Vietnam Journal of Earth Sciences, 38(4), 317-326, DOI: 10.1562 50866-71873848713 [630626]

Vietnam Journal of Earth Sciences, 38(4), 317-326, DOI: 10.1562 5/0866-7187/38/4/8713
317
(VAST)
Vietnam Academy of Science and Technology
Vietnam Journal of Earth Sciences
http://www.vjs.ac.vn/index.php/jse
Quantifying organic carbon storage and sources in
sediments of Dong Rui mangrove forests, Tien Yen
district, Quang Ninh province using carbon stable isotope
Pham Thao Nguyen, Nguyen Tai Tue*, Tran Dang Quy, Nguyen Dinh Thai
Faculty of Geology, VNU University of Science
Received 1 June 2016. Accepted 20 August 2016
ABSTRACT
The objective of this study is to quantify the organic carbon ( OC) storage and its sources in sediments of Dong
Rui mangrove forests, Tien Yen district, Quang Ninh province by analyzing TOC (total organic carbon), TN (total
nitrogen), C/N ratios, δ13C isotopes and sediment grain sizes. The results showed that th e fine-grained sedimen t
fraction (<63 µm) ranged from 8.58 to 82.10%; TOC, TN contents, C/N ratios and δ13C values ranged from 0.21 to
10.20%, 0.01 to 0.34%, 15.07 to 46.09 and –27.75 to –25.84‰, re spectively. The variation of δ13C and C/N ratios,
and the correlation between TOC, TN and the fine-grained sedime nt fractions indicated that mangrove forests play
important roles in OC sequestration and accumulation of fine-gr ained sediments. The OC storage in sediments varie d
from 16.7 to 78.3 MgC ha-1 with an average of 57.2±14.9 MgC ha-1. Nowadays, the mangrove forest area in Dong
Rui is about 2194.1 ha, thus, the total OC in sediment pool up to 45 cm in depth, contained 1.26(±0.3)×105 MgC,
equivalent to 4.6(±1.2)×105 Mg CO 2. These results demonstrated that the conservation of Dong Rui mangrove fores t
not only provides opportunities for coastal protection and disa ster mitigation and other provision values, but also
enhancing carbon seque stration and offsetting greenhouse gas em issions.
Keywords: mangrove forest, sediment, organic carbon, stable isotope, Dong Rui.
©2016 Vietnam Academy of Science and Technology

1. Introduction1
Mangrove forests distribute in the
transitional zone between the continent and
the sea in tropical and subtropical regions.
Vietnam has a coastline of 3,260 km and large river deltas, estuaries that provide favorable conditions for mangrove development.
                                                            
*Corresponding author, Email: [anonimizat]   Mangrove forests of Vietnam provide many
ecosystem functions and services such as
mitigation of natural hazards (i.e., typhoons,
floods and coastal erosion), supply food
sources for a large variety of invertebrates, fish and waterfowls, and contribute to
livelihoods for coastal communities (Alongi,
2011). Despite their importance to economic
and social development, mangroves have been
destroyed at an alarming rate since 1980s.

