Carbon and Nitrogen in the Terrestrial Environment

Carbon and Nitrogen in the Terrestrial

Environment

Carbon and Nitrogen

in the Terrestrial

Environment

R. Nieder and D.K. Benbi

R. Nieder D.K. Benbi

Institut für Geoökologie Department of Soils

Technische Universität Braunschweig Punjab Agricultural University

Braunschweig Ludhiana

Germany India

ISBN 978-1-4020-8432-4 e-ISBN 978-1-4020-8433-1

Library of Congress Control Number: 2008927744

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Preface

One of the biggest reality before us today is the global climate change resulting

from the emission of greenhouse gases (GHGs). There has been an unprecedented

increase in the concentration of carbon and nitrogen containing GHGs in the atmos￾phere, resulting primarily due to intervention in terrestrial carbon (C) and nitrogen

(N) cycles by human beings. Two anthropogenic activities viz. food production and

energy production are mainly responsible for perturbation of C and N cycles.

If drastic remedial measures are not taken, the concentration of GHGs is projected

to increase further. According to Kyoto Protocol, industrial countries are to reduce

their emissions of GHGs by an average of 5% below their 1990 emissions by the

first commitment period, 2008–2012. Therefore, there is an increased focus to look

for options for mitigating the emission of GHGs. Terrestrial C sequestration

through biotic processes is being viewed as a plausible option of reducing the rates

of CO2

emissions while abiotic processes of carbon storage and alternatives to fossil

fuel take effect.

The importance of the C and N transfer from soils to the atmosphere lies not only

in global warming, but also on soil quality and the potential of soils to perform

ecosystems functions some of which are related to the three major international con￾ventions on Biodiversity, Desertification, and Climate Change. Soil organic matter

(SOM) being the main reservoir of C of the continental biosphere, can either be a

source of CO2

during mineralization or a sink if C sequestration is favored. During

the last two centuries, soils have lost a considerable amount of C due to land use

changes and expansion of agriculture. These losses from soils are clearly of concern

in relation to future productivity and environment. To ensure sustainable management

of land, it is imperative that organic matter in the soil is maintained and sustained at

satisfactory levels through improved management practices.

As pool changes of C and N are often very slow, and the full impact of a change

in land management practice may take decades to become apparent, long-term

perspectives are required. The cycling of C and N is intimately linked and the two

cannot be studied effectively separately. This necessitates a thorough understanding

of the interdependent and dynamic pools and processes of C and N in the terrestrial

ecosystem. Models could help in formulating or assessing land use strategies,

generating scenarios for optimizing SOM conditions and minimizing emissions and

upscaling research findings at different levels of spatial and temporal aggregation.

v

Development and use of models require a comprehensive knowledge about several

interdisciplinary processes.

Most of the currently available books on C and N cycling either deal with a sin￾gle element of an ecosystem, or are limited to one or a few selected aspects. This

book fills the gap by presenting a comprehensive, interdisciplinary description of C

and N fluxes between the atmosphere and terrestrial biosphere, issues related to C

and N management in different ecosystems and their implications for the environ￾ment and global climate change, and the approaches to mitigate emission of GHGs.

This unique volume presents comprehensive literature drawn from books, journals,

reports, symposia proceeding and internet sources to document interrelationships

between different aspects of C and N cycling in terrestrial ecosystems. Following

an introductory chapter, Chapter 1 presents distribution of C and N in the various

terrestrial pools, with special emphasis on storage in plants and soils. Chapter 2

presents the basics of C and N cycling processes and a generalized overview of

fluxes in terrestrial ecosystems so as to develop an understanding of the complex

interrelationships among different processes and the emission pathways, which are

discussed in subsequent chapters. Soils, particularly soil organic matter, play an

important role in the bidirectional flow of C and N in terrestrial ecosystems.

Therefore, knowledge about the composition and characteristics of soil organic

matter, and its role in influencing soil functions is essential to exploit synergies

between management practices, GHG mitigation and sustainable productivity.

