ORGANIC CARBON STORAGE IN SOME

SOILS OF BANGLADESH

Course title: Project thesis Course No: SS-4106

SUBMITTED TO

Soil Science Discipline, Life Science School, Khulna University, in partial fulfillment of the requirements for the degree of four years Bachelor (Hons.) in Soil Science

BY Student ID: 061327 Session: 2005-2006

Soil Science Discipline Life Science School, Khulna University

Khulna, Bangladesh July, 2010

Soil organic carbon storage in some soils of Bangladesh

A Thesis By

Munia Akhtar Moni Examination Roll No: 061327

Session: 2006-2007

Supervisor Md. Sadiqul Amin Assistant Professor

Soil Science Discipline Khulna University

Chairman of Examination Committee

Afroza Begum Associate Professor and Head

Soil Science Discipline Khulna University

Khulna-9208, Bangladesh

Soil Science Discipline

Life Science School, Khulna University Khulna, Bangladesh.

July, 2010

ORGANIC CARBON STORAGE IN SOME SOILS OF BANGLADESH

DECLARATION

This project paper is entirely the candidates own investigations and no part of this paper has been accepted for any degree, nor

it is being concurrently submitted for any degree

SUPERVISOR

Md. Sadiqul Amin Assistant Professor

Soil Science Discipline Khulna University

SOIL SCIENCE DISCIPLINE LIFE SCIENCE SCHOOL, KHULNA UNIVERSITY

KHULNA, BANGLADESH. JULY, 2010

DEDICATED TO

MY BELOVED PARENTS &

BROTHER

Acknowledgement

At first, i would like to express my extreme and humble gratefulness and endless

praise to almighty Allah for bestowing me sound health and energy to complete this

project thesis paper successfully in due time.

It is an excellent opportunity for me to express my deepest sense of gratitude and

sincere indebtedness to Md. Sadiqul Amin, Assistant Professor, Soil Science

Discipline, Khulna University, for his whole hearted guidance, scholastic supervision,

constant encouragement, constructive criticisms and valuable suggestions throughout

the tenure of this thesis work and also in preparation of the manuscript.

It is an immense pleasure to express sincere respect and heartfelt thanks to all the

teachers of Soil Science Discipline, Khulna University, for their constant co-operation

and good wishes during the work.

I am thankful to Mr. Mahfuzur Rahman, lab technician of Soil Physics Laboratory,

Soil Science Discipline; Mr. Abdullah, Lab technician of Soil Chemistry Lab, Soil

Science Discipline, Khulna University for continuous assistance during performing

several tests.

My heartiest thanks go to all of my classmates for their best possible help during the

preparation of this thesis.

Finally, I respectfully remember my parents and brother for their support for my

higher education and without whose blessings this work would not have been

possible.

July, 2010 Author

Munia Akhtar Moni

i

CONTENTS

TITLE Page No. Acknowledgement i Contents ii

List of Tables v

List of Figures vi

Chapter 1 Introduction 1 Chapter 2 Literature review 3 2.1 Soil organic carbon 3

2.2 Soil organic matter dynamics 4

2.3 Soil Carbon sequestration and its impacts on global climate change and food security 5

2.4 Soil Organic Carbon storage 6

2.5 Long term storage of carbon 7

2.6 Changes in Soil organic carbon storage 7

2.7 Stabilization mechanism of soil organic carbon 9

2.7.1 Protected SOC pool 9

2.7.1.1 Chemical stabilization 10

2.7.1.2 Physical protection 10

2.7.1.3 Biochemical stabilization 11

2.7.2 Unprotected SOC pool 11

2.8 Mechanism of carbon emission from soil 12

2.8.1 Tillage 12

2.8.2 Erosion 13

ii

TITLE Page No.

2.8.2.1 Increase of soil degradation 13

2.8.2.2 Breakdown of macroaggregares into microaggregates 13

2.8.2.3 Transportation of easily mineralizable fractions 14

2.8.2.4 Exposition of carbonates 14

2.8.2.5 Increased rate of mineralization 14

2.9. Strategies to increase carbon sequestration 14

2.9.1 Soil management 16

2.9.1.1 Tillage 16

2.9.1.2 Mulching and residue management 17 2.9.1.3 Manuring 18 2.9.1.4 Compost 18 2.9.1.5 Fertilizer and nutrients 18 2.9.1.6 Restoring degraded soils 19

2.9.1.7 Forest soils 19

2.9.1.8 Urban soils 19

2.9.2 Crop management 20

2.9.2.1 Crop rotations and cover crops 20

2.9.2.2 Agroforestry 20

2.9.3 Irrigation 20

2.9.4 Pasture management 21

2.10 Carbon management 21

2.11 Consequences of organic carbon storage in soil 21

2.11.1 Soil quality and fertility 22

2.11.2 Environmental impacts 23

iii

TITLE Page No. 2.11.3 Biodiversity and soil biological functioning 23

2.11.4 Effect of climate change 23

Chapter 3 Materials and methods 25 3.1 Collection of Soil Samples 25 3.2 Processing of Soil Sample 25

3.3 Short description of soil samples 25

3.4 Laboratory Analyses 27

3.4.1 Particle size analysis 27

3.4.2 Bulk density 27

3.4.3 Soil reaction (pH) 27

3.4.4 Electrical conductivity (EC) 27

3.4.5 Soil organic carbon stock 27

Chapter 4 Results and discussion 28 4.1 General Analyses of Soil 28 4.2 Statistical Analyses 30 4.3 Relationship among soil properties 31 Chapter 5 Summary and conclusion 33 References 34

iv

List of Tables

Table No. Title Page No.

Table 2.1 Global estimates of potential net change in carbon

storage through improved management within land-use and change in land-use activities

07

Table 2.2 Possible cropland management practices for the maintenance or increase of soil organic carbon 16

Table 4.1 Physical properties of the studied soils 29

Table 4.2 Organic Carbon Stock, pH and EC of the soils 30

Table 4.3 Descriptive Statistics: BD, %Sand, %Silt, %Clay, SOC stock, pH, EC 31

Table 4.4 Correlations among bulk density, % sand, % silt, % clay, organic carbon stock 32

v

List of Figures

Fig. No. Title Page No.

2.1 Model of soil carbon dynamics 4

2.2 The terrestrial carbon cycle: Soil carbon and the global C budget 5

2.3 Hypothetical changes in soil organic C in two treatments(no-till and conventional tillage) 8

2.4 Conceptual model of SOM (soil organic matter) dynamics with measurable pools 10

2.5 Fractionation of soil organic matter based on aggregate hierarchy 11

2.6 Organic matter losses after various tillage practices 13

2.7 Conceptual model of organic C accumulation and organic C

mineralization, including the effects of tillage-induced disturbance

17

vi

1. Introduction

Storage of soil organic carbon (SOC) refers to the amount of organic carbon stored in

the soil, expressed as a percentage by weight (g C/kg soil). It is part of the soil organic

matter (SOM), which includes other important elements such as calcium, hydrogen,

oxygen, and nitrogen. Soil OM is a key soil component and plays a critical role in a

range of physical, chemical and biological soils processes. Organic material in the soil

is essentially derived from residual plant and animal material, synthesised by

microbes and decomposed under the influence of temperature, moisture and ambient

soil conditions. There are two groups of factors that influence inherent organic matter

content- natural factors (climate, soil parent material, land cover and/or vegetation

and topography), and human-induced factors (land use, management and

degradation). Soil OC, the largest component of the terrestrial carbon pool, plays a

vital role in the terrestrial carbon cycle. The approaches used to study and quantify the

contribution of terrestrial ecosystems to the global carbon balance can be categorized

as top-down and bottom-up (Houghton, 2003).

Top-down approaches start from atmospheric concentration measurements and

generally use inverse modeling. Bottom-up approaches are based on C flux-

measurements in the field, e.g., flux tower measurements or soil carbon concentration,

which are integrated over space and time, often involving process-based models.

Bottom-up and top-down approaches can be considered complementary, but since

they are based on different assumptions and measurements, the predicted carbon

fluxes may diverge strongly (Janssens et al., 2003). The amount of organic carbon in

soil is a balance between the build-up which comes from inputs of new plant and

animal material and the constant losses where the carbon is decomposed and the

constituents separate to mineral nutrients and gases, or are washed or leached away.