Pham Thao Nguyen, et al./Vietnam Journal of Earth Sciences 38 ( 2016)
318 Nearly 25% of the natural mangrove forests
have been lost as a result of human
intervention such as urbanization,
development of aquaculture and agriculture
(Spalding et al., 2010). Subsequently, the decline of mangrove forests has caused
adverse impacts on the environment, for
example, greenhouse gas emissions (Donato
et al., 2011; Lovelock et al., 2011),
biodiversity degradation (Nguyen et al., 2014),
and increased the damage by coastal erosion and loss of livelihoods of people in coastal areas (Richard et al., 2011).
Due to fast urbanization and industrial
development, the greenhouse gases (GHG) emissions are reported to increase with larger a m p l i t u d e ( R i c h a r d e t a l . , 2 0 1 1 ) . I n o r d e r t o restrain increase in GHG, concentrations in the atmosphere, the Kyoto Protocol (1997) and Paris Agreement on Climate Change (2015) were adopted by the international
community committed to reducing GHG emissions. These agreements become the legal basis for the implementation of the GHG reduction by using various solutions and mechanisms. In which, Clean Development Mechanism allows developed countries to achieve their targets on reducing GHG emissions through investigation of reforestation projects in the developing
countries.
Mangrove forests are suitable place to
sequester a larger carbon amount than the
other forests (Christensen, 1978) and
considered as crucial carbon sinks in globally
coastal systems (Kristen sen et al., 2008; Ong,
1993). On average, one hectare of mangrove
forests contains approximately 470 Mg C. W i t h a t o t a l a r e a o f 1 3 9 , 1 7 0 k m
2 globally,
m a n g r o v e f o r e s t s c a n s t o r e a b o u t 6 . 5 P g C ,
equivalent to 24 Pg of CO 2e (Siikamäki et al.,
2012). Particularly, a large proportion of
carbon storage is preserved in sedimentary
stratum (Donato et al., 2011; Nguyen et al., 2014). Therefore, the sedimentary organic
carbon (OC) plays an important role in the global carbon cycle and reducing GHG
emissions. However, study of the
quantification of OC storage and sources in
m a n g r o v e s e d i m e n t s h a s n o t b e e n w i d e l y
conducted in Vietnam. Therefore, the objectives of the present study are to quantify
OC storage and to examine the sources of OC
in sediments of Dong Rui mangrove forest
area, Tien Yen District, Quang Ninh province.
The outcomes will provide crucial
information for understanding the role of mangrove forests in sequestration of OC and offsetting GHGs.
2. Materials and methods 2.1. Study area
The present study was carried out in Dong
Rui mangrove forests, Tien Yen district,
Quang Ninh province. The Dong Rui
commune is located between Voi Lon and Ba
Che creeks (Figure 1). The area of Dong Rui commune is about 5,000 ha, of which
2194.1 ha are covered by mangrove forests.
The mangrove forests are unique primeval in
Vietnam. The floristic compositions dominate
by Bruguiera gymnorrhiza , Rhizophora
stylosa , Kandelia obovata , Avicennia marina ,
Aegiceras corniculatum w i t h t r e e ' s h e i g h t i s
l e s s t h a n 6 m , t h e m a x i m u m d i a m e t e r b r e a s t height ≤25 cm. The mangrove forests are obviously high in biodiversity level,
ecological functions and environmental
protection, on the one hand, and providing various economic values, directly supporting the livelihood for local communities (i.e., shrimp, bivalves, crabs, peanut worm, etc.), on the other.
2.2. Field sampling
Fieldwork was conducted from 24 to 29
March, 2016 in Dong Rui mangrove forests.
Three sediment cores were collected from
different mangrove forest types, consisting of the fringe mangrove forests along Voi Lon
creek (core C1), the mangrove forest in isle of

Vietnam Journal of Earth Sciences, 38(4), 297-306
319 Voi Lon River (core C2) and the mangrove
forest in downstream of Ba Che River adjacent to Mui Chua (core C3) (Figure 1).
Sediment cores were taken by a hand corer
with a PVC inner tube (30 cm in length and
10 cm diameter). The lengths of sediment
cores C1, C2, C3 were 45 cm, 32 cm and
40 cm, respectively. Immediately following collection, sediment cores were caped and
stored in upright position and maintained in
cool condition. Sediment cores were
processed within 12 h of collection by first
r e m o v i n g t h e o u t e r c r u s t ( 0 . 5 c m i n
thickness), then slicing into 3 and 5 cm intervals for the core depths intervals of 0-15
and 15-45 cm, respectively. The sediment
samples were packed in labeled polyethylene
b a g s f o r f u r t h e r a n a l y s i s . A l l s e d i m e n t
samples were maintained on ice in coolers and
transported to the laboratory where they were frozen at –20°C until further processing and
analysis.
Samples identifying the potential of OC
sources for mangrove sediments in the study
area were collected, consisting of five fresh
mangrove leave samples and five marine
phytoplankton samples (POM). The samples
were analyzed for C/N and stable carbon
isotope δ
13C values. Mangrove leaves were
carefully rinsed with distilled-deionized water
soon after collecting to remove any extraneous
m a t e r i a l s . P O M s a m p l e s w e r e c o l l e c t e d b y
fi l t e r i n g 0 . 5 – 1 . 0 L o f s u r f a c e w a t e r ( 0 – 2 m i n depth) through pre-combusted (at 550°C) 47 mm Whatman GF/F glass fiber filters using a
pressure pump. Filters were lightly rinsed by
distilled-deionized water to remove salt residue,
and stored on iceboxes then transported to the laboratory for further processing.
 