While Chapter 3 presents physical, chemical and morphological characterization of

soil organic matter, Chapter 4 enunciates the influence of SOM on soil quality and

its ability to perform ecosystem functions. To complement the information provided

in Chapter 1 on C and N forms, Chapter 5 presents the transformations of organic

and inorganic forms of carbon and nitrogen in soils and their role in influencing

C and N fluxes between soils and atmosphere. The impact of anthropogenic activities,

particularly land use and land use changes and agricultural management on C and

N dynamics is presented in Chapter 6. Chapter 7 discusses leaching of reactive

C and N forms from soils and contamination of groundwater. Chapter 8 provides a

detailed description of bidirectional biosphere-atmosphere interactions with current

estimates of GHG emissions, their sources, governing variables and the mitigation

options. Finally, Chapter 9 presents modeling approaches adopted to simulate

various components of C and N cycling processes. The use of models to upscale

measurements and generate scenarios on a regional and global scale vis-à-vis

management options are discussed.

We are thankful to the German Research Foundation (Deutsche Forschung￾sgemeinschaft) for funding the stay of D.K. Benbi at Braunschweig Technical

University. We appreciate our families: Alexandra, Raphaela and Petra (R. Nieder),

and Adwitheya and Meenu (D.K. Benbi) for their patience and understanding during

the preparation of this book. We are grateful to Hans P. Dauck for help in the preparation

of illustrations.

R. Nieder

D.K. Benbi

vi Preface

Contents

Preface .............................................................................................................. v

Introduction ........................................................................................................ 1

Chapter 1 Carbon and Nitrogen Pools in Terrestrial Ecosystems

1.1 Forms and Quantities of Carbon and Nitrogen

on Earth .............................................................................. 5

1.1.1 Carbon .................................................................... 5

1.1.2 Nitrogen .................................................................. 7

1.2 Carbon and Nitrogen in Terrestrial Phytomass................... 8

1.2.1 Estimates of Phytomass C and N Stocks for

Natural Ecosystem Types ....................................... 9

1.2.2 Estimates for Agroecosystems ............................... 20

1.2.3 Net Primary Production and Phytomass Stocks

in Different Climatic Zones .................................... 21

1.3 Carbon and Nitrogen in Soils ............................................. 22

1.3.1 Global Soil Organic Carbon and Nitrogen

Pools ....................................................................... 22

1.3.2 Global Soil Inorganic Carbon and Nitrogen

Pools ....................................................................... 36

1.4 Global Vegetation-Soil Organic Matter

Interrelationships ................................................................ 41

Chapter 2 Carbon and Nitrogen Cycles in Terrestrial Ecosystems ........... 45

2.1 The Global Carbon Cycle ................................................... 45

2.1.1 Biosphere-Atmosphere Exchange

of Carbon Dioxide .................................................. 45

2.1.2 Biosphere-Atmosphere Exchange of Methane,

Carbon Monoxide and Other C-Containing

Gases ...................................................................... 47

2.1.3 Ocean-Atmosphere Exchange of Carbon

Dioxide ................................................................... 47

vii

2.1.4 Transport of Carbon to Oceans via Fluvial

Systems ................................................................... 48

2.2 The Global Nitrogen Cycle ................................................ 49

2.2.1 N2

Fixation by Lightning ........................................ 49

2.2.2 Biological N2

Fixation ............................................ 49

2.2.3 Ammonia Production with the Haber-Bosch

Process .................................................................... 51

2.2.4 Atmospheric N Depositions ................................... 52

2.2.5 Emissions of NOx

, N2

O, N2

, NH3

and

Organic N ............................................................... 54

2.2.6 Leaching of Nitrogen to Groundwater ................... 55

2.2.7 Transport of Nitrogen to Oceans by Rivers ............ 55

2.2.8 Ocean N Budgets .................................................... 56

2.2.9 Summary of the Major Global N Fluxes ................ 57

2.3 Carbon and Nitrogen Cycling in Soils ................................ 58

2.3.1 Carbon and Nitrogen Cycling in Upland Soils ....... 59

2.3.2 Carbon and Nitrogen Cycling in Wetland Soils ..... 73

2.4 Global Climate Change and C and N Cycling .................... 79

Chapter 3 Soil Organic Matter Characterization ........................................ 81

