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Nghiên cứu khả năng tích lũy và trao đổi carbon trong rừng ngập mặn trồng

MINISTRY OF EDUCATION
AND TRAINING

MINISTRY OF AGRICULTURE
AND RURAL DEVELOPMENT

THUY LOI UNIVERSITY

HA THI HIEN

QUANTITATIVE STUDY ON CARBON STOCK AND FLUX
IN PLANTED MANGROVES AT
XUAN THUY NATIONAL PARK

Specialization: Soil and water environment
Code number: 9 44 03 03

SUMMARY OF DOCTORAL DISSERTATION

HANOI, 2018
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This scientific work has been accomplished at Thuyloi University

Supervisor: Assoc. Prof. Dr. Nguyen Thi Kim Cuc

Reviewer No. 1: Assoc. Prof. Dr. Le Xuan Tuan
Reviewer No. 2: Assoc. Prof. Dr. Pham Minh Toai
Reviewer No. 3: Assoc. Prof. Dr. Duong Thi Thuy

This Doctoral dissertation will be defended at …………………..……….. on
date ….…………………………………………………………………………...

This dissertation is available at:
- The National Library
- The Library of Thuyloi University
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INTRODUCTION
1. Rationale of the research
Mangroves are vegetation that exist along estuaries and coastlines of
(sub)tropical area [1]. The mangrove ecosystems protect the coastal zone from
natural disasters such as rainstorms, cyclones, waves, floods and tsunami [4],
[5]. Beside the coastal protection role, mangrove ecosystems provide numerous
ecosystem services and other commercial values [6]-[8]. In the last few
decades, mangrove forests have been recognized as important ecosystems in
the carbon cycle. The mangrove ecosystem act as a sink for atmospheric CO2,
and contribute as source of organic and inorganic carbons in the coastal areas.
Researches on carbon accumulation in planted mangroves in Northern Vietnam
were conducted by Nguyen Thanh Ha, Nguyen Thi Kim Cuc and Nguyen Thi
Hong Hanh [10] - [13]. The authors calculated carbon stocks in soils and
carbon accumulation in biomass of the planted mangroves (Kandelia obovata
Sheue, Liu & Yong; <13 years old). However, there was no research on carbon
stocks and flux in the planted mangroves at a higher stand age (~ 20 years old).
There was also no research focus on CO2 emissions from soil-air interface,
water-air interface as well as carbon exchange between mangroves and
surrounding water.
In order to continue with conducted studies for the planted mangroves under
13 years old, this research was implemented in the planted mangrove area at


Xuan Thuy National Park (XTNP) and completed the quantitative carbon
values. The study results will summary the carbon accumulation in planted
mangroves from the early stage to the 20 years old as well as assess the
greenhouse sink of this ecosystem. Therefore, the dissertation "Quantitative
study on carbon stock and flux in planted mangroves at Xuan Thuy National
Park" is a research that has high scientific and practical significances,
contribute to the determination of carbon stocks of planted mangroves in each
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stage. In addition, carbon exchange between the mangrove ecosystem and
surrounding environment (air and water) will be determined to complete
carbon cycle. The study results will be used to evaluate the carbon accumulated
function of mangroves in sustainable management of mangrove ecosystems.
2. Research objectives
The research objectives in the dissertation are to quantify the potential of
carbon accumulation in the K. obovata mangroves with stand age. In addition,
the study calculates the carbon exchange and carbon balance among the
environmental components in the mangrove forests at XTNP, Nam Dinh
province. The main objectives of this dissertation are listed below:
i. Quantify the amount of carbon accumulation in the planted forest (18 - 20
years old), include carbon accumulates in biomass and carbon stores in soils.
The study also determines the relationship between carbon stocks and stand
age;
ii. Quantify carbon exchange between mangrove ecosystem and surrounding
environment (water, air);
iii. Determine the relationship between carbon stock in soil, carbon in biomass
with carbon flux, then complete the carbon cycle in mangroves at XTNP.
3.
3.1.

Research object and scope
Research object

The research object of the dissertation is carbon accumulation and carbon flux
in planted K. obovata mangroves (18-20 years old) in the buffer zone of XTNP,
Nam Dinh province.
3.2.

Research scope

The study was implemented on planted K. obovata mangroves growing and
developing around a tidal creek connected to the main canal (20o13'37.6"N and
106o31'42.0"E, Fig. 2.1). This site belong to the buffer zone of XTNP, Nam
4


