling (Bellamy et al.
(2005)
x 1000 tonnes yr-1
18.78
Table 6: Estimates of annual organic carbon loss using various estimates of peat
wastage (see text for explanation).
Using a value of 6mm yr-1 a mean of the estimates by Brown et al. (2003) and
Brunning (2003), and similar, although lower than the rates estimated by Kechavarzi
(2009) this would represent a carbon loss of approximately 20,000 tonnes yr-1.
As discussed above the presence of a high water table could limit carbon losses and
therefore as the levels and moors lower due to wastage and erosion there is a
minimum level set by the water table below which wastage will not continue if the
water table remains at a constant level.
8 Present and Potential Sequestration Rates Under Changing
Land Use
Changes from one land use to another will result in changes to the carbon stocks
over time whereas no land use change will represent a steady state if the organic
content of the soil is not increasing (for example if no peat is forming) and the
organic production is not being used as a substitute for fossil fuels (according to
Kyoto rules and interpreted by the UK Government (DECC 2008). This is a
complicated issue and at present there is not clear guidance and so it must be
assumed that peat is a biofuel and so the calculations are based upon this
assumption and the land use that is in place prior to extraction as per LULUCF (see
Couwenberg undated for further discussion). Therefore the simplest approximation
to the sequestration of carbon into the Levels is based upon the OM/carbon
concentration of sediments and the flux rate under steady state conditions (e.g. no
climate change). The background to, and methodology for accounting this change is
provided by the IPCC Good Practice Guide (GPG IPCC 2003) for land Use, Land
Use Change and Forestry (LULUCF). Land use changes are reported to IPCC as
changes to or from any of 6 land use classes which are; A. Forestry, B cropland, C
12
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
grassland, D wetlands, E settlements and F other land. The Other land (F) class
contains open water (lakes, rivers etc.) and rocky areas and it is assumed that they
are C neutral (Mobbs & Thomson, 2005-2006; CEH 2008). These categories have
been used as comparisons with the net effect of peat extraction. This is a generic
approach and a few comments are warranted on the assumptions made and how
appropriate it is in the Somerset Levels context.
In essence the Somerset Levels are a series of wetlands that formed over the last
10,000 years (the Holocene) along a number of low-lying river valleys. The role of
wetlands, and floodplains, as net carbon sinks is indicated by the preservation of
organic material within wetland or floodplain fills. Alluvial wetland and floodplain
peats generally originally deposited in rheotrophic-Eutrophic fens, are typically 40-90
% organic matter (OM) whereas overbank silt clay deposits are typically 2-4 % OM
and ombrotrophic peats are typically 90-99% OM (Macaire et al 2006). As has been
shown by Brown et al. (2003) and palaeoecological studies (Housley, et al. 2000;
Aalbersberg, G. 2000) the peats of the Somerset levels are transitional between
alluvial-rheotrophic peats and ombrotrophic peats due to the existence of at least
two areas of raised mire in the Holocene – in the central Brue Valley and in the
central-north Parrett Valley (Housley et al. 2000).
Today the Somerset Levels are a mosaic of grassland (improved and unimproved),
open reed/sedge dominated fen and very small areas of wet woodland dominated by
alder (Alnus glutinosa) and willows (Salix sp.). The carbon uptake of grasslands is
dependant upon nitrogen availability typically varying between 2 and 6 tonnes of C
ha yr-1 (Luscher et al. 2004). Alder woodland can sequester 5 to 10 t C ha yr-1 and
sedge fen up to 20 t C ha yr-1 (Ramade 2003). These figures are significantly higher
than the in-situ C storage rates for wetlands soils such as for the present Rhine
floodplain of 0.05 to 0.17 t C ha yr-1 (Hoffman and Glatzel 2007). This is probably
due to the lack of incorporation into the estimate of CO2, CH4 and dissolved organic
carbon (DOC) losses from these environments (Sutherland 1998). Theses estimates
and the transition estimates used by LULUCF are used to provide estimates of
sequestration rates for other land uses in the Somerset Levels (Table 7).
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Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
Table 7: Flux estimates for land use conversion used in this study. Net losses of
organic carbon are in italics.
LULUCF Land Use Flux or
Class
Land Use Transition
flux
LULUCF
kg m2
flux estimates
sources and
comments
OC flux
Luscher et al.