In particular, the suitability of soil for sustaining plant growth and biological activity

is a function of physical (porosity, water holding capacity, structure and tilth) and

chemical properties (nutrient supply capability, pH, salt content), many of which are a

function of SOM content (Doran and Safley, 1997). In general, increases in SOM are

seen as desirable by many farmers as higher levels are viewed as being directly

related to better plant nutrition, ease of cultivation, penetration and seedbed

preparation, greater aggregate stability, reduced bulk density, improved water holding

capacity, enhanced porosity and earlier warming in spring (Lal, 2002). Soil OC is the

1

most often reported attribute from long-term agricultural studies and is chosen as the

most important indicator of soil quality and agronomic sustainability because of its

impact on other physical, chemical and biological indicators of soil quality (Reeves,

1997).

Total SOC content increased with precipitation and clay content and decreased with

temperature. The importance of these controls switched with depth, climate

dominating in shallow layers and clay content dominating in deeper layers, possibly

due to increasing percentages of slowly cycling SOC fractions at depth. In tropical

region like Bangladesh, SOC stock is not high because of the climatic condition

(temperature, rainfall etc.), cultural practices, land types, physiology etc.

A lot of experimental works have been performed on SOC stock in different countries

of the world and vast volume of literature on SOC stock investigation presents much

information about the SOC storage under different climatic and environmental

condition. However, in Bangladesh, only a few investigations have been made on the

storage pattern of SOC with different properties of soil though it is essentially related

with fertility status of soil and also global climate change and food security which is

an increasing alarm to us.

With these views in mind, the present study was initiated with some representative

soils of Bangladesh to meet the following objectives:

To assess the OC stock in some soils of Bangladesh. To study the relation among SOC stock and bulk density, %sand, %silt and

%clay.

2

2. Literature Review

The stupendous roles of soil organic carbon in soil processes, soil productivity and

sustainability of agriculture led scientists to study the storage of soil organic carbon in

different soils.

In connection with this present work, a brief review of literature relating to soil

organic carbon storage together with some other related characteristics of soil organic

carbon are presented in this chapter.

2.1. Soil organic carbon

Soils are undoubtedly important reservoirs for carbon (Watson et al., 2000). Soil OC

is closely related to the amount of SOM (soil organic matter). Accurately estimating

the SOC stocks in soils is difficult because SOC stocks vary over multiple spatial

scales due to the complexity of physical, chemical and biological processes that

influence C cycling in the soil (Trumbore et al., 1995). Variations in soil SOC stocks

are related to a number of natural factors (e.g. climate, parent material, landscape

position) and human-induced factors (e.g. land use type, management intensity) (Von

Ltzow et al., 2006). Soil organic C is a complex entity consisting of several fractions

of different physical and chemical characteristics and microbial degradability (Collins

et al., 1997). Physical separation of soil organic matter into pools with different ages

and turnover rates has shown potential to improve the analytical capacity to detect

organic C changes induced by land management, and to help understand soil C

dynamics (Christensen, 1992; Conant et al., 2003). Changes in the size of microbial

C, microbially respired C in short-term laboratory incubations, and associated

metabolic quotients have frequently been used to evaluate the influence of landuse

and management on the soil organic matter status and consequently soil biological

quality (Saggar et al., 2001). Dissolved organic C mostly contains the low molecular

weight, microbially available and mobile components of organic C, which could

readily be removed from a soil system by microbial respiration and/or by leaching. In

contrast, organic matter associated with clay and silt is considered more resistant to

effects of management, mainly due to physical protection and chemically recalcitrant

nature (Six et al., 2002). Additionally, the soil organic matter and its pools (including

living microbial biomass) that respond to management are often used as measures of

soil quality. These organic matter pools support plant production and influence many

3

important physical, chemical and biological parameters of soils (Kumar and Goh,

2000).

2.2. Soil organic matter dynamics

Carbon is a key ingredient in soil organic matter (57% by weight). Plants produce

organic compounds by using sunlight energy and combining carbon dioxide from the

atmosphere with water from the soil. Soil organic matter is created by the cycling of

these organic compounds in plants, animals, and microorganisms into the soil. Well-

decomposed organic matter forms humus, a dark brown, porous, spongy material that

provides a carbon and energy source for soil microbes and plants. When soils are

tilled, organic matter previously protected from microbial action is decomposed

rapidly because of changes in water, air, and temperature conditions, and the

breakdown of soil aggregates accelerates erosion. A soil with high organic matter is

more productive than the same soil where much of the organic matter has been

burned through tillage and poor management practices and transported by surface

runoff and erosion. However, organic matter can be restored to about 60 to 70% of

natural levels with best farming practices (Sundermeier et al., 2004). The stock of

organic carbon present in natural soils represents a dynamic balance between the input

of dead plant material and loss from decomposition (mineralization) (Fig. 2.1). In

aerobic soil conditions, most of the carbon entering soil is labile and only a very small

fraction (1%) of what enters the soil (55Pg yr-1) accumulates in the stable, humic

fraction (0.4Pg yr-1).

Fig. 2.1. Model of soil carbon dynamics (Balesdent et al., 2000).

4

2.3. Soil Carbon sequestration and its impacts on global climate change and food

security

Soil carbon sequestration is the process of transferring carbon dioxide from the

atmosphere into the soil through crop residues and other organic solids, and in a form

that is not immediately reemitted. The quantity of C stored in soils is highly

significant; soils contain about three times more C than vegetation and twice as much

as that which is present in the atmosphere (Batjes and Sombroek, 1997). The

terrestrial carbon cycle is presented in Fig. 2.2. In this cycle, soil organic carbon

represents the largest reservoir in interaction with the atmosphere and is estimated at

about 1500 Pg C to 1m depth (about 2456 Pg C to 2m depth). Inorganic C represents

around 1700 Pg, but it is captured in more stable forms such as caliches. Vegetation

(650 Pg) and the atmosphere (750 Pg) store considerably less C than soils do.

Fig. 2.2. The terrestrial carbon cycle: Soil carbon and the global C budget (Robert, 2001).

Emissions corresponding to change in land use (deforestation and increase in pasture

and cultivated lands) were around 140 Pg from 1850 to 1990 (from 0.4 Pg yr-1 in 1850

to 1.7 Pg yr-1 in 1990), with a net release to the atmosphere of 25 Pg C (Houghton,

1995). According to IPCC (2000), the historical loss from agricultural soils was 50 Pg

C over the last half century, which represents 1/3 of the total loss from soil and

vegetation. The increase in atmospheric concentration of CO2 by 31% since 1750

from fossil fuel combustion and land use change necessitates identification of

strategies for mitigating the threat of the attendant global warming. Depletion of SOC

pool has contributed 7812 Pg of C to the atmosphere. Some cultivated soils have lost

one-half to two-thirds of the original SOC pool with a cumulative loss of 30-40 Mg

C/ha (Mg=mega gram=10 6 G=1 ton). The depletion of soil C is accentuated by soil

5

6degradation and exacerbated by land misuse and soil mismanagement. Thus, adoption

of a restorative land use and recommended management practices (RMPs) on

agricultural soils can reduce the rate of enrichment of atmospheric CO2 while having

positive impacts on food security, agro-industries, water quality and the environment.

A considerable part of the depleted SOC pool can be restored through conversion of

marginal lands into restorative land uses, adoption of conservation tillage with cover

crops and crop residue mulch, nutrient cycling including the use of compost and

manure, and other systems of sustainable management of soil and water resources.

The cumulative potential of soil C sequestration over 25-50 years is 30-60 Pg (Lal,

2004).