Dinh province. Study quantifies carbon stock in soils, carbon accumulation in
biomass as well as carbon exchange from mangroves by the horizontal and
vertical. The study period is from February 2016 to the end of April 2018,
following by 18, 19 and 20 year-old plantations. In the study area, K. obovata
is the species that accounts for ~ 95% of the total number of individuals in the
planted area, the rest is the small proportion of R. stylosa. Therefore, the study
area was defined as the K. obovata plantations (mangroves).
The planted mangroves grow around the tidal creek connected directly with the
main canal. The flood tides bring nutrients to the plantations, and the ebb tides
export litterfalls from the mangrove floor, nutrients from substrate back to the
ocean water. Therefore, the research scope of the dissertation will focus on
carbon accumulation and carbon exchange in mangrove area surrounding the
tidal creek. Due to limited on study instruments, the research does not take into
account the amount of carbon exchanged (CO2) from mangrove trees to the
atmosphere and vice versa.
4. Research content
From the research objectives, the main study contents are as follows:
i. Study on biomass accumulation in the K. obovata mangroves including:
above ground biomass (ABG) and below ground biomass (BGB), carbon
accumulation in biomass and in soils with stand age. From the results of
carbon accumulation in biomass and carbon stocks in the soil, the study
determined the carbon accumulation rates in biomass, carbon burial rate in
soils with stand age;
ii. Determine the net primary productivity (NPP) of plantations and the carbon
values exported to the water environment through macro-export. Measure
and calculate the CO2 fluxes from mangrove soils and water to the
atmosphere, from mangroves to the surrounding water as DOC, POC and
DIC parameters on tidal cycles. From the obtained results, study calculates
carbon balance in planted mangroves;
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iii. Quantify the correlation between the results in study content (i) and study
content (ii) to complete the component carbon cycle of the planted
mangroves.
From the analyzed results, the study will clarify the contents with the main
protection points as follows:
(1) carbon accumulation in biomass and in soils increase with stand age;
(2) carbon emission to the air and carbon exchange to surrounding water
depend on the carbon stock in soils and other characteristics (climate, tide,
hydrology and plant species).
5.
5.1.

Scientific and practical significances
Scientific significance

The dissertation results completed the quantitative estimation of carbon
accumulation in mangroves with stand age (18-20 years old). The study also
contributes scientific arguments and backgrounds for calculating the
greenhouse gases (CO2) absorbed by planted mangroves, and how this function
varies in each growing stage of the tree. The study results explain carbon
exchange between soil-water-air interfaces and clarify the role of mangroves
as a greenhouse gas (CO2) sink, reducing greenhouse gas emissions.
5.2.

Practical significance

The study results provide quantitative data on carbon stocks accumulated in
mangrove forests, these can be used to determine the exact amount of carbon
(or CO2eq) that can be fixed in the planting programs at the coastal area.
Quantitative results of the research can be used for the payment ecosystem
service on the evaluation of the carbon sequestration of mangrove ecosystem,
and thus help to manage sustainably these ecosystems. The results also
contribute plans to restore, protect and develop mangrove areas with the aims
to reduce the impacts of climate change mitigation.
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The dissertation also provides information on teaching subjects such as
environment and ecology as well as some related subjects in universities and
colleges.
6. Dissertation structure
Apart from the introduction and conclusions, the dissertation consists of three
chapters:
Chapter 1: Overview on carbon stock and flux in mangroves
Chapter 2: Study site and methodology
Chapter 3: Results and discussions
CHAPTER 1 OVERVIEW ON CARBON STOCK AND FLUX IN
MANGROVES
1.1

Overview about mangroves

Mangroves distributed in estuaries mainly in the (sub)tropical areas, from 25oN
to 25oS [14]. Mangroves around the world have great diversity of structures
and functions, mainly the results of factors such as substrate, latitude, tidal
regime, hydrology and climate [20]. Due to the variation in height and stem
diameter, AGB is very variable, ranging from about 8 Mg ha-1 in the dwarf or
scrub mangroves to over 500 Mg ha-1 in the mangrove forests near estuaries in
the Indo-Pacific region [21].
The variation of carbon accumulated in mangrove soils were similar to the
variation of plant biomass, depending on mangrove species, mangrove types and
mangrove structures. Studies showed that the carbon stocks in the mangrove
soils found mainly in 3 m depths, and carbon stocks in the soils accounted for
49-98% of the total carbon stocks of the mangrove ecosystem [15].
Beside, studies also focus on the carbon stocks in mangrove ecosystems on
global scale. Some studies also calculated carbon fluxes from mangroves to
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coastal waters. Because mangrove areas located in different latitudes with
different environmental factors, then carbon exported from mangroves to
surrounding environment (and vice versa) depending on many factors such as
tide, climate, vegetation, biotic factor, etc. These factors strongly influenced
the NPP, carbon stocks as well as carbon flux in mangroves.
1.2

Overview on carbon accumulation in mangroves

In the past few decades, carbon stocks in mangrove ecosystems have been
studied extensively in the world. The AGB, BGB, and the carbon sequestration
have been studied in mangrove forests on latitude. The study results showed
the lower latitude, the higher carbon stocks.
In the Northern Vietnam, the studies on carbon accumulation in the biomass
and the soil have been conducted in newly planted mangroves by Nguyen
Thanh Ha, Nguyen Thi Kim Cuc and Nguyen Thi Hong Hanh since 2000s
[10]–[12]. The results showed that total belowground carbon accumulation
increased with stand age. However, the study results showed only for those
conducted on newly planted areas up to the age of 13 years old, and there were
no results for the higher plantations in this area.
1.3