2004
+2-6
(Thomson
et al.
2008)
C
Cropland or C neutral
LU to improved
grassland
+23
Hofffman &
Glatzel. 2007
tonnes ha
yr-1
(+0.05-0.17)
2-3
MAFF (NZ)
A
Cropland or neutral LU
to wet woodland
(Alnus)
+25
Ramade 2003
+5-10
F
Grassland with high
water-table (saturated)
-
assuming steady
state and no peat
growth and no
wastage
0
F
fishing lake
0
F*
commercial reeds beds
Ramade 2003
+5-20
Sphagnum peat bog
(theoretical)
+0.20-0.37
20-37 g m-2 yr-1
(Clymo et al 1998)
-
40-70 g m-2 yr-1
(Cannell et al.
1993)
+0.40-0.70
Sphagnum/herbaceous peat growth estimated
from the average peat
accumulation rate
0.75 mm yr-1
(Brown et a.
2003)
B
Ploughing of grassland
– with low water-table
-23
2.3% C store loss, -3.0
or 0.3 kg m-2
(Hargreaves et al.
2003)
C*
Grassland with peat
extraction
-
using this study
(with average
peat depth) and 0
grassland net flux
-541
peat wastage
-
Brown et al.
(2003)
-0.08
Brunning (2003)
14
+0.12
-0.72-1.30
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
As can be seen from Table 7 there is considerable variation in the estimates of flux
rates from different land uses and therefore from land use conversion. This is both
due to differences in the method of estimation and also the high degree of variability
in the processes involved as discussed below.
In theory standing freshwater can act as both a sink and source. It can act as a sink
through the uptake of bicarbonates in water by aquatic organisms and direct fixation
of atmospheric CO2 by aquatic plants and by algae (including phytoplankton). This
carbon then may accumulate at the bottom of the pond or lake and can amount to a
sequestration rate of 0.15 t C ha yr-1 (Campbell et al. 2000). The extreme example of
this is the peat Norfolk Broads with are Medieval peat cuttings filled with organic-rich
lacustrine sediments. However, this carbon accumulation is dependant upon the
biological productivity of the water and this is controlled by water chemistry and
depth. It may also not be desirable for other reasons that restoration ponds be filled
with algae as it can cause a nuisance and poison both fish and animals. For several
reasons therefore the restoration of peat workings to lakes or ponds in the Somerset
Levels is unlikely to cause significant carbon sequestration and so the assumption in
LULUCF of water being carbon neutral is justified.
Reed beds either fringing water bodies or infilling old workings are known to
sequester carbon from the atmosphere, although there is considerable variation in
the estimates of the magnitude of this flux from 5 to 20 t C ha yr-1 (Table 7). If the
reeds are cut or cropped the result in carbon accounting terms depends upon their
eventual use, if used for thatching or mulching then that cropped component
becomes a carbon source, but if it is used for biofuel effectively replacing fossil fuels
or compost which replaces a peat product, then that is counted as a carbon offset.
In the case of grasslands the rate of carbon sequestration is known to be sensitive to
drainage and nitrogen supply (Kechavarzi et al. 2007; Van Groenigen, et al. 2006)
and a net effect will only be gained on conversion (from a neutral or negative flux
land use) unless the crop is taken and used for biofuel effectively substituting for
fossil fuels.
In the case of arable land and ploughing the figures are variable as it is believed to
vary with the organic content of the soil and so in highly organic peats would be at
the highest rates. It is not possible, however, to use the estimates based upon a
percentage of the carbon stock in the soil, because in deep peatlands, unlike most
15
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
soils, most of the OC lies beneath the plough zone and so the results would be far
too high. It is more realistic to take measured absolute flux rates and regard them as
minimum estimates. There has also been recent work on the effect of tillage and soil
erosion which suggests that the net effect of soil erosion is probably neither a source
or sink (Van Oost et al. 2007).
In the case of peatlands, variable estimates would be expected due to the variation
in peat types (upland/lowland, herbaceous/wood/Sphagnum etc.) and the
observations that CO2 exchange can vary with temperature (Hargreaves et al. 2003)
and water-table conditions (Worrall, et al. 2003). Additionally the flux of CH4 from
peatlands is also temperature and water table sensitive (Best and Jacobs 1997; Van
den Pol-Van Dasselaar et al. 1999).