2.4. Soil Organic Carbon storage

Newly photosynthesized C is added regularly in the form of plant litter, and existing

SOC is gradually decomposed back to CO2 by soil biota. Management or

environmental conditions that change the relative rates of inputs and decomposition

will effect a change in the amount of SOC stored. Changes in SOC can only be

reliably measured over a period of years or even decades (Post et al., 2001). Since the

distribution of SOC in space is inherently variable, temporal changes (e.g.,

attributable to management practices, environmental shifts, successional change) must

be distinguished from spatial ones (e.g., attributable to landform, long-term

geomorphic processes, non uniform management). Temporal changes in SOC can be

defined in two ways as an absolute change in stored C (SOC at t x minus SOC at t

0). Table 2.1, illustrates the potential range for C stock increases through some

broadly defined activities. It provides data and information on C stock changes for

some candidate activities for the year 2010. The greatest potential for C sequestration

occurs when land-use becomes more sustainable, with the largest dividend estimated

when arable land is changed to agroforestry.

Table 2.1 Global estimates of potential net change in carbon storage through improved management

within land-use and change in land-use activities. Values shown are average rates during this period of

accumulation (Haile et al., 2009).

2.5. Long term storage of carbon

Composting may result in long-term carbon storage in the form of undecomposed

carbon compounds. The composting process leads to increased formation of stable

carbon compounds (e.g., humic substances, aggregates) that then can be stored in the

soil for long (>50 years) periods of time. Humic substances make up 60-80 percent of

soil organic matter and are made up of complex compounds that render them resistant

to microbial attack. In addition to humic substances, soil organic carbon may be held

in aggregates (i.e., stable organo-mineral complexes in which carbon is bonded with

clay colloids and metallic elements) and protected against microbial attack. The

application of compost produces a multiplier effect by qualitatively changing the

dynamics of the carbon cycling system and increasing the retention of carbon from

noncompost sources. The carbon increase apparently comes not only from the organic

matter directly, but also from retention of a higher proportion of carbon from residues

of crops grown on the soil. This multiplier effect could enable compost to increase

carbon storage by more than its own direct contribution to carbon mass accumulation

(Franklin et al., 1994).

2.6. Changes in Soil organic carbon storage

Management or environmental conditions that change the relative rates of inputs and

decomposition will effect a change in the amount of SOC stored. Rates of change in

SOC (typically less than 0.5 Mg C ha_1 year_1) are quite small, however, compared to

the large amounts of SOC often present (as high as 100 Mg C ha_1, or more, in the top

Additional Activities

Total area (106

acre)

Area under

activity (%)

Net annual change (Metric

tons of CO2 acre-1 yr-1)

Estimated net change in 2010 (106 Metric

tons of CO2 yr-1)

Improved management within land use Crop land 3212 30 0.4 385 Forest land 10008 10 0.6 600

Grazing land 8401 10 1 840 Agroforestry

area 988 20 0.4 79

Urban land 247 5 0.4 5 Change in land use

Agroforestry 1557 20 4.6 1432 Grassland 3707 3 1.2 133

7

30 to 60 cm soil layer). Thus changes in SOC can only be reliably measured over a

period of years or even decades (Post et al., 2001). Since the distribution of SOC in

space is inherently variable, temporal changes (e.g., attributable to management

practices, environmental shifts, successional change) must be distinguished from

spatial ones (e.g., attributable to landform, long-term geomorphic processes, non

uniform management).

Temporal changes in SOC can be defined in two ways, (Fig. 2.3) as an absolute

change in stored C (SOC at t x minus SOC at t 0), or as a net change in storage

among treatments (SOC in treatment A minus SOC in treatment B, after x years). The

former provides an estimate of the actual C exchange between soil and atmosphere;

the latter provides an estimate of the C exchange between soil and atmosphere,

attributable to treatment A, relative to a control (treatment B). Both expressions of

temporal change may be available from manipulative experiments with appropriate

samples collected at establishment (assesses spatial variability) and at various

intervals (5 to 10 years) thereafter. For measuring the change in C storage, either

absolute or net, typically for periods of 5 years or more, the method needs to: measure

organic (not total) C, provide estimates of C stock change (expressed in units of C

mass per unit area of land to a specified soil depth and mass), be representative of the

land area or management treatment under investigation, and provide an indication of

confidence in the measurements (Ellert et al., 2008).

Fig. 2.3. Illustration of hypothetical changes in soil organic C in two treatments, A and B. For

treatment A, the absolute change is the difference in SOC at timex, compared to that at time0. The

net change is the difference between SOC in treatment A and that in treatment B, at timex, assuming

that SOC was the same in both treatments at time0. The latter approach is often used to measure the

effect on SOC of a proposed treatment (e.g., no-till) compared to a standard control (e.g.,

conventional tillage) (Ellert et al., 2008).

8

2.7. Stabilization mechanism of soil organic carbon

Whole SOC is commonly separated into labile (active) and stable (passive) pools

(Parton et al., 1987). For soils to act as a C sink, organic C needs to be stabilized in

stable C pools (Paustian et al., 1997a). Due to different protection mechanisms, the

degree and the duration of stabilization of SOC within macroaggregates and

microaggregates differ (Tisdall and Oades, 1982). Macroaggregates contain C that (i)

functions as transient binding agents holding microaggregates together and (ii) is

occluded within microaggregates, which results in greater absolute C contents of

macroaggregates than microaggregates (Tisdall and Oades, 1982; Elliott, 1986).

Microaggregates have a lower C storage potential, but as they sequester C in the long

term (Six et al., 2000); the degree of stabilization is greater.

2.7.1. Protected SOC pool

Stabilization means a decrease in the potential for SOM loss by respiration, erosion or

leaching. Stabilization may begin before plant tissues reach the ground, or even before

they die. Three main mechanisms of SOM stabilization have been proposed: (1)

chemical stabilization, (2) physical protection and (3) biochemical stabilization

(Christensen, 1996). Soil organic matter can be protected from decomposition through

physical protection within soil aggregates, chemical protection through association

with mineral particles, and biochemical stabilization through biotic or abiotic creation

of decay-resistant compounds. Most SOC models assume a linear increase in C

content with C input, and thus C sequestration can continue regardless of the amount

of organic C already contained in each SOC pool (Paustian et al., 1997b). These

process-oriented models are based on first-order kinetics in which the decomposition

rate is defined as a constant. Hassink (1997) assumed that the protective capacity of

soil to store organic C is limited and that the rate of additional C sequestration

depends on the degree to which the protective capacity of the silt plus clay fraction

has been reached. Based on this concept, Six et al. (2002) further developed a

conceptual SOM model of C saturation that includes physical, chemical, and

biochemical protection mechanisms (Fig. 2.4). They defined functional C pools that

are measurable and argued that the C sequestration in each of these pools depends on

their saturation deficit (i.e., the difference between saturation level and the actual SOC

content).

9

Fig. 2.4. Conceptual model of SOM (soil organic matter) dynamics with measurable pools. The soil

processes of aggregate formation/degradation, SOM adsorption/desorption and SOM

condensation/complexation and the litter quality of the SOM determine the SOM pool dynamics (Six et

al., 2002).

2.7.1.1. Chemical stabilization: Silt- and clay-protected SOM

Chemical recalcitrance may be important in the slowest turnover `stable' pools (which

contribute little to soil dynamics) and possibly at high latitudes, where lignin can

incorporate nitrogen to form condensation products leading to the build-up of thick

layers of non-peat humus (Stevenson, 1982).

2.7.1.2. Physical protection: Microaggregate-protected SOM

Physical protection is one of the important ways to stabilize organic carbon in soils

(Fig. 2.5). Aggregates physically protect SOM by forming physical barriers between

microbes and enzymes and their substrates and controlling food wed interactions and

consequently microbial turnover (Elliot and Coleman, 1998).

The physical protection exerted by macro and /or microaggregates on POM

(particulate organic matter) C is attributed to

1. the compartmentalization of substrate biomass (Killham et al., 1993)

2. The reduced diffusion of oxygen into macro and especially micraaggregates (Sexton

et al.,1985) which leads to a reduced activity within the aggregates (Sollins et al.,

1996), and

10

3. the compartmentalization of microbial biomass and microbial grazers (Elliot et al.,

1980).

Fig. 2.5. Fractionation of soil organic matter based on aggregate hierarchy (Jastrow et al., 2004).