Figure 1. Sampling sites in Dong Rui mangrove forest

Pham Thao Nguyen, et al./Vietnam Journal of Earth Sciences 38 ( 2016)
320 2.3. Sample preparation and analysis
2.3.1. Sediment grain size analysis
Sediment grain size was analyzed using an
automatic laser diffraction particle size
analyzer LA-950V2 (HORIBA Co.). After booting 30 minutes, a computer program of
the LA-950V2 was operated. About 1g of
fresh sediment sample was loaded into the machine chamber. After measurement, results
of sediment grain size compositions, were
printed or exported as pdf or csv files.
2.3.2. Organic matter analysis
W e t s e d i m e n t s a m p l e s ( 1 0 – 2 0 g ) w e r e
weighed in porcelain cups and dried in an oven at 60°C, then ground in an agate mill.
During grinding, extraneous materials,
including tree branches, roots and inorganic
matter were manua lly removed.
The organic matter (OM) content was
obtained via loss on ignition measurement. Two grams of the pulverized sediments was
fi r s t d r i e d a t 1 0 0 ° C i n a d r yi n g o v e n f o r 2 h
a n d t h e n h e a t e d a t 5 5 0 ° C i n a t e m p e r a t u r e –
monitored muffle furnace for 5 h. OM content is calculated as the difference between the
weight from before and after combustion at
550°C divided by the initial sample weight times 100%.
2.3.3. Total organic carbon, total nitrogen
and stable isotope δ
13C analysis
A total of 0.2 g of pulverized sediment
sample was placed in an Eppendorf tube, added about 3 ml of 1N HCl, mixed
thoroughly using a vibrating mixer, then left
at room temperature for 24 h to remove carbonates. After acid treatment, the samples
were repeatedly rinsed with 4 ml milli-Q
water, mixed in an ultrasonic bath, then centrifuged to separate from solutions. This
process was repeated four times before being dried in an oven at 60°C for 48 h. About 10
mg of the treated sediment sample was taken
and wrapped into a tin cup sized 6×4 mm.
S t a b l e i s o t o p e δ
13C , T O C a n d T N v a l u e s
were measured using an elemental analyzer
(EA-EuroVector) connected to a stable
isotope ratio mass spectrometry system (IRMS) (Nu Perspective). The samples were
placed into the auto-sampler tray and
introduced into the combustion chamber. In the combustion chamber, the sediments are converted into CO
2 a n d N O 2. The gases are
sent to reduction column for making pure CO 2
and N 2 gases and separated by a gas
chromatographic column and carried to the
IRMS system. Here, the gases are ionized,
passed through the electro-magnetic field and the carbon and nitrogen stable isotopes are
collected and counted by the Faraday cups.
The precision and accuracy of the analysis
were evaluated using a Urea isotopic standard
B2174 (Microanalysis Co.). δ
13C value was
expressed in ‰ (permil) deviations from the
referenced standard value by the following Eq. 1:
sample 13
st a nd a r dRC1 1‰ 000R   
 (1)
where R = 13C/12C, R sample is the isotope ratio
of the sample and R standard is the isotope ratio
of a standard referenced to Pee Dee Belemnite (PDB) limestone carbonate. The analytical error was ± 0.1‰ for δ
13C.
2.4. Ecosystem C storage estimation procedure
The sediment C pool was calculated
following Eq. 2 (Nguyen et al., 2014):
Sediment C pool (MgC ha-1) = ρ (gcm-3) ×
TOC (%) × h (cm) (2)
In which: ρ is the bulk sediment density,
T O C i s t h e t o t a l o r g a n i c c a r b o n a n d h i s t h e
depth interval. In this study, bulk sediment d e n s i t y w a s m e a s u r e d t h r o u g h T O C v a l u e s
based on the Eq. 3 (Nguyen et al., 2014):