3.1 Chemical Characterization of Soil Organic Matter ............ 82

3.1.1 Non-Humic Substances .......................................... 83

3.1.2 Humic Substances .................................................. 85

3.2 Physical Characterization of Soil Organic Matter .............. 97

3.2.1 Particulate Organic Matter...................................... 98

3.2.2 Organomineral Complexes ..................................... 100

3.3 Morphological Characterization of Soil

Organic Matter .................................................................... 104

3.3.1 Classification of Terrestrial Humus Forms ............. 104

3.3.2 Characterization of Terrestrial Humus Forms ........ 106

3.3.3 Humus Form Development in a Forest

Succession .............................................................. 110

3.3.4 Ecological Features of Humus Forms .................... 110

Chapter 4 Organic Matter and Soil Quality ................................................ 113

4.1 Soil Quality ......................................................................... 114

4.1.1 Definition and Concept ........................................... 114

4.2 Impact of SOM on Soil Physical, Chemical

and Biological Properties ................................................... 117

4.2.1 Physical Properties ................................................. 118

4.2.2 Chemical Properties ............................................... 122

4.2.3 Biological Properties .............................................. 126

4.3 Evaluation of Organic Components as Soil

Quality Indicators ............................................................... 130

viii Contents

4.3.1 Soil Organic Matter ................................................ 130

4.3.2 Soil Microbial Biomass .......................................... 132

4.3.3 Soil Enzymes .......................................................... 132

4.4 Use of Combined Biological Parameters for Soil

Quality Estimation .............................................................. 133

4.4.1 Indexes Developed from Two Measured

Parameters .............................................................. 133

4.4.2 Indexes Developed from More than Two

Measured Parameters .............................................. 134

Chapter 5 Carbon and Nitrogen Transformations in Soils ......................... 137

5.1 Transformations of Organic Components .......................... 138

5.1.1 Methods of Mineralization-Immobilization

Measurement .......................................................... 139

5.1.2 Mineralization-Immobilization Measurements

in the Field .............................................................. 142

5.1.3 Results from 15N Field Studies ............................... 145

5.1.4 Long-Term C and N Mineralization

and Accumulation ................................................... 148

5.2 Transformations of Inorganic Components ........................ 148

5.2.1 Formation of Secondary Carbonates ...................... 148

5.2.2 Nitrification ............................................................ 152

5.2.3 Fixation and Defixation of Ammonium ................. 156

Chapter 6 Anthropogenic Activities and Soil Carbon

and Nitrogen .................................................................................. 161

6.1 Land Use Changes .............................................................. 161

6.1.1 Land Use Area Distribution

and Its Global Change ............................................ 161

6.1.2 Change in SOC and SON Following Land

Conversion .............................................................. 172

6.1.3 Land Use Changes and Greenhouse

Gas Emissions ........................................................ 187

6.1.4 Fire Regimes ........................................................... 192

6.2 Agricultural Management ................................................... 194

6.2.1 Soil Tillage ............................................................. 194

6.2.2 Fertilization ............................................................ 200

6.2.3 Introduction of Fallow Systems .............................. 205

6.2.4 Crop Rotation Effects ............................................. 207

6.3 Ecosystem Disturbance ...................................................... 209

6.3.1 Erosion and Deposition Effects .............................. 209

6.3.2 Mine Spoil Reclamation ......................................... 212

6.3.3 Salinization ............................................................. 214

6.3.4 Soil Acidification .................................................... 214

Contents ix

Chapter 7 Leaching Losses and Groundwater Pollution ........................ 219

7.1 Dissolved Organic Carbon .................................................. 220

7.2 Dissolved Organic Nitrogen ............................................... 223

7.3 Nitrate Leaching ................................................................. 226

7.3.1 Reducing Leaching Losses ..................................... 230

Chapter 8 Bidirectional Biosphere-Atmosphere Interactions .................... 235