2.7.1.3. Biochemical stabilization: Biochemically-protected SOM

Biochemical stabilization or protection of SOM occurs due to the complex chemical

composition of the organic materials. The biochemically protected C pool turns over

very slowly. This complex chemical composition can be an inherent property of the

plant material (referred to as residue quality) or to be attained during decomposition

through the condensation and complexation of decomposition residues, rendering

them more resistant to subsequent decomposition. Using 14C dating, it has been

found that, in the surface soil layer, the non-hydrolyzable C is approximately 1300

years older than total soil C (Paul et al., 2001).

2.7.2. Unprotected SOC pool

Labile SOM pools are characterized by a rapid turnover, mainly consist of young

SOM, and are sensitive to land management and environmental conditions (Parton et

al., 1987). Due to these characteristics, labile SOM pools play an important role in

short-term C and N cycling in terrestrial ecosystems (Schlesinger, 1990). The most

commonly isolated labile C pools are the light fraction (LF) and particulate organic

matter (POM). The LF appears in soils as free POM (Golchin et al., 1994; Six et al.,

1998). When free POM is further decomposed, it can be incorporated into aggregates,

where its decomposition is restricted (Puget et al., 1995). In this case, POM becomes

part of a less labile pool, but is easily decomposable when it is set free again; hence

the degree of protection of labile C is dependent on aggregate turnover. Stabilization

11

of occluded organic matter is greatest when aggregate stability is high and aggregate

turnover is slow (Six et al., 1998).

2.8. Mechanism of carbon emission from soil

The depletion of the SOC pool upon cultivation is attributed to three processes:

1. oxidation or mineralization due to breakdown of aggregates leading to exposure of

carbon, and change in temperature and moisture regimes,

2. leaching and translocation as DOC (dissolved organic carbon) or POC (particulate

organic carbon) and

3. accelerated erosion by water runoff or wind (Lal, 2001).

2.8.1. Tillage

Crop cultivation generally depletes soil organic C due to accelerated mineralization,

leaching, translocation and erosion (Lal, 2002), and reduces aggregate stability as a

result of organic matter loss and tillage breakage (Bronick and Lal, 2005). Tilling the

soil is disruptive and can promote soil erosion, high moisture loss rates, degradation

of soil structure and depletion of soil nutrients and C stocks. Following long-term

tillage soil C stocks can be reduced by as much as 20-50% (Haas et al., 1957).

Conservation tillage reduces the negative impacts of tillage, preserves soil resources

and can lead to accrual of much of the soil C lost during tillage (Paustian et al., 1997a,

b).

As indicated in Fig. 2.6, moldboard plowing causes the fastest decline of organic

matter, no-till the least. The other three types of tillage are intermediate in their ability

to foster organic matter decomposition. The moldboard plow increases the soil surface

area, allowing more air into the soil and speeding the decomposition rate. The

horizontal line on Fig. 2.6 represents the replenishment of organic matter provided by

wheat stubble. With the moldboard plow, more than the entire organic matter

contribution from the wheat straw is gone within only 19 days following tillage.

12

Fig. 2.6. Organic matter losses after various tillage practices (Reioosky et al., 1995).

2.8.2. Erosion

The processes of soil erosion are caused by following mechanisms.

2.8.2.1. Increase of soil degradation

Soil erosion increases soil degradation and reduces biomass production on-site.

Erosion reduces production through adverse effects on soil structure, aeration,

effective rooting depth, available water-holding capacity, and nutrient reserves; the

reduced production, in turn, further reduces the soil carbon pool. Erosion decreases

net primary productivity (NPP) on eroded sites, increases oxidation of soil organic

matter, and reduces net ecosystem productivity (NEP). The gains in the soil carbon

pool in depressional sites rarely compensate for losses on eroded sites in view of

reduced NEP and increased mineralization (Renwick et al., 2004).

2.8.2.2. Breakdown of macroaggregares into microaggregates

Erosion causes the breakdown of macroaggregates into microaggregates and, possibly,

complete soil dispersion, exposing hitherto encapsulated organic matter to microbial

processes. The outer layer of macroaggregates has more soil organic matter than the

inner core; that outer organic matter is progressively peeled off and transported with

the sediments, because aggregation and soil structure control decomposition of

organic matter in soil. Changes in soil moisture and temperature also increase the rate

of decomposition of the remaining organic matter at the eroded site. Eroded soils have

different radiative and thermal properties, leading to increased soil temperature, an

important factor controlling CO2 emission from soil (Van Oost et al., 2004).

13

2.8.2.3. Transportation of easily mineralizable fractions

Sediments are often enriched in SOC, because SOC has low density and is

concentrated in the vicinity of the soil surface. Most of C transported with sediment is

the labile fraction, which is easily mineralizable; the mineralizable fraction in

translocated organic matter may range from 29% to as high as 70%. Thus, assuming

that the mineralizable fraction in eroded and redeposited material is close to zero can

lead to erroneous conclusions. In most cases, sediment deposited may lead to higher

emissions (CO2, CH4, and N2O) from depositional sites. The fate of carbon deposited

in burial and depressional sites is governed by complex processes. The deposition may

decrease the rate of mineralization by reaggregation of dispersed clay and silt and

burial of carbon-rich material and calciferous layer. On the other hand, the rate of

mineralization may also be increased in depressional sites because of the high

proportion of mineralizable fraction. Depending on soil moisture and temperature

regimes, depositional sites may also undergo methanogenesis with release of CH4 and

denitrification with release of N2O. The rate of mineralization on erosional phases

strongly depends on soil temperature (Lal et al., 2004).

2.8.2.4. Exposition of carbonates

In truncated soil profile characterized by carbonaceous subsoil horizons, exposed

carbonates may react with acidiferous materials, such as fertilizers, and release CO2in

to the atmosphere (Lal et al., 2004).

2.8.2.5. Increased rate of mineralization

The fate of carbon deposited in burial and depressional sites is governed by complex

processes. The deposition may decrease the rate of mineralization by reaggregation of

dispersed clay and silt and burial of carbon-rich material and calciferous layer. On the

other hand, the rate of mineralization may also be increased in depressional sites

because of the high proportion of mineralizable fraction. Depending on soil moisture

and temperature regimes, depositional sites may also undergo methanogenesis with

release of CH4 and denitrification with release of N2O. The rate of mineralization on

erosional phases strongly depends on soil temperature (Renwick et al., 2004).

2.9. Strategies to increase carbon sequestration

Soil organic carbon has been depleted through (1) the long-term use of extractive

farming practices and (2) the conversion of natural ecosystems (such as forest lands,

14

prairie lands, and steppes) into croplands and grazing lands. Such a conversion

depletes the soil organic carbon pool by increasing the rate of conversion of soil

organic matter to CO2, thereby reducing the input of biomass carbon and accentuating

losses by erosion. Most agricultural soils have lost 30 to 40 mt of carbon per hectare,

and their current reserves of soil organic carbon are much lower than their potential

capacity. Soil carbon sequestration involves adding the maximum amount of carbon

possible to the soil. The technical potential for this process is higher in

degraded/desertified soils and soils that have been managed with extractive farming

practices than it is in good-quality soils managed according to recommended

management practices (RMPs). Thus, converting degraded/desertified soils into

restorative land and adopting RMPs can increase the soil carbon pool. The rate of soil

carbon sequestration through the adoption of RMPs on degraded soils ranges from

100 kilograms per hectare (kg/ha) per year in warm and dry regions to 1,500 kg/ha per

year in cool and temperate regions. A recent estimates of the technical potential of

soil organic carbon sequestration through adoption of RMPs for world cropland soils

(1.5 billion hectares) is 0.4 billion to 1.2 billion mt of carbon per year. Examples of

soil and crop management technologies that increase soil carbon sequestration include

no-till (NT) farming with residue mulch and cover cropping;

integrated nutrient management (INM), which balances nutrient application with

judicious use of organic manures and inorganic fertilizers;

various crop rotations (including agroforestry);

use of soil amendments (such as zeolites, biochar, or compost); and

improved pastures with recommended stocking rates and controlled fire as a

rejuvenate method.