Vietnam Journal of Earth Sciences, 38(4), 297-306
321 Bulk sediment density = 1.539е(-0.289 × TOC)
(R2 = 0.62, p <0.001, n = 225) (3)
The total C storage of Dong Rui mangrove
forest was scaled up by multiplying the mean ecosystem C storage (MgC ha
-1) with the total
area of mangrove forest (2194.1 ha) and converted to carbon dioxide equivalents
(CO
2e) using a factor of 3.67 (Kauffman and
Donato, 2012).
3. Results and discussion
3.1. Sediment characteristics
Mangrove sediments were composed
mainly of fine sand, silt and clay. The fraction smaller than 63 µm or mud content varied from 9.48% to 56.07%, with an average of 26.2% and tended to decrease with depth of
the sediment cores. Average mud content in
core C1, C2 and C3 was 37.36%, 37.43% and 24.83%, respectively (Figure 2). High mud content in sediment cores located in fringe mangrove forests of Voi Lon river and in
downstream Ba Che river (near Mui Chua
estuary, see Figure 1) indicated that fine-grained sediments being transported from Ba Che river mainly deposited in the fringe mangrove forests. Tide is an important factor
for transporting particles in Voi Lon creek and
Ba Che river into mangrove forests. From the fringe forest to interior forest, the decrease of t i d a l c u r r e n t e n e r g y w a s r e d u c e d b y vegetation density and mangrove root systems
of K. obovata , B. gymnornitreza a n d A.
corniculatum (Furukawa and Wolanski, 1996).
Consequently, the coarse-grained sediments mainly deposited at the fringe forest and only the fine-grained sediments were transported
further into the interior forest areas (Van
Santen et al., 2007).
Figure 2. Mud content profiles in three sediment cores
from Dong Rui mangrove forest
3.2. Total organic carbon, organic matter
content, C/N ratios and carbon stable isotope (δ
13C)
The TOC contents ranged from 1.91 to
8.28%, 0.63 to 10.20% and 0.21 to 8.57% for core C1, C2 and C3, respectively (Figure 3). The TN contents varied from 0.09 to 0.24%, 0.04 to 0.34% and 0.01 to 0.25% for core C1, C2 and C3, respectively. Both TOC and TN
contents tended to increase from the surface to
30 cm in depth and slightly decrease to the
core bottom (Figure 3). Strongly positive
correlation between TOC and TN (Figure 4)
p o s s i b l y r e f l e c t t h e s o u r c e s i m i l a r i t y o f T O C
and TN (Nguyen et al., 2011). Additionally, t h e r e g r e s s i o n l i n e o f T O C a n d T N approached very close to the origin (0,0), suggesting that the inorganic nitrogen content in mangrove sediments was insignificant and the TN was mostly in organic nitrogen. Therefore, TN can be used to calculate atomic
C / N r a t i o a n d a s c e r t a i n t h e o r i g i n s o f
sedimentary OC (Lamb et al., 2006).