9 Sensitivity of the System to Climate Change and Land Use
Substitution
From the figures presented above an approximate area of conversion required to
compensate for current rates of peat extraction can be calculated (Table 8). Due to
the variation in estimates of sequestration and the comments made previously these
figures must be regarded as preliminary estimates and only indicative.
Land use conversion
Minimum OC Maximum
flux (tonnes OC flux
ha yr-1)
(tonnes ha
yr-1)
Estimated required
conversion to offset
current annual peat
extraction rate (ha)
Arable or open water to
reed bed/sedge fen
5
20
70 - 280
Grassland to reed-bed
3
18
80 - 460
Arable to wet woodland
5
10
140 - 280
Grassland to wet
woodland
3
8
170 - 460
Arable to grassland
2
6
230 - 690
Arable to peat land
0.12
0.4
1350 - 4500
Table 8: Estimates of the area required to offset 84,000m3 of peat extraction
using the data presented in Table 7. The extracted area is assumed to be left as
a lake with 0 flux.
16
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
From Table 8 it can be seen that using these estimates the best option for carbon
offsetting, in order to minimise land conversion, would be to encourage conversion of
arable land or grassland (drained), to reed bed or sedge fen. This would require
conversion of a minimum of 70ha. Based on the Cradlebridge application site where
approximately 12,000m3 peat would be extracted from 1 hectare of land, an
equivalent of 7 hectares are currently extracted annually, i.e. at best 10 hectares of
land is required to be converted to reed beds for every 1 hectare of extracted peat.
The very high areas of conversion to Sphagnum bog or peatland accumulation that
would be required reflect the low measured carbon net storage flux (sequestration)
of peats, although these rates are probably unrealistically low in comparison with
favourable conditions, which might maximise peat growth. The rate of peat growth
has varied over the years but if ideal conditions were created on the Levels and
Moors a rate of just over 1mm/yr could be achieved. More research is required into
the on-site mitigation of peat extraction, through Sphagnum peat re-growth, as it is
clearly a highly attractive option if it can be shown to be effective.
In the absence of better figures for peat re-growth the best option from a carbon
sequestration perspective is clearly to restore workings to reed-beds (e.g.
Phragmites) but only if the reeds are left to decay in situ or, if they are cropped, the
crop used as biofuel to replace fossil fuel sources or used for reed compost and
therefore replacing a peat product. In theory the use of reeds for thatching or for
mulching would, over a cut-cycle, have no net effect on carbon storage and so not
represent any mitigation of the carbon loss due to extraction. As mentioned
previously carbon offset is only accounted for on conversion although future carbon
offset may be achieved depending on future land management.
17
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
Figure 4: The total carbon emission or removals across the U.K due to soil
losses calculated within administrative area. From Mobbs & Thomson 20052006.
From the analysis above comparisons can be made with the National Inventory
estimates for countywide carbon losses due to soil loss. Somerset is estimated to be
in the category of total 15 to 25 t C km-2 emissions (from Mobbs & Thomson, 20052006; Figure 4) which is equivalent to a loss of approximately 70,000 t C yr-1. The
effect of peat extraction at current rates of 84,000 m3 yr-1 when averaged over the
area of the Levels and Moors is 7 t C km yr-1. Over the County as a whole it
contributes approximately 2% of the County’s carbon emissions through soil losses
18
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
although the area worked per year is only around 0.001% of the county. The losses
through soil wastage and peat extraction in the Levels and Moors accounts for
around 30% of the county’s soil carbon losses whilst only covering 6% of the county.
There is an opportunity for the offsetting of carbon loss due to peat extraction by
changes to land use within the Somerset Levels or elsewhere (within the same
spatial carbon accounting unit) such as conversion of grassland or arable to
commercial reed beds. However, a strong note of caution is required. Firstly the
methodology and scientific basis upon which many of the current carbon accounting
procedures is based is questionable and will be the subject of revision in the next
few years. A good example of this is revision of the role of tillage and soil erosion in
the carbon storage and flux of arable fields (Van Oost et al. 2007). The assumption
made by LULUCF of open water bodies being carbon neutral is also questionable
and dependant upon the management and biological productivity of the resultant
water bodies. Secondly any mitigation through land use change may have other
effects that could easily outweigh the advantages in carbon accounting terms. The
most attractive option is clearly to maximise the carbon capture of in situ peat since if
any carbon accumulation above that for the surrounding land use would represent a
direct reduction of the carbon loss and over a number of years could mitigate a
proportion of organic carbon lost from the Somerset Levels and emitted to the
atmosphere.