Overview on carbon flux in mangroves

Part of carbon stores in mangrove soils is mineralized and forms greenhouse
gases (GHG), including CO2. This GHG may emitted directly to the air through
the soil-air interface, or dissolved in underground water, then seepage to the
surrounding waters as DOC, DIC, POC [3], [30], [83]. The carbon flux and
carbon exchange are highly variable in mangrove soils, depending on factors
such as plant species, stand age, mangrove topography, and their relationship
to tidal amplitude, allochthonous source, season, temperature [3], [82], [84]–
[86]. The latest estimates by Bouillon et al. (2008) [3] on carbon balance in
mangroves showed that the carbon flux, carbon storage and carbon

8


mineralization accounts for only ~ 50% of the carbon fixation by mangroves
through photosynthesis.
In the Northern Vietnam, there have no study on carbon stocks and carbon flux
in mangrove forests since 2010. In detail, there were no study focus on CO2
emissions from mangrove soils, from water interface as well as carbon
exchange between mangroves and surrounding water under their existent
parameters (DOC, POC, DIC and carbon exported from litterfall).
Consequently, the hypotheses in this study are to calculate carbon
accumulation, carbon burial rate and carbon flux for a specific study area (with
plant species and forest type).
1.4

Summary of chapter 1

Carbon accumulation in mangrove ecosystems (in the latitudes) in Vietnam and
around the world has been studied in different locations. To my best
knowledge, carbon exchange and from mangroves to the surrounding
environment have been reported in some studies, focus on the factors that
influence CO2 emissions from soil-air interface in mangroves at low tide [67],
[82], [84]. However, there were lack of studies on carbon flux and transfer
within mangrove areas, especially research on CO2 emissions in different parts
of the world as well as in different mangrove types (plantation and natural
mangroves). The authors have predicted carbon transfers in mangrove
ecosystems, but not enough data to complete the carbon cycle for a particular
study area.
In Vietnam, most of the mangroves are plantations (~ 66%) [80], and several
studies have been evaluated the carbon accumulation in biomass and in the
soils. However, there are no study combining carbon accumulation and carbon
flux in planted mangrove ecosystems, especially in Northern Vietnam.
Therefore, this study is needed to quantify carbon accumulation and carbon
flux in this ecosystem. Research in the dissertation will address the
shortcomings in quantifying the total carbon accumulation and carbon
9


exchange in mangrove ecosystems, then completing the carbon cycle for the
planted mangroves in the Northern, Vietnam.
CHAPTER 2 STUDY SITE AND METHODOLOGY
2.1.

Study site

The study site was chosen in the
mangrove area located in the south
bank of the Red River, Nam Dinh
province,

Northern

Vietnam.

Mangrove forest in XTNP is
a combination of planted forest and
natural regeneration one with three
main tree species: K. obovata, S.
caseolaris and R. stylosa. The study
area is located in the buffer zone of
XTNP, where the mangrove has
been planted since 1998. The plots
were selected in the mangroves
surrounding a tidal creek connected
to the main canal (mangroves 18
years old, 2016; 19 years old , 2017
and 20 years old, 2018; Figure 2.1).

Figure 2.1. Map of study site with
sample locations in mangroves of Xuan
Thuy National Park.

2.2. Research object
The research object of the dissertation is the carbon stock and carbon flux in
planted mangroves (18-20 years old) in the buffer zone of XTNP, Nam Dinh
province. K. obovata is a small tree species, growing in Vietnam, China,
Taiwan, Japan and in the Natuna Islands, Indonesia. K. obovata tree grows on
mud and muddy sand with varied salinity. This species is resistant to wide
10


range of temperature and salinity. In the north of Viet Nam, K. obovata
distributed with large proportion of mangrove areas (both natural and
plantation), along with a small proportion of S. caseolaris and R. stylosa.
2.3.

Study time

The research was conducted from February 2016 to the end of April 2018. In
each study year, carbon accumulation in soils and in biomass of mangroves
were determined. In addition, CO2 emissions from soils, from water and carbon
exchange (DOC, DOC, POC) from the K. obovata mangrove soils to coastal
waters (and vice versa) are also identified in two main seasons: dry and wet.
The results were used to calculate carbon import and export as well as carbon
balance in mangroves. In each season, the study was conducted in two tidal
cycles: spring tide and neap tide in order to determine carbon exchange in each
tidal cycle.
Litterfall samples in the mangroves were taken once a month between April
2016 and April 2018. Similarly, the litterfall samples exported from the
mangroves to the tidal creek and surrounding waters are collected once a month
to calculate the total carbon export (macro-export).
2.4.