8.1 Atmospheric Nitrogen Depositions .................................... 236

8.1.1 Wet and Dry Deposition ......................................... 236

8.1.2 Effect of N Deposition on Ecosystems ................... 240

8.2 Carbon Fixation via Photosynthesis ................................... 243

8.2.1 Photosynthetic Pathways ........................................ 243

8.2.2 Global Distribution of C3

and C4

Pathways ............ 244

8.2.3 Response of C3

and C4

Pathways to Increasing

Atmospheric CO2

Concentration ............................ 245

8.3 Biological N2

Fixation ........................................................ 246

8.3.1 N2

Fixation by Non-symbiotic Bacteria ................. 247

8.3.2 N2

Fixation by Symbiotic Bacteria ......................... 248

8.3.3 Global Estimates of Biological N2

Fixation ........... 250

8.4 Carbon Dioxide Emission ................................................... 251

8.4.1 Carbon Dioxide Emissions from Biomass

Burning and Soils ................................................... 254

8.4.2 Carbon Dioxide Emission Mitigation Options ....... 255

8.4.3 Role of Forests in CO2

Mitigation .......................... 256

8.4.4 Potential for C Sequestration by Agriculture ......... 260

8.5 Methane Emission .............................................................. 265

8.5.1 Methane Emission from Rice Agriculture .............. 268

8.5.2 Methane Production in Rice Soils .......................... 269

8.5.3 Factors Regulating Methane Emission

from Rice Fields ..................................................... 271

8.5.4 Mitigation Options for Agricultural Emission

of Methane .............................................................. 273

8.6 Emission of Oxides of Nitrogen: N2

O and NO .................. 276

8.6.1 Nitrous Oxide Emissions ........................................ 276

8.6.2 Nitric Oxide Emissions .......................................... 281

8.6.3 Factors Regulating Emission of N2

O and NOx

....... 284

8.6.4 Nitrogen Oxide Emission Mitigation Options ........ 291

8.7 Ammonia Emission ............................................................ 291

8.7.1 Ammonia Emission Mitigation Options ................. 294

8.7.2 Ammonia Emission from Plants ............................. 294

8.8 Global Climate Change and Crop Yields ........................... 295

8.8.1 Projected Demand of Crop Yields .......................... 295

8.8.2 Influence of Climate Change on Crop Yields ........ 296

8.8.3 Potential to Increase Global Production ................. 297

x Contents

8.9 Economics of Carbon Sequestration .................................. 298

8.9.1 Methods for Calculating Carbon

Sequestration Costs ................................................ 299

8.9.2 Economics of Carbon Sequestration

in Forestry ............................................................... 301

8.9.3 Economics of Carbon Sequestration

in Agriculture ......................................................... 304

8.9.4 Secondary Benefits from Carbon Sequestration

Measures ................................................................. 304

8.9.5 Leakage of Emissions Beyond Project

Boundaries .............................................................. 305

Chapter 9 Modeling Carbon and Nitrogen Dynamics in the

Soil-Plant-Atmosphere System .................................................... 307

9.1 Carbon Dioxide Exchange from Soils ................................ 307

9.2 Methane Emissions from Rice Fields

and Natural Wetlands .......................................................... 312