Pham Thao Nguyen, et al./Vietnam Journal of Earth Sciences 38 ( 2016)
322
Figure 3. The variation of the TOC (%), TN (%), C/N
ratio (mol/mol) and δ13C of the sediment cores. (a)
fringe mangrove forest along Voi Lon creek (C1); (b)
mangrove forests in isle of Voi Lon River (C2); (c) m a n g r o v e f o r e s t i n d o w n s t r e a m o f B a C h e R i v e r adjacent to Mui Chua (C3)
Fine-grained sediment fractions were
closely correlated with TOC (R2 = 0 . 4 8 ) a n d
TN (R2 = 0.40) contents (Figure 5). The low
correlation coefficients indicated that other factors and fine-grained sediment fractions could also affect accumulation of OM contents in mangrove sediments, including sedimentation rates (Hedges and Keil, 1995;
Reimers and Suess, 1983) and maturity levels
of mangrove forests and intertidal elevation (Alongi et al., 2004). For example, the sedimentation rates could be an important factor for the preservation and storage of OM (Hedges and Keil, 1995; Reimers and Suess,
1983). These authors suggested that if the sedimentation rates are high, the OM will be
burial faster and escaped from the microbial decomposition and preserve better. Maturity
level of mangrove forests also affect
sedimentary OM contents with higher
sedimentary OM contents in mature mangrove
forests in comparison to young forests (Alongi
et al., 2004).
The OM content in mangrove sediments
ranged from 2.56 to 17.31%, with an average
of 7.63 ± 4.9%. The OM is positively
correlated with TOC content (Eq. 4, Figure 6):
TOC (%) = 0.41 × OM (%) + 0.17 (R2 = 0.79) (4)
The results indicated that the TOC contents
in sediments of Dong Rui mangrove forests
could be estimated by the OM contents
following Eq. 4. The linear regression model
(Eq. 4) suggested that the application of the
loss- on- ignition method (LOI method) for
determining TOC in mangrove sediments by using Bemmelen factor of 0.58 (OC
accounting 58%) (Schumacher, 2002) should
be performed with cau tion, because it can
considerably overestimate the TOC content by
approximately 17%.

Figure 4. Linear regression of the TOC (%) and TN (%)
for the mangrove sediments
The δ13C in core C1 ranged from –27.50 to
–26.24‰, with an average of –26.76‰. The δ
13C values were the lowest at surface
sediments (–27.50‰) and gradually increased
with sediment depth. The C/N ratios varied from 15.07 to 46.09, with an average of 24.45. The ratios slightly increased from the surface

Vietnam Journal of Earth Sciences, 38(4), 297-306
323 to the bottom of the core and reached the
highest value of 46.09 at 30 cm in depth (Figure 3a).

Figure 5. Correlation between TOC, TN and fine-
grained sediment fraction in mangrove sediments

Figure 6. Correlation between TOC and OM in
sediments from Dong Rui mangrove forest
F o r c o r e C 2 , t h e C / N r a t i o s r a n g e d f r o m
18.88 to 29.57, with an average of 24.56.
From the surface to the bottom core, C/N
tended to increase and reached the maximal
values of 29.57 at 25 cm in depth. The δ13C
values ranged from –27.25 to –26.43‰ with
an average of –26.85‰. From the surface to
12 cm in depth, δ13C slightly decreased from –
27.25 to –27.08‰, then rose to –26.57‰ at 20
cm in depth, but thereafter decreased toward
the bottom of the core (Figure 3b).
The C/N ratios in core C3 ranged from
15.56 to 32.29, with an average of 22.82. From the surface to 25 cm, C/N increased from 17.53 to 27.72, then decreased to the
lowest value (15.56) at 35 cm in depth, then fluctuated to the core bottom. The δ
13C values
ranged from –27.75 to –25.84‰, with an average of –26.83‰. From the surface to the depth of 35 cm, δ
13C values slightly increased
and reached the maximum value of –25.84‰, then tended to increase to the core bottom (Figure 3c).
The variation of the C/N ratios and δ
13C
values with depth in mangrove sediment cores indicated the change in the origin of OC and the degree of OM decomposition (Nguyen et al., 2011) and the absorption nitrogen by fine-grained sediments at the top layers of
mangrove sediment cores (Lamb et al., 2006).
3.3. Sources of organic carbon in sediments
Organic matters in the mangrove
sediments were derived from local plants
(termed as autochthonous sources) and other
sources such as terrestrial organic matter,
marine phytoplankton, and others, termed as
allochthonous. Therefore , the determination of
the OC sources in mangrove sediments will
elucidate the role of mangroves in the carbon
sequestration processes.
In mangrove forests, biomass of
microalgae is generally low due to limited
light and rich in tannin compounds released
from the decomposition process of mangrove
litters. Therefore, the m ajor sources of OC in
the mangrove sediments may consist mainly
of mangrove litters and marine phytoplankton.
Generally, δ
13C values of mangrove litters
ranged from –29.4 to –27‰ and of the marine
phytoplankton ranged from –22 to –18‰
(Bouillon et al., 2008; Bouillon et al., 2003).
In addition, due to their high cellulose
content, C/N ratios of mangrove litters are
commonly greater than 12, whereas marine
phytoplankton is rich in nitrogen content and
has relatively low C/N ratios, ranging from 5
to 7 (Lamb et al., 2006). Thus, C/N ratios and
δ13C values were used to determine the
sources of sedimentary OC.
In this study, the mean δ13C values of
mangrove leaves and C/N were –28.84 ±
1 . 0 9 ‰ ( n = 5 ) , 4 4 . 4 0 ± 1 7 . 1 2 ( n = 5 ) ,
respectively. The δ13C value of mangrove