Methodology of measurements

The research used the inheritance method from published researches in the
same fields (articles, books, research reports). In addition, the data and
information from primary and secondary sources were collected and analyzed
in order to set up the arguments to prove the hypotheses of the research. The
study results was analyzed by statistical method to build up carbon cycle in K.
obovata mangrove forests.
Research included knowledge in many sciences (biology, ecology, chemistry,
geology and environment), so study results need the contributions of many
major scientists. Then, the expert method was used through scientific
conferences and seminars in order to improve the contents of the dissertation.
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The study used survey to choose study sites, field measurements, measured
plantation structure in each year, measured CO2 concentrations between
environmental interfaces, analyses physicochemical properties of soil and
water, measuring carbon in soil and water, calculating carbon exchange
between mangroves and creek. Soil samples, water samples and plant biomass
samples were collected, stored and analyzed according to standard laboratory
methods. All data calculated after the experiments were analyzed using
analysis of variance (ANOVA), T-test, multiple regression, principal
component analysis (PCA) methods. All statistical tests were performed using
R software (Version R.3.3.2).
CHAPTER 3 RESULTS AND DISCUSIONS
3.1. Aboveground biomass of planted mangroves
Based on the dry weight of the components in the samples collected at the study
sites for three years, combining with the results in Nguyen Thi Kim Cuc et al.
(2007) [103], the study found the correlations between the dry mass of wL&P,
wS&B, wT (dry weight of leaf and propagule, stem and branch, total biomass,
respectively) with tree size D0.32·H. The correlations between wL&P, wS&B, wT
and D0.32·H are given as below:
Leaf and propagule (g):
wL&P = 5.464 × (D0.32·H)0.8943 (R2 = 0.94; P < 0.0001)

(3-1)

Stem and branch (g):
wS&B = 28.120 × (D0.32·H)0.9655 (R2 = 0.96; P < 0.0001)

(3-2)

Total biomass (g):
wT = 33.931 × (D0.32·H)0.9585 (R2 = 0.96; P < 0.0001)

(3-3)

In the above equations, D0.3 is the stem diameter measured at 30 cm above the
soil surface (cm); H is the tree height (m) and w is the dry weight of each
component (g). The correlation equation between D0.32·H and wT (wT = 33.931
× (D0.32·H) 0.9585) in this study is in line to those reported in Khan et al. (2005)
with K. obovata mangroves in the Manko wetland, Okinawa, Japan (wT (g) =
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32.03 × (D0.12·H)
2

(D0.1 ·H)

1.058

) [118]; with B. gymnorrhiza stand (wT (g) = 28.04 ×

1.063

) in the study of Deshar et al. (2012) [119]; with mangroves in

Biscayne National Park, Florida, USA [120].
Using equations from 3-1 to 3-3 to calculate biomass of K. obovata from 18 to
20 years old stand in 1 ha of mangroves (Table 3.3). The total biomass of K.
obovata increased with stand age, from the value of 79.95 Mg ha-1 (18 years
old) to 87.66 Mg ha-1 (20 years old). The carbon accumulation in the biomass
also increased with plantations, from 35.44 ± 2.79 MgC ha-1 in 18 years old
stand to 39.16 ± 5.20 MgC ha-1 in 20 years old stand.
Table 3.3. The component biomass, total biomass and carbon accumulation
in biomass of K. obovata from 18 to 20 years old stands.
Stand age
(year)

Leaf and
propagule
(Mg ha-1)

Stem and branch
(Mg ha-1)

Total biomass
(Mg ha-1)

Carbon in total
biomass
(MgC ha-1)

18

9.55 ± 0.82

70.40 ± 5.41

79.95 ± 6.31

35.44 ± 2.79

19

9.95 ± 1.55

73.86 ± 11.36

83.81 ± 13.03

37.15 ± 5.78

20

10.05 ± 0.79

77.61 ± 10.79

87.66 ± 10.54

39.16 ± 5.20

The study has established the correlations between biomass and stand age,
where t is stand age (year):
Leaf and propagule (Mg ha-1): wL&P = 0.51 × t + 0.23 (R2 = 0.98)
-1

Stem and branch (Mg ha ):
-1

Total biomass (Mg ha ):

2

wS&B = 3.85 × t + 0.54 (R = 0.99)
wT = 4.44 × t + 0.53

2

(R = 0.99)

(3-4)
(3-5)
(3-6)

The linear relationship of total biomass with the stand age shows that at the age
of the 20th, K. obovata is still in the development process. The percentage of
leaf and propagule collected in K. obovata mangroves is quite stable (~ 12%).
This proportion is consistent with the published studies in other mangrove
species (both natural and planted forests) in the world [3].

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The results of AGB and aboveground carbon accumulation show that the both
values increase with stand age. The AGB rate in the K. obovata mangroves in
Northern Vietnam was 4.60 Mg ha-1 year-1, and this value is lower than other
values in mangrove species in different parts of the world [55], [58], [59], [61].
Study found the linear and positive correlations between biomass components,
total biomass with stand ages of K. obovata mangroves. These equations can
be used to estimate AGB and carbon accumulation in planted mangrove areas
in Northern Vietnam.
3.2. Litterfall of planted mangroves
The litterfall collected in hanging traps at mangrove plots were quite stable in
the two years, with values were 5.98 ± 1.28 and 6.83 ± 1.89 Mg ha-1 year-1,
respectively. The annual mean litterfall is 6.41 ± 1.59 Mg ha-1 year-1, those
value equivalences to 2.32 ± 0.57 MgC ha-1 year-1 of carbon accumulation.
Compared with leaf biomass of these ages (18-20 years old), litterfall
accounted for 65% of total leaf biomass (~ 10 Mg ha-1 year-1, Table 3.3).
OC export to creek