Another good strategy for soil carbon sequestration is the restoration of

degraded/desertified soils (about 2 billion hectares), which can be achieved through

afforestation and reforestation. The technical potential of soil carbon sequestration

through restoration of degraded/desertified soils is 0.6 billion to 1 billion mt of carbon

per year. The establishment of energy plantations can also improve ecosystem carbon

pools. It is estimated that afforestation and establishment of energy plantations can

offset 25 billion mt of carbon between 2000 and 2050. The technical potential of

carbon sequestration in world soils may be 2 billion to 3 billion mt per year for the

15

next 50 years. Thus, the potential of carbon sequestration in soils and vegetation

together is equivalent to a draw-down of about 50 parts per million of atmospheric

CO2 by 2100 (Lal, 2009). Management of soils to increase SOC content therefore

involves measures that reduce losses and/or measures that increase inputs (Table 2.2).

Table 2.2: Possible cropland management practices for the maintenance or increase of soil organic

carbon (Eleanor Milne, 2008).

SOC loss reduction SOC input increase

Reduced tillage/no-tillage Organic manures, Composts

Cover crops Cover crops

Erosion reduction measures (mulching,

contour cultivation on sloping land) Crop residue returns

Reduced fallow periods Increase crop productivity through

irrigation/chemical fertilizer

Reduced burning of crop residues

2.9.1. Soil management

If amounts of C entering the soil exceed that lost to the atmosphere by oxidation, SOC

increases. Such an increase can result from practices that include improved: (1) tillage

management and cropping systems, (2) management to increase amount of land

cover, and (3) efficient use of production inputs, e.g. nutrients and water. Among the

most important contributors is conservation tillage (i.e., no-till, ridge-till, and mulch-

tillage) whereby higher levels of residue cover are maintained than for conventional-

tillage (Follett, 2001).

2.9.1.1. Tillage

Numerous studies of replicated, long-term field experiments comparing conventional

tillage (Fig. 2.7) and no-tillage have demonstrated that most soils, following

conversion to no-tillage, show an increase in aggregation and soil carbon content

relative to tilled soils, when the measurements are integrated over the full depth of

soil affected by tillage (typically the top 20-30 cm) (Paustian et al., 1997, West and

Post, 2002). In general, positive soil C responses are obtained first after several years

of no-till management (Six et al., 2004) and after 20-30 years, the relative rates of C

accumulation tend to decline as soil C levels approach a new equilibrium level under

no-till conditions (West and Post, 2002).

16

Fig. 2.7. This conceptual model depicts organic C accumulation and organic C mineralization in

conjunction with the formation and turnover of soil aggregates, including the effects of tillage-induced

disturbance. At t1, fresh residue is incorporated into macroaggregates and forms coarse intra-aggregate

particulate organic matter (iPOM). Subsequently (from t1 to t2), the iPOM is further decomposed and

fragmented into fine iPOM within the aggregates, which forms the core of a new microaggregate and is

physically protected from decomposition. At t3, carbon is depleted and microbial activity and

production of binding agents decrease. This cessation results in a destabilization and potential

disaggregation of the macroaggregates. Upon disaggregation, microaggregates, the mineral fraction,

and POM are released. These fractions may be reincorporated into new macroaggregates when fresh

residue is added. In CT some macroaggregates go through the same sequence. However, the majority

of them are disrupted from cultivation or slaking in the field at t2 and go through a shorter cycle

resulting in faster turnover. This faster turnover results in fewer macroaggregates reaching t3 and less

fine iPOM is formed in CT compared to NT. When iPOM is released from the aggregates, it becomes

exposed to microbial decay which leads to a loss of iPOM and an increased CO2 flux in CT compared

to NT. This loss precludes re-incorporation of the iPOM in to macroaggregates and thereby accounts

for the differences in composition of macroaggregates in NT and CT (Six et al., 1999).

2.9.1.2. Mulching and residue management

Carbon sequestration in terrestrial ecosystems has two distinct but related

components: sequestration in biomass (primarily trees comprising both the above

ground and below ground components) and soil. A fraction of the biomass returned to

the soil is converted into stable humic substances and related organo-mineral

17

complexes with a long residence time. The effectiveness of soil C sequestration

depends on the quantity and quality of biomass returned to the soil. Crop residues are

also a principal source of C, which constitutes about 40% of the total biomass on dry

weight basis. Increase in rate of application of biomass C increases the SOC pool. In

cropland soils, a principal source of biomass is the crop residues (Lal, 2008).f

2.9.1.3. Manuring

Manure is an excellent soil amendment, providing both organic matter and nutrients.

The application of manure can increase SOM and used in order to restore soil fertility

and obtain a satisfactory yield, because SOM contributes to nutrient supply,

improvement of soil physical properties, protection from erosion, and promotion of

biological activity (Jimenez et al., 2002). Application of organic manure, either alone

or in combination with mineral fertilizers, is more effective in increasing SOM and its

fractions than mineral fertilizers alone (Wu et al., 2004). Additionally, lots of carbon

would be added to the soil, resulting in no loss of soil organic matter. High crop

residues grown from this manure application would also contribute organic matter.

2.9.1.4. Compost

Adding compost can raise soil carbon levels by increasing organic matter inputs. Soils

degraded by intensive crop production, construction, mining, and other activities lose

organic matter when decomposition rates and removals of carbon in harvests exceed

the rate of new inputs of organic materials. Adding compost shifts the balance so that

soil organic carbon levels are restored to higher levels. Some of the compost carbon is

retained by the system (Franklin et al., 1994).

2.9.1.5. Fertilizer and nutrients

Fertilization is often recommended to increase SOM level and carbon sequestration in

soils of highly managed multiple cropping systems (Yang et al., 2005). Fertilization

could promote aggregate formation (Sleutel et al., 2006) and stabilization (Blair et al.,

2006), then enhance the spatial inaccessibility for decomposing organisms (Kgel-

Knabner et al., 2008). Fertilization could affect the population, composition, and

function of soil microorganisms (Marschner et al., 2003), and soil microbial biomass,

but mineral fertilizers had relatively less effect on soil microbial biomass than organic

fertilizers (Plaza et al., 2004). Mineral fertilizer could increase crop residue and root

exudates which provide organic matter for microorganisms.

18

2.9.1.6. Restoring degraded soils

The potential for sequestering C through the rehabilitation of drylands is substantial

(FAO, 2001b). The magnitude of the potential for sequestering C in soils in terrestrial

ecosystems at 50-75 percent of the historic carbon loss (Lal, 2000). Furthermore, Lal

hypothesized that annual increase in atmospheric CO2 concentration could be

balanced out by the restoration of 2 000 000 000 ha of degraded lands, to increase

their average carbon content by 1.5 tonnes/ha in soils and vegetation. Enhancing CS

in degraded agricultural lands could have direct environmental, economic, and social

benefits for local people. The effects of soil degradation and desertification affect the

global C cycle. A decline in soil quality leads to a reduction in the soil organic C pool,

and an increase in the emission of CO2 to the atmosphere. The decline in soil quality

and structure leads to a loss in the capacity to retain water, and therefore in plant

productivity (Lal, 2000).

2.9.1.7. Forest soils

Plantations can sequester significant amounts of carbon and are generally considered

to be carbon sinks, unless they replace natural forests, which are usually richer in

carbon. The largest potential carbon gains for plantations are on marginal agricultural

land and degraded soils (Lal, 2004b). However, in some cases plantations deplete soil

carbon stocks and careful management is therefore necessary. By increasing the

rotation period for cutting and implementing site improvement strategies, soil carbon

stocks can be replenished and more carbon sequestered by the vegetation.

2.9.1.8. Urban soils

Urban areas represent one of the single largest sources of C emissions, but such

environments can be managed to foster greenhouse gas management to mitigate

climate change. Accompanying urbanisation and development is a rapid increase in

the area of turfgrass, i.e. home lawns, parks, recreational facilities, and golf courses

(Bandaranayake et al., 2003). Some scientists (Quian and Follet, 2002) suggested

turfgrass may be making substantial contributions to sequester atmospheric C, due to

high productivity and lack of soil disturbance. Others (Bandaranayake et al., 2003)

also found that such grasslands may act as C sinks, absorbing more C than they

release, resulting in C sequestration.