Pham Thao Nguyen, et al./Vietnam Journal of Earth Sciences 38 ( 2016)
324 leaves indicated the characteristics of C3
terrestrial plants and consistent with other
mangrove species in Vietnam (Nguyen et al.,
2 0 1 2 ) . T h e δ13C value and C/N ratio of
phytoplankton were –23.46 ± 0.34‰ and 9.20
± 0.10, respectively (Figure 7). The
relationship between C/N and δ13C indicated
that sedimentary OC in Dong Rui mangrove
forest was composed of marine phytoplankton
and mangrove litters (Figure 7). The δ13C
values and C/N ratios indicated that the
sources of cores C1 and C2 were mangrove
leaves, while the δ13C values and C/N ratios in
core C3 were close to those of phytoplankton
(Figure 7). These results suggested that the
sources of OC in cores C1 and C2 were
delivered mostly from mangrove litters, while
the sources of OC in core C3 were mainly
from the marine phytoplankton (Figure 7).
The finding was similar to previous report for
mangrove forests in Tien Yen Bay, Quang
Ninh (Tran and Nguyen, 2011).
The proportional contribution of mangrove
litters and marine phytoplankton to sediments was determined by equation Eq. 5 (Nguyen et
al., 2011):
13 13
tt p
MG 13 13
MG pCCf 100CC  (5)
With pM Gf (%) 100% f where, f p, fMG are the proportional contribution
of mangrove litters and marine phytoplankton, respectively; δ
13Csed, δ13Cp, δ13CMG are the
carbon stable isotope values of sedimentary
OC and marine phytoplankton, respectively.

Figure 7. Comparison of C/N ratios and δ13C values in
mangrove sediments
The proportional contributions of the
mangrove litters and marine phytoplankton in Dong Rui mangrove forests are shown in Figure 8. The mangrove litter proportion in sedimentary OC varied from 44.32 to 79.72% with an average of 62.4%. The marine
phytoplankton contribution in sedimentary
OC ranged from 20.28 to 55.68% with an average of 37.6%. For all sediment cores, the mangrove litter contribution was higher in the sedimentary formation from surface to the 20 cm in depth and slightly decreased to the core
bottom.

Figure 8. The percentage of OC contribution of mangrove litters (f MG) and phytoplankton (f p)

Vietnam Journal of Earth Sciences, 38(4), 297-306
325 3.4. Ecosystem carbon storage of the
mangrove forest
The ecosystem C storage significantly
decreased, respectively, 78.27, 46.72, 46.62
MgC ha-1 in estuarine mangrove forests,
mangrove forests in river isle and mangrove
forest adjacent to Mui Chua (Figure 9).