9.2.1 Oxidation of Atmospheric Methane in Soils .......... 317

9.3 Nitrogen Trace Gas Emission ............................................. 317

9.4 Modeling Nitrogen Dynamics in Soils ............................... 324

9.4.1 Denitrification ......................................................... 324

9.4.2 Ammonia Volatilization .......................................... 325

9.4.3 Nitrate Leaching ..................................................... 327

9.4.4 Nitrogen Mineralization Kinetics ........................... 328

9.4.5 Nitrification ............................................................ 333

9.5 Modeling Organic Matter Dynamics in Soils ..................... 333

9.5.1 Measured Versus Functional Soil Organic

Matter Pools ........................................................... 337

9.5.2 Classification of Models ......................................... 339

9.5.3 Evaluation and Use of Soil

Organic Matter Models ........................................... 340

References ........................................................................................................ 343

Index ................................................................................................................. 417

Contents xi

Introduction

Carbon (C) and nitrogen (N) are the building blocks of life on earth. Carbon delivers

the framework for carbohydrates, fats and proteins and N as component of proteins

is present in amino acids, enzymes and nucleic acids. These organic forms occur in

living and dead organic materials of plants, animals and humans and are also

important constituents of soil organic matter (SOM). Both C and N also exist in

inorganic forms and are present in all ecosystems. In the atmosphere, carbon is

present as carbon dioxide (CO2

). Minor amounts of gaseous C occur as methane

(CH4

), carbon monoxide (CO) and other higher molecular C-containing gases. In

the lithosphere C is a major constituent of limestone, occurring as carbonates of

calcium and magnesium (CaCO3

and CaMg (CO3

)2

). In ocean and fresh water, it is

present as dissolved carbonates. Flow of carbon occurs between different spheres,

leading to what is generally termed as carbon cycle. The dominant fluxes of the

global C cycle are those that link atmospheric CO2

to land biosphere and oceans.

About 98% of the world’s nitrogen is found in the solid earth within rock, soil and

sediment. The remainder moves in a dynamic cycle involving the atmosphere, ocean,

lakes, streams, plants and animals. Nitrogen in the atmosphere mainly exists as

molecular nitrogen (N2

), which comprises 78% of the atmospheric gases. Trace

amounts of nitrogen oxides, gaseous ammonia, ammonium compounds, nitric acid

vapor, particulate nitrate and organic nitrogen circulate through the atmosphere.

Atmospheric nitrogen compounds cycle to the land and water through wet and dry

deposition. Nitrogen is capable of being transformed biochemically or chemically

through a number of processes termed as the nitrogen cycle. Most N transformations

involve the oxidation or reduction by biological and chemical means. In the hydro￾sphere, N exists as soluble organic or inorganic nitrogen.

The global C cycle is one of the most important, complex and challenging

cycles on earth as it influences several physical and biological systems directly

and through its effect on global temperatures. The interest in the global C cycle

has increased tremendously in the last 2 decades because of its role in global cli￾mate change and the recognition that human activities are altering the carbon cycle

significantly. As early as 1896, Arrhenius indicated the importance of CO2

in the

air on the global temperature and calculated the alteration of temperature that

would follow with the increase in CO2

concentration. But the topic did not feature

prominently in research agenda until 1958 when continuous measurements of CO2

R. Nieder, D.K. Benbi, Carbon and Nitrogen in the Terrestrial Environment, 1

© Springer Science + Business Media B.V. 2008

concentrations were initiated at Mauna Loa in Hawaii. However, the real impetus to

C cycling research was provided in 1980s by the revelations of ocean core sediments

and ice-core measurements, that atmospheric CO2

concentrations were much lower

in cold stages as compared to contemporary ones. These results brought to focus

potential climatic consequences of human induced elevated CO2

levels. New ice

core records show that the present atmospheric concentrations of CO2

, or indeed of

CH4

, are unprecedented for at least 650,000 years, i.e. six glacial- interglacial cycles

(Denman et al., 2007). The increasing trend in the atmospheric CO2

concentration

still continues and over the last 250 years its concentration has increased globally

by 100 ppm (36%) from about 275 ppm in the preindustrial era (AD 1000–1750) to

379 ppm in 2005 (Denman et al., 2007). The increase in global atmospheric CO2

is

mainly due to human activities; primarily combustion of fossil fuel and cement

production though there is substantial contribution from land use changes and man￾agement such as deforestation, biomass burning, crop production and conversion of

grassland to croplands. This has serious implications for all forms of life in terres￾trial ecosystems. It has been predicted that there will be an increase in the Earth’s

average surface temperature, shifts in weather patterns, and more frequent extremes

in weather events. Because of these concerns there is a tremendous effort underway

to better understand the global C cycle, reduce anthropogenic emissions and to miti￾gate the atmospheric CO2

concentration.