19

2.9.2. Crop management

Strategies for SOC restoration by adoption of RMPs (recommended management

practices) include conversion from conventional tillage to no-till, increasing cropping

intensity by eliminating summer fallows, using highly diverse rotation, introducing

forage legumes and grass mixtures in the rotation cycle, increasing crop production,

and increasing carbon (C) input into the soil (Desjardins et al., 2001). West and Post

(2002) observed that changing plow till to no-till increased SOC pool at the rate of 57

g C m-2 year-1 (or 570 kg ha-1 year-1). Relatively lower rates (14 g C m-2 year_1) of

increase in SOC pool were associated with adoption of complex crop rotations. The

efficiency of a no-till system for SOC sequestration is enhanced when used in

combination with high intensity crop rotations and elimination of summer fallow

(Potter et al., 1997). Converting a monoculture to a crop rotation also leads to SOC

sequestration, which increases residue inputs to the soil (Robinson et al., 1996; Drury

et al., 1998).

2.9.2.1. Crop rotations and cover crops

Aggregate dynamics vary among different crops, crop rotations and cover crops

(Jarecki and Lal, 2003). Crop rotation is a sequence of crops grown in regularly

recurring succession on the same area of land. Varying the type of crops grown can

increase the level of soil organic matter. However, effectiveness of crop rotating

depends on the type of crops and crop rotation times (Peter, 2004). Many types of

plants can be grown as cover crops include: rye, buckwheat, hairy vetch, crimson

clover, subterranean clover, red clover, sweet clover, cowpeas, millet, and forage

sorghums.

2.9.2.2. Agroforestry

One management practice with a high potential for carbon sequestration in tropical

areas is agroforestry (Lal, 2002).

2.9.3. Irrigation

Irrigation increases C input to soils via increased litter and root production. When

assessing the potential of irrigation of arid or semiarid land to increase C storage in

soils, one needs to assess C loss from CO2 emitted to the atmosphere as a result of I)

fertilizer manufacture, storage, transport and application, 2) fossil- fuel CO2 emitted

from pumping irrigation water, 3) farm operations, such as tillage and planting, and 4)

20

CO2 lost via dissolved carbonate in irrigation water (West and Marland, 2001). In

addition, C may be lost as CO2 from the irrigation water itself. When water is applied

to the soil, CaCO3 can precipitate, releasing CO2 into the atmosphere.

2.9.4. Pasture management

On a global basis, grasslands occupy 3400 M ha. Restoring degraded grazing lands

and improving forage species is important to SOC pool. In addition to converting

marginal croplands to pastures can also sequester C. Similar to cropland; management

options for improving pastures include judicious use of fertilizers, controlled grazing,

sowing legumes and grasses or other species adapted to the environment,

improvement of soil fauna and irrigation (Follett et al., 2001).

2.10. Carbon management

Increasing soil C stocks requires increasing C inputs and/or reducing soil

heterotrophic respiration. Management options that contribute to reduced soil

respiration include reduced tillage practices (especially no-till) and increased cropping

intensity. Reconstructions of global landuse change suggest that terrestrial ecosystems

have contributed as much as half of the increases in CO2 emissions from human

activity in the past two centuries (Post et al., 1990; Houghton and Skole, 1990). Of

the past anthropogenic CO2 additions to the atmosphere, about 50 Pg has been

contributed by cultivated soils (Paustian et al., 1997a), through the mineralization of

soil organic carbon (SOC). Physical disturbance associated with intensive soil tillage

increases the turnover of soil aggregates and accelerates the decomposition of

aggregate-associated SOM. No-till increases aggregate stability and promote the

formation of recalcitrant SOM fractions within stabilized micro- and macroaggregate

structures. Experiments using 13C natural abundance show up to a two-fold increase

in mean residence time of SOM under no-till vs. intensive tillage. Greater cropping

intensity, i.e., by reducing the frequency of bare fallow in crop rotations and

increasing the use of perennial vegetation, can increase water and nutrient use

efficiency by plants, thereby increasing C inputs to soil and reducing organic matter

decomposition rates (Paustian et al., 2000).

2.11. Consequences of organic carbon storage in soil

Organic materials are important soil additives to improve soil physical properties.

This is important to sustain the productivity of soils particularly in semi-arid regions

21

where there is low input of organic materials. Soil aggregate formation and aggregate

stability have an important role in crop production and sustainable agricultural

management. Soil aggregate formation has an important role concerning seedsoil

relation, hydraulic conductivity and root respiration, the diffusion of gases within the

soil and plant growth. Furthermore, water-stable aggregates in soil prevent erosion,

which is one of the main factors of soil degradation (Dinel et al., 1991). Structural soil

degradation occurs mostly due to the decrease in soil organic matter caused by

excessive soil cultivation (Grandy et al., 2002). The slaking of aggregates increased

with the rate of wetting. Faster wetting leads to more entrapped air and greater

differential movement of particles due to swelling (Emerson, 1954). On the other

hand, soil structures under the integrated cropping and organic regime were the most

stable (Buciene et al., 2003). A good structure is important for sustaining long-crop

production on agricultural soils because it influences water status, workability,

resistance to erosion, nutrient availability and plant growth and development.

Organic matter is an important agent responsible for binding soil mineral particles

together creating an aggregate hierarchy (Oades and Waters, 1991). The organic

matter is still considered to provide the flexible links between the external surfaces of

clay domains (Emerson et al., 1986). The main bonding polymers in aggregation are

considered to be carbohydrates (Emerson and Greenland, 1990). Soil aggregate

stability was highly correlated with soil organic matter content but the addition of

crop residues and manure were not alone sufficient to restore soil physical quality

(Spaccini et al., 2002).

2.11.1. Soil quality and fertility

The concentration of SOC is also commonly used as a soil quality index (Sikora and

Stott, 1996). Soil organic matter is considered to be the most important factor that

contributes to soil quality because of the significant influence it has on soil chemical,

physical, and biological properties. Organic matter is of particular interest for tropical

soils with low activity clays, i.e. having a very low cation exchange capacity. Cation

exchange capacity in function of the increase in organic matter. Bioavailability of

other important elements, such as phosphorus will be improved and the toxicity of

others can be inhibited by formation of chelate or other bonds, for example, aluminum

and OM (Robert, 1996). In agriculture with low plant nutrient inputs, recycling of

22

nutrients (N, P, K, Ca) by gradual decomposition of plant and crop residues is of

crucial importance for sustainability (Sanchez and Salinas, 1982).

2.11.2. Environmental impacts

Carbon sequestration in agricultural soils counteracts desertification process through

the role of increased soil organic matter in structural stability (resistance to both wind

and water erosion) and water retention, and the essential role of soil surface cover by

plant, plant debris or mulch in preventing erosion and increasing water conservation.

Air quality is mainly concerned with the decrease in atmospheric CO2 concentration.

Wetland rice culture represents the most complex system in relation to carbon

sequestration. The overall effect of increasing OM in soil is an improvement of soils

buffer capacity and resilience to different kinds of degradation or stress (FAO, 2001).

2.11.3. Biodiversity and soil biological functioning

Soil biota, primarily at a functional group level, is known to regulate vital ecosystem

processes such as decomposition (the modification of complex organic compounds

into organic nutrients available for plant growth), carbon sequestration, and nutrient

cycling. The rate of decomposition is dependent on the interaction of climate, biota

and the quality and quantity of organic matter (Swift et al., 1979). Earthworms,

termites and ants, which are the main groups composing the macro fauna (>1 cm)

increases generally with an increase in OM and with a decrease in soil disturbance

(no-tillage). They are good indicator of soil biological quality (Lavelle, 2000; Lobry

de Bruyn, 1997).

2.11.4. Effect of climate change

A proportion of the SOC present in undisturbed soils has been burned lost through

agriculture practices. This has contributed to the global rise of CO2 in the atmosphere.