Figure 9. OC storage in Dong Rui mangrove forest. C1:
fringe mangrove forest along Voi Lon creek; C2:
mangrove forest in isle of Voi Lon River; C3: mangrove forest in downstream of Ba Che River adjacent to Mui Chua.
The overall mean of ecosystem C storage
was estimated to be 57.2±14.9 MgC ha-1 in the
Dong Rui mangrove forests that was similar
to previous report for planted mangrove
forests in the Giao Thuy district, Nam Dinh
province (Mai and Nguyen, 2009). The
mangrove forest area of 2194.1 ha in Dong
Rui mangrove forests was estimated store
upwards to 1.26×105 M g o f C a t 4 5 c m i n
depth, which can be converted to 4.6×105 Mg
of carbon dioxide equivalent (CO 2e). If the
whole mangrove forest area is to convert into
other land uses (i.e., shrimp ponds or
agricultural lands), a large amount of GHGs
may be emitted into the atmosphere (Lovelock
et al., 201 1). Ou r results indicated th at Dong
Rui mangrove forests plays an important role
in carbon sequestration and OC storage in
sediments, reducing GHGs and mitigating
effect of climate change. Therefore, the
conservation of mangroves not only brings
about visible benefits (i.e., wood and fishery production) but also lends effort in reduction
of GHG emissions from deforestation.
Mangrove forests, therefore, should be
included in the program of ‘reducing
emissions from deforestation and forest
degradation’ (REDD +) (Donato et al., 2011)
and suitable use schemes in the coastal zone
(Hoang et al., 2012).
4. Conclusions
The present study showed that the fine
sediment grained size fractions (< 63 µm) varied in a large range from 8.58 to 82.10%, and TOC and TN contents ranged from 0.21
to 10.20% and 0.01 to 0.34%, respectively.
The TOC and TN contents tended to decrease with sediment depth and the fine sediment grained size fractions. The total OC storage in sediments is up to 45 cm in depth of Dong Rui mangrove forest accounting for
1.26(±0.3)×10
5 Mg C, equivalent to
4.6(±1.2)×105 M g C O 2. The δ13C values and
C/N ratios ranged from –27.75 to –25.84‰
and 15.07 to 46.09, respectively. The results
indicated that sedimentary OC was dominantly originated from the mangrove
litters. The proportion of mangrove litters in
the sedimentary OC ranged from 44.32 to
79.72%, with an average of 62.4%. The
present study also demonstrated that the Dong
Rui mangrove forests play an important role in the accumulation process of fine sediments
and OC sequestration. Therefore, conservation
of mangrove forests provides not only coastal
protection and disaster mitigation and other
provision values, but also enhances carbon
sequestration and offsetting GHG emissions.
Acknowledgements
We thank two anonymous reviewers for
their critical comments that significantly
improved this manuscript from an early
version. This research is funded by Vietnam
National Foundation for Science and
Technology Development (NAFOSTED)
under grant number 105.08-2015.18.