4
3

2.17

2.47

2.32

2
1

Litterfall (MgC ha-1 yr -1)

Litterfall (MgC ha-1 yr -1)

OC in mangroves
0.4
0.3

0.26

0.27

0.26

2017-2018

Mean

0.2
0.1
0.0

0
2016-2017

2017-2018

Mean

2016-2017

Figure 3.8. Carbon in litterfall of K. obovata mangroves and carbon in
macro export to the water surrounding.
The litterfall exported from the mangroves to the tidal creek waters has
correlation with the amount of litterfall collected on traps in planted
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mangroves. In the two studied years, the litterfall values in traps were 0.77 and
0.83 Mg ha-1 year-1, with an average value of 0.80 Mg ha-1 year-1, which is
equivalent to 0.26 MgC ha-1 year-1 of organic carbon exported to the creek
water (Figure 3.8).
Compared to the litterfall of planted mangroves at the age of 18-20 years old
(2.32 MgC ha-1 year-1), the value of litterfall exported from the mangroves to
the creek (0.26 MgC ha-1 year-1) only accounted for 11.21% of total mangrove
litterfall. The low macro exported may due to the high density structure of the
planted mangroves (~ 20,000 trees ha-1). The results showed that most of the
litterfall (88.79%) were trapped in the mangroves, partly consumed by crabs as
food sources, then followed by bio-disturbances to accumulate in soil layers.
The other part stores in soil and decays to contribute organic carbon for
mangrove soils.
3.3. Belowground carbon of K. obovata mangroves
3.3.1. Carbon stocks in roots
The root biomass of the K. obovata at XTNP showed that the carbon content
in mangrove root biomass is different in the soil layers. In general, carbon
content in root biomass decreased with soil depth, The highest root densities
were observed between 20 and 60 cm depth, and the lowest in the bottom layer
(70-100 cm). The upper layer, down to 60 cm depth, accounted for 86% of root
biomass, suggesting that deep roots were scarce. In the 19 to 20 years old
mangrove stands, root biomass is higher than the 18 years old stand in the 40100 cm soil depth. The distribution of root biomass in this study is in broad
agreement with several studies in mangrove forests that showed the highest
root densities were observed in the upper soil horizons [10], [ 11], [49], [131],
[132]. In particular, dead root biomass rose from 18 to 20 years old stands. In
our study, the total carbon stocks in roots of the planted K. obovata up to 100
cm depth increased with stand age, with values ranging from 12.67 MgC ha-1
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(18 years old) to 15.35 MgC MgC ha-1 (19 years old) and 16.71 MgC MgC ha1

(20 years old; Table 3.7).
Table 3.7. Root biomass in 18 to 20 years old K. obovata mangroves in
XTNP, Nam Dinh Province.
Stand age
(year)

Live root
(MgC ha-1)

Dead root
(MgC ha-1)

Total carbon
(MgC ha-1)

Total biomass in soil
(Mg ha-1)

18

6.69

5.98

12.67

38.07

19

7.40

7.95

15.34

45.90

20

7.37

9.34

16.71

50.37

The carbon accumulation rate in the root biomass of the K. obovata mangroves
during the period from 18 to 20 years is 2.38 MgC ha-1 year-1. The higher stand
age of the mangroves, the higher root biomass accumulation.
3.3.2. Carbon stocks in soils of mangroves and in the adjacent bare land
Organic carbon distribution at depths in 19 and 20 years old K. obovata
mangroves are in board agreement with the trend of the 18-year-old stand, with
the highest carbon contents in the soil layers from 0-60 cm. Carbon contents
increased intensively with depths of 30-60 cm in the 19-20 years old stands. In
the total depth of 100 cm, the carbon stocks increased slightly from 18 to 20
years old stand (Table 3.9).
Table 3.9. Carbon sequestration in soil (MgC ha-1) in K. obovata mangroves
at 18 - 20 years old.
Stand age (year)

Live root

Dead root

Soil

Total carbon

Bare land (2016)

-

-

87.59

87.59

Bare land (2017)

-

-

89.83

89.83

Bare land (2018)

-

-

91.14

91.14

18

6.69

5.98

146.78

159.45

19

7.40

7.95

151.31

166.66

20

7.37

9.34

152.31

169.02

In the bare land, the highest carbon content recorded in the soil layer of 20 cm
depth and the lowest value recorded in the deepest layer (100 cm). Statistical
16


data show that there is a significant difference in organic carbon content
between soil layers (P < 0.01). Up to 100 cm depth in the bare land (next to 18
years old stand), the total organic carbon was 87.59 ± 1.08 MgC ha-1, and this
value increased over time, reaching 91.14 ± 6.84 MgC ha-1 (next to 20 years
old K. obovata; Table 3.9).
The results obtained in this study for 18 - 20 years old K. obovata mangroves
showed that there was positive correlation between carbon in soil and carbon
in roots with depths. Carbon accumulation increased with stand age, with the
highest value recorded at the stand of 20 years old (152.31 MgC ha-1).
Belowground carbon is the sum of carbon in roots and carbon in soil. Carbon
in soil accounts for ~ 90% of the total belowground carbon, while carbon
accumulation in roots only accounts for ~ 10%. The results showed that carbon
stored in the soil partly derived from root biomass, partly from mangrove
litterfall and partly from river and ocean sediments.
3.3.3. Belowground carbon burial rates
Statistical data show that there is a strong correlation between belowground
organic carbon and stand age. The carbon sequestration rate is shown in
Equation (3-7), where C and t denote to belowground carbon (MgC ha-1 year1