The annual global rate of photosynthesis is generally balanced by decomposition and

represents one tenth of the carbon in the atmosphere or one twentieth of the carbon in

soils (Lal et al., 1998). Global warming could lead to an increase in heterotrophic

respiration and decomposition of organic matter, and consequently to a decline in the

sink capacity of terrestrial ecosystems (Schimel, House and Hibbard, 2001). Another

important effect of CO2 increase is a decrease in transpiration from the stomata which

results in increased water use efficiency, particularly in C4 plants. So far as water is

23

concerned, the net effect of CO2 on the reduction of plant transpiration is favorable

(Gregory et al., 1998).

24

3. Methods and Materials

In order to investigate the stock of organic carbon in soils, the soil samples were

collected from different locations and were processed for analyses.

3.1. Collection of Soil Samples

Soil samples were collected from different locations of Bangladesh (Batiaghata,

Dumuria, Tala, Hogladanga, Pirojpur Sadar and Srimangal). From each sampling

spot, soils were collected in bulk from 0-25 cm depth. The collected soils were packed

in a polyethylene bag and labeled properly by a marker pen.

3.2. Processing of Soil Sample

Each of the collected soil samples was dried in air by spreading on separate sheet of

paper after it was transported to the laboratory. After air drying, the larger and

massive aggregates and gravel were broken by crushing them gently with a wooden

hammer. Dry roots, grasses and scrubs were discarded from the sample. Soil samples

were dry sieved by hand to collect aggregate size classes (8-2mm, 2-0.25 mm and

0.25-0.05 mm). Collecting each size class individually allows for each size class to be

wet sieved or analyzed individually.

3.3. Short description of soil samples

A short description of soil series as well as sampling sites is given below.

Bajoa Series: Sample no. 1 and 7 were under this series. Samples were collected

from Batiaghata upazila and Pirojpur Sadar, respectively. The Bajoa series comprises

seasonally shallowly to moderately deeply flooded, poorly drained soils developed in

Ganges tidal deposits. They have a grey to olive grey, calcareous silty clay loam

subsoil with moderate to strong blocky structure in the B horizon. Elevation of this

soil series is typically basin. Land use pattern is transplanted aman-fallow.

Jhalakathi Series: Sample no. 2 was collected from Batiaghata upazila. The Jhalakati

series comprises seasonally shallowly flooded, poorly drained soils developed in

Ganges tidal deposits. They have a grey silty clay loam subsoil with strong to

moderate prismatic and blocky structure with dark grey cutan along ped faces in the

horizon. Elevation of this soil series is typically upper slope of nearly level low ridge.

Land use pattern is followed by transplanted aus-transplanted aman-fallow.

25

Dumuria Series: Sample no. 3 was under Dumuria series collected from Ghutudia,

Dumuria. Dumuria series consists of seasonally shallowly to moderately deeply

flooded soils developed in tidal clay deposits. They have grey, calcareous silty clays

with moderate to strong prismatic and blocky structure with patchy olive grey cutan

along ped faces in the B horizon. Land use pattern is transplanted aman-fallow.

Garuri Series: Sample no. 4 was under Garuri series collected from Sagda, Tala. The

Garuri series comprises seasonally shallowly to moderately deeply flood. Poorly

drained soils developed in older Gangetic alluvium. They have dark grey non-

calcareous clay subsoil with strong coarse prismatic breaking into strong coarse and

medium blocky structure with ped cutan along vertical and horizontal ped faces. They

are calcareous below 12-30 inches depth. Land use pattern is jute (bankin)-t.aman

(BR-11)-rabi crops (lentil and others).

Harta Series: Sample no. 5 was under Harta series collected from Hogladanga,

Batiaghata. The Harta series comprises seasonally flooded poorly to very poorly

drained soils developed in Ganges tidal floodplain. They have a dark grey to grey,

silty clay to clay soil overlying an organic layer at 10-20 inches depth. The series

consists of Broad basin. Land use pattern is followed by Mixed aus and broadcast

aman-fallow.

Sara Series: Sample no. 6 was under Sara series collected from Kulbaria, Dumuria.

Sara series comprises intermittently and shallowly flooded, imperfectly to poorly

drained soils developed in Gangetic alluvium. They have olive-brown to light olive

brown color, calcareous silt loams with weak prismatic and blocky structure in the B

horizon. Elevation of this soil series is typically upper slope of high ridge. Land use

pattern is followed by aus-rabi crops.

Srimangal Series: Sample no. 8 and 9 were under Srimangal series collected from

Srimangal, Sylhet. The Srimangal series consists of well drained soils developed in

old Holocene or late Pleistocene piedmont terrace deposits. They occur on deeply

dissected, nearly level (normal phase) to steep (strongly dissected phase) terrace

remnants in association with small patches of Lakhaichara series; with Itkhola, These

soils have a brown to yellowish brown, very friable to friable fine sandy loam to

sandy clay loam topsoil with subangular blocky to granular worm cast structure (no

crumbs), very strongly acid, overlying a yellowish brown to brown faintly mottled

26

yellowish red to dark brown, friable silty clay loam to sandy clay subsoil with

subangular blocky and granular worm cast structure, very strongly acid.

3.4. Laboratory Analyses

In the laboratory chemical and physical properties of the soils were determined

according to the conventional methods. The methods that were used in the experiment

are as follows:

3.4.1. Particle size analysis: The particle size analysis of the soils was done by

combination of sieving and hydrometer method as described by Gee and Bauder

(1986). Textural classes were determined using Marshalls Triangular Coordinates as

devised by the United States Department of Agriculture (USDA, 1951).

3.4.2. Bulk density: Bulk density was determined by obtaining an undisturbed soil

sample of known volume and dividing the oven-dry soil mass by the volume of the

sample. A Daiki core sampler was used for obtaining cylindrical core samples.

Precautions were taken to avoid compaction. The soil was trimmed to the exact

volume of the cylinder and oven-dried at 105C (Black, 1965).

3.4.3. Soil reaction (pH): The pH of the soil was measured electrochemically by

using a pH Meter. The ratio of soil to water was 1:2.5 as suggested by Jackson (1973).

3.4.4. Electrical conductivity (EC): Electrical conductivity of the soil was measured

at soil to water ratio of 1:5 using EC meter as described by USSL STAFF (1954).

3.4.5. Soil organic carbon (SOC) stock: Percent soil organic carbon was determined

by wet oxidation of Walkley and Blacks method as described by Piper (1950) and

Jackson (1973). Percent soil organic carbon then converted to SOC content. Stock of

soil organic carbon (SOC) can be evaluated by the following equation:

SOC Stocks (Fixed Depth):

where SOCFD is the SOC stock to a fixed depth (Mg C ha-1 to the specified depth), Dcs

is the density of core segment (g cm_3), Ccs is the organic C concentration of core

segment (mg C g_1 dry soil), and Lcs is the length of core segment (cm) (Ellert et al.,

2008).

27

4. Results and Discussion

4.1. General Analyses of Soil

Particle size distribution: Soil texture refers to the relative proportion of sand, silt

and clay. Soil texture detects the physical, chemical and biological properties of soils.

SOC storage is largely influenced by soil texture.

Soil texture of studied soil samples varied between silt loam and clay. The variation

of textural classes of the studied soils is presented in Table 4.1. In most of soil

sample, silt was dominant fraction, followed by clay and sand. The percentage of silt

in soil samples was higher than sand and clay percentage. Soil sample no. 1, 2, and 4

were found to be silty clay loam. The texture of 3 and 7 no. sample was found to be

silty clay. Textural class of Srimangal series is sandy clay loam. This may be due to

high percentage of sand (60%) as because of high content of iron and aluminum and

high BSP.

The standard deviation of sand, silt and clay data indicates sand of the soil samples

varied from 4% to 60%, silt of the soil samples was ranges from 18% (lowest) to

84% (highest) and the Clay of the soil samples varied from 12% (lowest) to 52%

(highest) (Table 4.3). It showed different percent of sand present in same series at

different location. Highest percent of sand, silt and clay were found in Srimangal,

Harta and Dumuria series, respectively and the lowest percent of sand was found in

Jhalakathi (Sample no. 2), Harta (Sample no. 5) and Bajoa (Sample no. 7) series.

Lowest percent of silt and clay were found in Srimangal (Sample no. 8) and Harta

(Sample no. 5) series, respectively.