Pham Thao Nguyen, et al./Vietnam Journal of Earth Sciences 38 ( 2016)
326 References
Alongi, D.M., 2011. Carbon payments for mangrove
conservation: ecosystem constraints and uncertainties of
sequestration potential. Environmental Science Policy 14, 462-470.
Alongi, D.M., Sasekumar, A., Chong, V.C., Pfitzner, J., Trott,
L.A., Tirendi, F., Dixon, P ., Brunskill, G.J., 2004.
Sediment accumulation and organic material flux in a managed mangrove ecosystem: estimates of land – ocean – atmosphere exchange in peninsular Malaysia. Marine
Geology 208, 383-402.
Bouillon, S., Connolly, R., Lee, S., 2008. Organic matter
exchange and cycling in mangrove ecosystems: recent insights from stable isotope studies. Journal of Sea
Research 59, 44-58.
Bouillon, S., Rao, A.V.V.S., Koedam, N., Dahdouh-Guebas, F.,
Dehairs, F., 2003. Sources of organic carbon in mangrove sediments: variability. Hydrobiologia 495, 33-39.
Christensen, B., 1978. Biomass and primary production of
Rhizophora apiculata Bl. in a mangrove in southern
Thailand. Aquat Bot 4, 43-52.
Donato, D.C., Kauffman, J.B., Murdiyarso, D., Kurnianto, S.,
Stidham, M., Kanninen, M., 2011. Mangroves among the most carbon – rich forests in the tropics. Nature Geoscience 4, 293-297.
Furukawa, K., Wolanski, E., 1996. Sedimentation in mangrove
forests. Mangroves Salt Marshes 1, 3-10.
Hedges, J.I., Keil, R.G., 1995. Sedimentary organic matter
preservation: an assessment and speculative synthesis. Marine Chemistry 49, 137-139.
Hoang Van Tuan, Tran Dang Quy, Nguyen Van Vuong, Mai
Trong Nhuan, 2012. Orientation of functional zoning for
sustainable use of environment and natural resources in
Tien Yen Bay. Vietnam Journal of Earth Sciences 34, 486-494 (In Vietnamese).
Kauffman, J.B., Donato, D.C., 2012. Protocols for the
measurement, monitoring and reporting of structure,
biomass and carbon stocks in mangrove forests. Center for
International Forest Research, Bogor, Indonesia, 86 pp.
Kristensen, E., Bouillon, S., Dittmar, T., Marchand, C., 2008.
Organic carbon dynamics in mangrove ecosystems: A review. Aquat Bot 89, 201-209.
Lamb, A., Wilson, G., Leng, M., 2006. A review of coastal
palaeoclimate and relative sea-level reconstructions using δ
13C and C/N ratios in organic material. Earth Science
Reviews 75, 29-57. Lovelock, C.E., Ruess, R.W., and Feller, I.C., 2011. CO 2 efflux
from cleared mangrove peat. PLoS ONE 6, 1-4.
Mai Sy Tuan, Nguyen Thi Hong Hanh, 2009. Carbon
accumulation of Kandelia obovata (Sheue, Liu & Yong)
plantation in the coastal area of Giao Thuy district, Nam Dinh province. Journal of Biology 31, 57-65 (In Vietnamese).
Nguyen Tai Tue, Luu Viet Dung, Mai Trong Nhuan, Omori, K.,
2014. Carbon storage of a tropical mangrove forest in Mui
Ca Mau National Park, Vietnam. Catena 121, 119-126.
Nguyen Tai Tue, Hamaoka, H., Sogabe, A., Tran Dang Quy,
Mai Trong Nhuan, Omori, K., 2011. The application of δ
13C and C/N ratios as indicators of organic carbon sources
and paleoenvironmental change of the mangrove
e c o s y s t e m f r o m B a L a t E s t u a r y , R e d R i v e r , V i e t n a m .
Environmental Earth Sciences 64, 1475-1486.
Nguyen Tai Tue, Nguyen Thi Ngoc, Tran Dang Quy, Hamaoka,
H., Mai Trong Nhuan, Omori, K., 2012. A cross-system analysis of sedimentary organic carbon in the mangrove ecosystems of Xuan Thuy National Park, Vietnam. Journal
of Sea Research 67, 69-76.
Ong, J.E., 1993. Mangroves – a carbon source and sink.
Chemosphere 27, 1097-1107.
Reimers, C.E., Suess, E., 1983. The partitioning of organic
carbon fluxes and sedimentary organic matter decomposition rates in the ocean. Marine Chemistry 13,
141-168.
Richard, M., Angus, M., Tim, H., 2011. The potential for
mangrove carbon projects in Vietnam. SNV REDD+, Hanoi.
Schumacher, B.A., 2002. Methods for the determination of total
organic carbon in soils and sediments. U.S. Environmental
Protection Agency, Washington.
Siikamäki, J., Sanchirico, N., Jardine, J., McLaughlin, S.,
Morris, D.F., 2012. Blue carbon: global options for reducing emissions from the degradation and development of coastal ecosystems. Resources for future, Washington.
Spalding, M., Kainuma, M., and Collins, L., 2010. World atlas
of mangroves. Earthscan from Routledge, London.
Tran Dang Quy, Nguyen Tai Tue, 2011. Spatial distribution of
total organic carbon (TOC), total nitrogen (TN), TOC/TN
ratio, and stable carbon isotopes value (δ13C) in surface
sediments of Tien Yen Bay, northeast Vietnam. Vietnam
Journal of Earth Sciences 33, 615-624 (In Vietnamese).
Van Santen, P., Augustinus, P., Janssen-Stelder, B., Quartel, S .,
Nguyen Hoang Tri, 2007. Sedimentation in an estuarine mangrove system. Journal of Asian Earth Sciences 29, 566-575.