19
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
10 Conclusions
This report presents the first estimation of carbon stocks and losses (emissions) due
to past and present peat extraction in the Somerset Levels and Moors. Estimates
have been based on the available peat area data, known areas and volumes of
extraction and conversions of peat to organic carbon using analytical data from a
comprehensive survey of the peats of the Levels by Brown et a. (2003). The results
indicate that the rates of extraction are small in relation to the total carbon stored in
the levels but do, along with wastage rates, represent a significant net carbon loss.
Combined extraction and wastage amount to about 21,400 tonnes yr-1 or about 0.2%
of the total storage in the Levels. The extraction component (approximately 1,400
tonnes yr-1) could be offset by conversion of other areas of the Levels, or elsewhere
in the County, to land uses with higher carbon storage, in vegetation or in soils; or
under the Kyoto rules, the use of biomass to offset fossil fuel usage. The best option
to minimise land use change required to offset, based on all the available data,
would be conversion to reed bed from arable land or water bodies. Using current
extraction rates this would require at least 70 ha per year to be converted, around 10
times the area of land extracted for peat. Although not part of this report
observations that the rate of carbon loss form organic soils, and probably the overall
peat wastage rate, is sensitive to climate change could significantly increase the loss
of carbon from the peat store, especially in drained areas outside the zones of watertable management. Unlike peat extraction which generally removes the full thickness
of peat including below the water table, peat wastage and erosion is limited by the
depth to the water table. In West Sedgemoor where high water tables have been
maintained this has been the case.
It must be noted that all the estimates in this report are based upon average depth
assumptions and approximate extraction rates. Further refinement is required of
several components of the carbon budget, and a more complete analysis of
appropriate carbon land use conversion estimates, as recommended below, in order
to provide estimates which could be used to provide a firm policy basis.
20
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
11 Recommendations
This report has prompted a number of technical recommendations for further work
which Somerset County Council could consider in order to improve its estimates of
carbon flux and peat extraction.
1. One source of potential inaccuracies could be reduced by producing a Somerset
Levels OM to OC carbon relationship and conversion factor. This would be possible
using a C auto-analyser and cores archived at the University of Southampton from
the Brown 2003 survey augmented by cores from the current and proposed
extraction areas.
2. It would be possible to make more accurate estimates of the carbon loss through
peat extraction by combining the extraction areas (worked, or being worked or with
permission to work) with the peat depth mapping of Cope and Colborne (1981). This
would involve re-interpolating their data and then overlaying with the relevant area
boundaries and the calculation of volumes from the two layers. This would be
possible using ArcGIS.
3. The peat survey upon which this report is based (Cope and Colborne, 1981) did
not cover all the areas of known peat in the Somerset Levels. Data from various
sources including Aalbersberg (2000), Housley et al (2000) and Brown, et al (2003)
indicate other areas of significant peat preservation. It would be possible to improve
the peat survey using both these data (and other stratigraphic data) and targeted
peat depth coring in order to provide a more complete and accurate estimate of peat
volumes. This would be required in order to produce a complete estimate of peat
within the South Somerset Levels and therefore more reliable estimates of the
relative carbon loss to store ratio.
4. The production of accurate estimation of possible carbon off-setting by land use,
and land management changes, in the Somerset Levels using specified areas. This
would involve the modelling of the current situation using contemporary land use
data, or generating that data from remote sensing, combined with the areas of
potential change.
5. A more detailed assessment of the alternative methods of restoring worked areas
in order to maximise carbon sequestration. The alternatives to be evaluated would
21
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
include, Sphagnum re-growth, carbon (OM) burial, the afforestation with wetland
trees, reed-beds (of various densities) and open water.
Work on these five recommendations would allow Somerset County Council to
evaluate more fully the aggregate effect of peat extraction on the county’s carbon
budget and what mitigation or other matters might be taken into consideration in both
minerals and land use planning.
22
Carbon Storage and Sequestration in the Somerset levels, UK: A. G. Brown
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