Bulk density: Bulk density is defined as the mass per unit volume of dry soil. Bulk

density of soils is, therefore, influenced by texture, structure, cultivation, cultural

practices etc. Bulk densities of the soils are presented in Table 4.1.

The bulk density varied between 0.68 to1.65 g cm-3. Bulk density of the soil samples

were measured at 0-25cm depth. Soils of sample no. 9 had highest bulk density (1.65

g cm-3). Sample no. 5 showed the lowest bulk density (0.68 g cm-3).

28

The results of the physical and chemical analyses of soil samples are represented in

Table 4.1 and 4.2.

Table 4.1 Physical properties of the studied soils

Sample no. Soil Series Location

Bulk Density (g cm-3)

Particle size distribution Texture

%Sand %Silt %Clay

1 Bajoa Batiaghata 1.16 9.00 55.00 36.00 Silty Clay Loam

2 Jhalakathi Batiaghata 1.18 4.00 58.00 38.00 Silty Clay Loam

3 Dumuria Ghutudia, Dumuria 1.54 5.00 43.00 52.00 Silty Clay

4 Garuri Sagda, Tala 1.61 5.00 61.00 34.00 Silty Clay Loam

5 Harta Hogladanga, Batiaghata 0.68 4.00 84.00 12.00 Silt Loam

6 Sara Kulbaria, Dumuria 1.49 30.00 31.00 39.00 Clay Loam

7 Bajoa Guamaria, Pirojpur Sadar 1.41 4.00 47.00 49.00 Silty Clay

8 Srimangal Srimangal, Sylhet 1.61 60.00 18.00 22.00 Sandy Clay

Loam

9 Srimangal Srimangal, Sylhet 1.65 60.00 19.00 21.00 Sandy Clay

Loam

Soil Reaction (pH): The pH of studied soil samples were shown in Table 4.2. Some

soil samples exhibit alkaline pH and some exhibit nearly neutral between 6.6 to 7.6

and Srimangal soil series exhibit pH in acidic ranges (5.1 to 5.8).

Electrical Conductivity (EC): The EC values of the soils varied from 0.78 to 17.11

dS m-1(Table 4.2). The highest EC value found for the soil sample 5 was 17.11 dS m-

1. The EC value in Srimangal series was in the range of non- saline (0.78 to 1.45).

Most soils shows slightly saline to moderately saline behavior.

29

Table 4.2 Organic Carbon Stock, pH and EC of the soils

Sample no. Soil Series pH EC (dS m-1) Organic Carbon Stock (Kg C m-2)

1 Bajoa 7.64 5.57 0.48

2 Jhalakathi 7.89 4.75 0.42 3 Dumuria 7.97 7.40 0.81

4 Garuri 7.77 5.50 0.49 5 Harta 5.18 4.50 0.91

6 Sara 8.04 4.75 0.35

7 Bajoa 6.79 4.24 0.49 8 Srimangal 5.19 0.78 0.35

9 Srimangal 5.72 0.80 0.43

Soil organic carbon (SOC) stock: Results of organic carbon percent in the collected

soil samples are presented in Table 4.2. The highest organic carbon percent was

observed in soil sample 3 and 5 (2.11 and 5.37, respectively) and lowest value

observed in sample 8 and sample 9 that is Srimangal series (0.88 and 1.03,

respectively). SOC stock in the soils is shown in Table 4.2.

The SOC stocks of the soil samples of 1 and 7 of Bajoa series were 0.48 and 0.49 Kg

C m-2 respectively. It showed nearly same pattern of SOC stock in same series.

In sample 2, the soil organic carbon stock of the Jhalakathi series was 0.42 Kg C m-2.

In sample 3, the SOC stock of the Dumuria series was 0.81 KgCm-2.

In sample 4, the SOC stock of the Garuri series was 0.49 Kg C m-2. In sample 5, the

SOC stock of the Harta series was 0.91Kg C m-2. Highest amount of SOC stock was

found in this soil. In sample 6, the SOC stock of the Sara series was 0.35Kg C m-2.

Lowest amount of SOC stock was found in this soil. In sample 8, the SOC stock of

the Srimangal series was 0.35Kg C m-2. Lowest amount of SOC stock was found in

this soil. In sample 9, the SOC stock of the Srimangal series was 0.43Kg C m-2.

4.2. Statistical Analyses

Statistical analysis is performed by MINITAB (Release 13.20).

30

Table 4.3 Descriptive Statistics: BD, %Sand, %Silt, %Clay, SOC stock, pH, EC

Variable Mean Median St. Dev. Minimum Maximum

Bulk Density 1.370 1.49 0.31 0.68 1.65

%Sand 20.11 5.00 24.08 4.00 60.0 %Silt 46.22 47.0 21.34 18.0 84.0

%Clay 33.67 36.0 13.18 12.0 52.0 SOC stock 0.52 0.48 0.19 0.35 0.91

pH 7.02 7.64 1.09 5.18 8.04 EC 4.25 4.75 2.17 0.78 7.40

*St. Dev. = Standard Deviation

4.3. Relationship among soil properties

SOC stock was negatively correlated with % sand and % clay but SOC stock and %

silt were positively correlated. SOC stock was also negatively correlated with bulk

density (Table 4.4).

The SOC stock of studied soils varied from 0.35 to 0.91 Kg C m-2 in different soil

series. SOC stock is higher in silt loam soil, lower in clay loam and sandy clay loam

soils. In many soils, most of the C is stored in the clay-size fraction (Christensen,

1996) and consequently the concept of an inverse functional relationship between

particle size and C storage capacity was developed. Further size separation in coarse

(0.22 m) and fine (

recognized as a mechanism for storing organic matter in soils on long timescales.

However, clay content alone is not a good predictor of the total amount of organic

matter; other factors, such as clay mineralogy (Torn et al., 1997). In 1970, Jeffrey

suggested that the relationship between SOM and bulk density might be universal.

Table 4.4. Correlations: Bulk density, %Sand, %Silt, %Clay, SOC stock

BD %Sand %Silt %Clay %Sand 0.520 P-Value 0.151 %Silt -0.806 -0.838 0.009 0.005 %Clay 0.353 -0.470 -0.087 0.351 0.202 0.823 Soc stock -0.574 -0.516 0.639 -0.091 0.106 0.155 0.064 0.815 Cell Contents: Pearson correlation P-Value Bulk density tends to decrease as SOM concentration increases. In Harta series, SOC

stock has a tendency to increase as bulk density decreases. In this soil SOC shows the

highest value which is 0.91 Kg C m-2 and bulk density was 0.68 g cm-3 which is the

lowest value. In Srimangal soil series (sample no. 9) the highest value of bulk density

was found when SOC stock was 0.43 Kg C m-2.

32

5. Summary and Conclusion

Laboratory investigations were conducted in Soil Science discipline, Khulna

University to study the amount of SOC in some soils of Bangladesh. Nine

representative soil samples from different locations of Bangladesh differing in

physical, chemical and other properties were collected for this purpose. The soils

belong to Bajoa, Jhalakathi, Dumuria, Garuri, Harta, Sara, Srimangal series. Most of

the representative soils were Silty Clay Loam to clayey in texture which influenced

the SOC stock. The percentage of silt in soil samples was higher than sand and clay

percentage. Highest percent of sand, silt and clay were found in Srimangal, Harta and

Dumuria series, respectively and the lowest percent of sand was found in Jhalakathi

(Sample no. 2), Harta (Sample no. 5) and Bajoa (Sample no. 7) series. Lowest percent

of silt and clay were found in Srimangal (Sample no. 8) and Harta series, respectively.

The SOC stock varied between 0.42 and 0.49 Kg C m-2 in silty clay loam soils, 0.49

and 0.81 Kg C m-2 in silty clay soils and 0.35 and 0.43 Kg C m-2 in sandy clay loam

soils. The highest value of SOC stock in studied soils was 0.91 Kg C m-2 and the

lowest value was 0.35 Kg C m-2. The highest bulk density was1.65 g cm-3 in

Srimangal soil series (sample no 9) and the lowest bulk density was 0.68 g cm-3 in

Harta soil series (Sample no 5).

33

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