) and stand age (year).
C = 48.524 × e0.0646 × t

(3-7)

The results showed that the belowground carbon burial rates were very high
during K. obovata growth in 20 years (mean of 7.08 MgC ha-1 year-1). Carbon
burial rates were higher in the early stages (5-8 years) and more stable in later
periods. At the stands from 18 to 20 years old, the mean burial rate is 6.91 MgC
ha-1 year-1. Carbon burial rate is quite stable during 18 to 20 years old stand
showed that carbon stored in the soil is derived from mangrove’s NPP (2.32
MgC ha-1 year-1), from carbon in the root biomass (2.38 MgC ha-1 year-1).
However, the sum of these two components (4.70 MgC ha-1 year-1) is still lower
17


than the soil carbon stock (6.91 MgC ha-1 year-1). Thus, carbon stored in the
soil will be partly derived from river and ocean sediments.
3.4.

CO2 emissions from mangrove soils to the atmosphere

CO2 emissions from the soil surface fluctuate significantly in both mangroves
and bare land. In dark condition, the CO2 fluxes ranged from 37.16 ± 14.60 to
228.43 ± 78.50 mmol m-2 day-1 in mangrove soil; while in bare land, these values
ranged from 16.34 ± 14.33 to 87.87 ± 32.71 mmol m-2 day-1. The mean CO2
fluxes in the mangroves (95.53 ± 89.28 mmol m-2 day-1) was more than double
folds of the one measured in the bare land (42.42 ± 32.73 mmol m-2 day-1).

Figure 3.21. CO2 fluxes (mmol m-2 day-1) with biofilm (FCO2(WB)-dotted
line) and without the biofilm (FCO2(WoB-line)) and soil temperature of
mangrove soil and bare land in dry and wet seasons in XTNP.
After having removed the upper 2 mm of the soil surface, CO2 fluxes were
higher than in dark condition for both sites. The CO2 fluxes measured in
mangrove soil were nearly double the values measured in bare land, with the
mean values of 122.22 ± 90.25 mmol m-2 day-1 and 73.76 ± 39.85 mmol m-2
day-1, respectively. The analysis of variance shows that there are significant
18


differences in the CO2 fluxes measured between mangrove soil and bare land,
with biofilm (P < 0.001) and without the biofilm (P < 0.01; Fig. 3.21).
Using multiple regression analyses, the study found a positive correlation
between CO2 fluxes and soil temperature as well as soil organic content with
and without the biofilms on the soil surface. The equations are shown below:
FCO2_WB (with biofilm) =
8.809 × Tsoil +121.135 × TOC – 301.689 (R= 0.61; P < 0.001)

(3-8)

FCO2_WoB (without biofilm) =
10.990 × Tsoil + 127.209 × TOC – 338.021 (R= 0.74; P < 0.0001)

(3-9)

F is the CO2 fluxes (mmol m-2 day-1), T is the soil temperature (⁰C), TOC is the
organic carbon content in soil (%).
In summary, CO2 fluxes in mangrove soils were higher than the values
measured in the bare land. This result is due to the higher carbon stock in
mangrove soil compared to that of bare land. The study found that the total
amount of CO2 emitted annually from mangrove soils and bare land were 1.75
± 0.76 MgC ha-1 year-1 and 0.93 ± 0.52 MgC ha-1 year-1, respectively. The
results showed that the CO2 emissions increases with the rise of air and soil
temperatures, with the Q10 values calculated for mangrove soil was 2.63 and
for bare land was 2.75. The Q10 values identified in this study were equivalent
to many published studies, with values ranging from 1.70 to 4.05 [67], [153] [155]. The variations of CO2 fluxes depends on the variation of abiotic factors
but also on biotic ones. And biofilms would be considered as one of the key
biotic factors that limits CO2 emissions from soil surface to the atmosphere.
3.5.