In addition to CO2

, methane (CH4

) and nitrous and nitric oxides (N2

O and NO)

are also considered to cause global warming. In 2005, the global average abundance

of CH4

was 1,774 ± 1.8 ppb (Forster et al., 2007), which is more than three times the

concentration during glacial periods. In recent years atmospheric growth rate of CH4

seems to stagnate, or even decline but the implications for future changes in its

atmospheric burden are not clear. While emissions from natural sources dominated

the preindustrial global budget of atmospheric CH4

, anthropogenic emissions domi￾nate the current CH4

budget. Wetlands account for about 80% of the total natural

emissions with small contributions from oceans, forests, wildfires, termites, and

geological sources. The anthropogenic sources include rice agriculture, livestock,

landfills and waste treatment, ruminants, biomass burning, and fossil fuel combus￾tion. Since irrigated rice contributes about 70–80% of the CH4

emission from global

rice fields it provides the most promising target for mitigation strategies.

Nitrous oxide, N2

O, constitutes 6% of the anthropogenic greenhouse effect and its

concentration in the atmosphere has been increasing by about 0.25% per year, from

about 270 ppb in preindustrial times to 319 ppb in 2005. Nitrous oxide is emitted into

the atmosphere both from natural (soil, ocean and atmospheric NH3

oxidation) and

anthropogenic sources. Anthropogenic emissions of N2

O originate from biological

nitrification and denitrification in soils and biomass burning. Nitric oxide (NOx

= NO

+ NO2

) emissions, which are also environmentally important originate from surface

and troposheric sources. The surface sources include fossil fuel and biomass burning

and biogenic emissions from soils. For alleviating biogenic emissions of nitrogen

oxides from soils, it is important to adopt practices leading to improved N use

efficiency. The higher the N recovery efficiency in plants, the lesser is the amount of

mineral N available for emission to the atmosphere.

2 Introduction

Burning of fossil fuel and activities related to land use, primarily tropical defor￾estation and biomass burning cause major perturbation to terrestrial C and N

cycles. During the 1990s deforestation occurred at a rate of about 13 million hec￾tares year−1 and over the 15 year period from 1990 to 2005, the world lost 3% of

its total forest area (FAO, 2007). Most of the C stored in the earth’s biota and soils

is associated with forests, when cleared and burned, much of this C ends up in the

atmosphere as CO2

. During the period 1990–2005, C stocks in forest biomass

decreased by about 5.5% at the global level (FAO, 2007). Obviously, through their

destruction, forests can be serious sources of greenhouse gases but through their

sustainable management they can be important sinks of the same gases. Conversion

of forest cover to agriculture also leads to loss of C and N stocks from the land

biosphere. During 1961–2002, agricultural land gained almost 500 million hectares

from other land uses; on average annually 6 million hectares of forest land and 7

million hectares of other natural land were converted to agricultural land, particu￾larly in the developing countries. The net effect of these land use changes is the

reduction in C and N stocks in the landscapes. Agriculture also contributes to the

emission of methane and nitrous oxide from livestock wastes, burning pastures and

crop residues, rice paddies and the application of nitrogen-based fertilizers, besides

contributing to other environmental issues such as groundwater pollution by

nitrates and eutrophication of surface waters. Adoption of more sustainable pro￾duction methods could minimize the negative impacts of agriculture and could also

help in mitigating climate change through C sequestration in soils and vegetation.