Carbon exchange between mangroves and surrounding water

The CO2 emissions from surface water to the atmosphere is closely related to
the water physicochemical parameters, especially the DIC, DO and pH. CO2
emissions from water - air interface increased at low tide, and CO2 emissions
were stable at flood tide and high tide. The results showed that CO2 fluxes from
19


the water-air interface were strongly emitted at the spring tides, especially in
the rainy season. The mean CO2 emissions from the water - air interface is 0.15
MgC ha-1 year-1. The CO2 emitted from the water - air interface is 11.67 times
lower compared to the CO2 emissions from soil - air interface (1.75 MgC ha-1
year-1).
Carbon imported in mangroves was calculated from the average concentration
of substances during flood tide to high tide and tidal discharges. Carbon
exported from mangroves was determined by the same method during ebb tide
to low tide. In general, total organic carbon (DOC and POC) imported to
mangrove forest from tidal currents was higher than that of exported values
from mangroves to coastal water. In contrast, dissolved inorganic carbon (DIC)
exported from the mangroves was greater than the imported values. The values
of carbon imported in and exported out of mangroves according to the tidal
cycles and seasons are shown in Figure 3.34.
C export from mangroves
(MgC ha-1)
6.68

C import in mangroves
(MgC ha-1)

13.39
1.70
18.53
22.59
1.77
DIC

DOC

POC

DIC

DOC

POC

Figure 3.34. Imported and exported carbon by tidal waters. DIC: dissolved
inorganic carbon; DOC: dissolved organic carbon; POC: particulate organic
carbon.
Organic carbon imported annually in 1 ha of mangroves was 15.16 MgC ha-1,
of which POC was 13.39 MgC ha-1 and DOC is 1.77 MgC ha-1. In the ebb tide
period, organic carbon exported were 6.68 and 1.70 MgC ha-1, respectively.
20


The results showed that the total amount of organic carbon retained in 1 ha of
mangroves was 6.78 MgC ha-1, of which particulate organic carbon (POC) was
6.71 MgC ha-1 (98.97 %) and dissolved organic carbon (DOC) accounts for
0.07 MgC ha-1 (1.03%). In contrast, dissolved inorganic carbon (DIC) exported
from mangroves was higher than imported values with 4.06 MgC ha-1. The DIC
values are increased particularly after the surface water has been run out of
from the surface of the mangrove floor; at this time, water seepage from the
mangrove area is mainly porewater.
Compared to published studies in the world, the results obtained in this study
are consistent with those of Romigh et al. (2006) [165]. The authors reported
that the concentrations of DOC imported in and exported out of mangroves
were influenced by the season factor, by the river flows as well as by the tidal
amplitude. However, according to Alongi (2014) [31], the carbon
concentrations exported from the mangroves in ebb tide were higher than the
values imported in flood tide; and in most cases, due to higher carbon
concentrations in mangrove waters compared to ocean waters. Thus, the carbon
values exported out of mangroves are always greater than that imported values.
In general, tidal hydrological regimes and tidal amplitudes are important
factors affecting to organic and inorganic carbons exported from mangroves.
The organic carbon imported in the mangroves was larger than the exported
one, and the main component imported was POC (6.71 MgC ha-1 year-1). In
contrast, DIC values exported to coastal waters were higher than imported
(4.06 MgC ha-1 year-1) and accounted for the largest component of all exported
carbon from the mangroves.
3.6.

Carbon cycle in planted mangroves

Carbon accumulation in mangrove ecosystems consists of two components:
carbon from mangrove NPP and carbon derived from tidal currents (POC, DOC
and DIC). After balancing imported and exported carbon in the planted
mangrove area through tidal currents, the amount of carbon imported into
21


mangroves includes DOC and POC with mean values of 0.07 and 6.71 MgC
ha-1 year-1, respectively (Table 3.18).
Table 3.18. Carbon imported inand exported out of mangrove system. Mean
values and standard deviations (MgC ha-1 yr-1).
Carbon import

Mean

SD

Carbon export

Mean

SD

Litterfall

2.32

0.21

Litterfall

0.26

0.01

AGB

2.03

0.18

Sequestration in soil

6.91

0.98

BGB

2.38

0.40

CO2 emissions from soil

1.75

0.76

DOC

0.07

0.11

CO2 emissions from water

0.15

0.03

POC

6.71

4.70

DIC

4.06

3.49

Total import

13.51

5.60

Total export

13.13

5.27

Thus, the total carbon sequestration in mangrove each year is 13.51 MgC ha-1
year-1, in which carbon in NPP is 6.73 MgC ha-1 year-1 (49.82%) and carbon
from the tides is 6.78 MgC ha-1 year-1 (50.18%). Carbon exported from the
mangrove ecosystem is composed of many components: carbon exported from
tidal water (micro export: DOC, DOC, POC), carbon exported from litterfall
(macro export), CO2 emissions from soil - air and water - air interfaces.
According to the results calculated in Table 3.18, the total value of carbon
exported from mangrove forests is 6.22 MgC ha-1 year-1. DIC is the largest
exported component of mangroves, with a value of 4.06 ± 3.49 MgC ha-1 year1

(65.70% of the total carbon exported). The second component of carbon

exported is CO2 emissions from the mangrove soil surface with an average of
1.75 ± 0.76 MgC ha-1 year-1 (28.32% of the total carbon exported). Exported
carbon from litterfall with the amounts of 0.26 ± 0.01 MgC ha-1 year-1, and the
smallest carbon emissions from the water surface (0.15 ± 0.03 MgC ha-1 year1

). Thus, DIC and CO2 emissions from soil were the two major components

(94%) of carbon emissions to the water and air environments, leaving only a
small portion (6%) of carbon emissions on the water surface and litterfall.

22


From the result analysis, the study combined carbon components of K. obovata
mangroves at XTNP, Nam Dinh Province is shown in Figure 3.35.

Figure 3.35. Component carbon cycle in K. obovata mangroves at XTNP,
Nam Dinh province. Study used the carbon cycle of Bouillon et al. (2008) [3].
Comparing the total carbon stored in the soils (6.91 ± 0.98 MgC ha-1 year-1)
with the total carbon exported from the components (6.22 ± 3.99 MgC ha-1
year-1), it can be seen that the proportion of carbon stored in soil was higher
than that of carbon exported. The total of the two above values were 13.13 ±
5.27 MgC ha-1 year-1, which was still lower than the total accumulated carbon
in mangrove and imported carbon (13.51 ± 5.60 MgC ha-1 year-1). After
calculating the carbon equivalence in mangrove from imported and exported
values, carbon accumulation in soil and biomass of K. obovata mangroves is
7.29 MgC ha-1 year-1.
3.7. Summary of chapter 3
The study results calculated the carbon accumulation in biomass, carbon stocks
in mangrove soil as well as the relationship between carbon accumulation rates
with stand age. The results quantified the types of carbon exchange between
mangroves and surrounding environments (soil, water, air) and their quantities.
The research has calculated the carbon balance in mangrove ecosystem and the
net accumulation rate over time. From the results, the research has completed
the component carbon cycle of K. obovata mangroves. The cycle has
23


generalized carbon balance in mangrove ecosystem at the XTNP. In this cycle,
carbon stocks in the soil account for the largest proportion (6.91 MgC ha-1 year1

) during the K. obovata developing period (18-20 years).

CONCLUTIONS AND SUGGESTIONS
1. Conclusions of the dissertation
(1) The study identified the correlations between the AGB and tree size. Total
biomass accumulation increases with stand age, and the highest value is 138.03
Mg ha-1 in the K. obovata of 20 years old (87.66 and 50.37 Mg ha-1 for AGB
and BGB, respectively). Organic carbon in soils also increased with mangrove
plantations, with 159.45 MgC ha-1 in the 18 years old and 169.03 MgC ha-1 in
the 20 years old. In which, carbon in soil accounted for over 90%, and carbon
in roots accounted for less than 10%. The decrease in T/R ratios with the
increase in stand age also suggests that, the rate of biomass accumulation is
rising rapidly in belowground and this process continues two decades after
planting. The increase in biomass accumulation on the ground reached an
average of 4.60 Mg ha-1 year-1, corresponding to the organic carbon
accumulation of 2.03 MgC ha-1 year-1. For the period from 18-20 years old, the
annual average root biomass accumulated value was 2.38 MgC ha-1 year-1.
(2) The mean litterfall in the two-year observation of the K. obovata forest (1820 years old) was 6.41 ± 1.59 Mg ha-1 year-1, this equivalences to the average
carbon value of 2.32 ± 0, 57 MgC ha-1 year-1. The amount of carbon exported
from the litterfall by the tidal current was 0.26 ± 0.01 MgC ha-1 year-1. Thus,
the amount of carbon stored in mangroves corresponds to 2.06 MgC ha-1 year1

. The total amount of carbon emitted (as CO2) from mangrove soils was 1.75

± 0.76 MgC ha-1 year-1, and in bare land was 0.93 ± 0.52 MgC ha-1 year-1. The
carbon emissions from soil is much higher than the value emitted from waterair interface (0.15 ± 0.03 MgC ha-1 year-1).

24


(3) The results indicated that mangroves have the ability to retain a large
amount of suspended organic carbon per unit area, with a retained POC value
of 6.71 ± 4.70 MgC ha-1 year-1, while DOC retained a negligible with only 0.07
± 0.11 MgC ha-1 year-1. In contrast, dissolved inorganic carbon (DIC) exported
from mangroves was relatively large, with 4.06 ± 3.49 MgC ha-1 year-1. The
DIC exported from the mangroves mainly at spring tides and accounts for the
largest proportion of all carbon types. After calculating the carbon balance in
the mangroves, carbon accumulated in the soil and biomass of the K. obovata
reached an average value of 7.29 MgC ha-1 year-1, of which the large proportion
is the belowground carbon.
2. New contributions of dissertation
The study identified the following:
- Quantify values on the carbon stocks and calculate the correlations between
carbon in biomass, carbon in soils with stand age.
- Quantify CO2 emissions from mangrove soil to the atmosphere, from water
environment to the atmosphere, calculate the amount of carbon export from
mangroves to surrounding water and vice versa.
- Clarify the relationship between the total carbon stocks in mangrove forests
in XTNP and the carbon emissions from mangroves.
3. Limitation and future research
Carbon concentrations in biomass and in soil were analyzed by traditional
methods. These methods have higher error values than modern analytical
methods by CN analyzer. However, the methods used in this study are still
standard methods, which are widely used in many countries around the world.
Due to the limit of the total study time of the thesis, the measurements are only
made at the stand age of 18 to 20 years old. With the stand age under 10 years
old, the research has consulted some published results of Nguyen Thi Kim Cuc

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