Currently, improved agriculture is being viewed as a potential route to the mitiga￾tion of climate change.

The importance of the C and N transfer between soils and the atmosphere lies

not only in global warming, but also on soil quality and the potential of soils to

produce food, fibre, and fuel. Soil organic matter, which is the main reservoir of C

and N, influences soil functional ability and its response to environmental and

anthropogenic influences. To ensure sustainable management of land and advanc￾ing food-security for resource-poor farmers, it is imperative that organic matter in

the soil is maintained and sustained at satisfactory levels. At the beginning of per￾manent agriculture, fields were cropped for 2 years, followed by a fallow year that

served to revamp soil fertility. As population pressure on land increased and the

fallow was eliminated, soil organic carbon (SOC) and nitrogen (SON) declined on

cultivated land. As a consequence, new management practices were introduced to

augment soil fertility, and legume crops like clover and alfalfa became common

rotation crops. In many agricultural systems, important means to maintain or

increase soil organic matter (SOM) have been incorporation of crop residues, ani￾mal wastes and green manures and conservation tillage. In the 20th century, their

significance has altered dramatically due to increased use of mineral N fertilizers.

Globally, soils contain about double the amount of C present in the atmosphere

and most of it is in organic form. It has turnover times ranging from months to mil￾lennia, with much of it around several years and decades. Depending on the input￾output balance, SOM can be both a source and sink of atmospheric CO2

. A soil

source results when net decomposition exceeds C inputs to the soil, either as a

Introduction 3

4 Introduction

result of human activities such as clearing of forests for agriculture or because of

increased decomposition rates due to global warming. Net sinks of C in soils are

postulated from increased C input to the soil through enhanced biomass production

and exogenous supply of organic materials, and decreased output/losses through

adoption of improved management practices for reducing soil respiration. Turnover

of SOC and SON has been measured on both, short (within year) and long (years,

decades) term scales, but it is the long-term trends that determine whether SOM

will act as a net source or sink for C in ecosystems with respect to global environ￾mental change. Changes in climate are likely to influence the rates of accumulation

and decomposition of SOM, both directly through changes in temperature and

moisture, and indirectly through changes in plant growth and rhizodepositions.

Changes in agricultural management practices, land use and soil degradation may

have even greater effects on terrestrial C and N pools, especially on SOM. As pool

changes of C and N are often very slow, and the full impact of a change in land

management practice may take decades to become apparent, long-term perspec￾tives are required. In order to assess the impact of land management practices on

organic matter turnover in soils several physical, chemical, biological, and func￾tional pools have been postulated. Efforts have been made to relate some of the

functional or conceptual pools to measurable soil organic matter fractions. This

necessitates a thorough understanding of the interdependent and dynamic pools

and processes of C and N in the terrestrial ecosystem. Much effort has gone into

modeling potential soil-atmosphere-climate interactions. Models have been used in

formulating/assessing land use strategies and generating scenarios for optimizing

SOM conditions. Though a number of models have been developed, but their role

in C and N optimization on a regional scale needs further elaboration.

During the last 2 decades, our knowledge on C and N pools and cycling has

increased tremendously, particularly in relation to soil and environmental quality.

Availability of improved measurement techniques have provided new and rela￾tively precise estimates of global C and N fluxes. New computing tools and the

development of several Atmospheric General Circulation Models have led to

scenarios of unprecedented magnitude in the area of C and N cycling in terres￾trial ecosystems. In efforts to develop strategies for mitigating the emission of

greenhouse gases from soils, several process based models have been used to

study the influence of management practices on emission of greenhouse gases

and fertilizer use efficiency in different ecosystems. Meeting the challenge of

sustainable management of C and N requires the widening of knowledge through

basic and applied research.

This book provides a holistic and up to date view of all the aspects related to

C and N cycling in terrestrial ecosystems. We hope that the book will be of immense

value to ecologists, environmentalists, soil scientists, agronomists, action agencies,

consultants, extension workers, and students.