Climate Change and Managed Ecosystems - Chapter 6 docx

balance, aeration for root/microbial respiration, soil reaction, and optimization ofthese properties and processes through anthropic interventions involving tillage orlack of it, water t

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CONTENTS

6.1 Introduction 127

6.2 Soil as Moderator of Earth’s Climate 128

6.2.1 Soils and the Global Carbon Cycle 130

6.2.2 Soil Carbon Dynamics 132

6.3 Soil Carbon Sequestration 134

6.3.1 Aggregation 135

6.3.2 Illuviation 136

6.3.3 Secondary Carbonates 136

6.4 Technological Options of Carbon Sequestration in Agricultural Soils 138

6.5 Rates of Soil Carbon Sequestration 139

6.5.1 Measurement Issues Related to Soil Carbon Storage 139

6.6 Conclusions 141

References 141

6.1 INTRODUCTION

Soils are integral component of natural ecosystems The functioning of ecosystems, including the cycling of elements and transfer of mass and energy, is moderated through soils Yet, the focus of soil science research during the 19th and 20th centuries has mainly been on two principal soil functions: (1) the medium for plant (crops, pastures, and trees) growth, and (2) the foundation for civil structures (roads, buildings, hydraulic dams) The unprecedented growth in human population during the 20th century led to (1) widespread adoption of the “Green Revolution” technol-ogies based on intensive management of plant nutrients and available water capacity

of the soil in the effective root zone for enhancing crop yields, (2) rapid expansion

of urban centers, which often necessitated use of fertile topsoil for brick making, along with development of highways, parking lots, airports, and recreational facil-ities, and dams to create water reservoirs and hydroelectric generation facilities Consequently, the 20th century witnessed rapid advances in soil science with focus

on soil as a medium for plant root growth (e.g., edaphology) with specific reference

to water and nutrient movement through the soil and uptake by plants, elemental

balance, aeration for root/microbial respiration, soil reaction, and optimization ofthese properties and processes through anthropic interventions involving tillage orlack of it, water table management, precision farming, fertigation, etc Equallyimpressive advances were made in soil mechanics and soil hydrology to meet therequirements for engineering functions of soil.

The human society faces different challenges at the onset of the 21st century.While there is no cause for complacency in the pursuit of agricultural intensificationfor advancing food production to meet the needs of human population growing at1.3%/annum and expected to reach 8.5 to 9.0 billion by 2050 and eventually stabilize

at the 10 to 11 billion level by 2100 Yet there are other pressing demands on worldsoil resources that also require attention In addition to the two usual functions,human needs in the 21st century demand prioritization of research on several otherfunctions of soil Important among these are (1) repository for industrial, urban, andnuclear waste, (2) storehouse of germ plasm and biodiversity, (3) archive of humanand planetary history, (4) biomembrane to denature pollutant and purify water, and(5) moderate the climate This chapter specifically focuses on the function of soil

as moderator of the climate, through its influence on the global C cycle World soilsconstitute the third largest global C pool comprising 1550 Pg of soil organic carbon(SOC) and 950 Pg of soil inorganic carbon (SIC) Indeed, the soil C pool of 2500

Pg is 3.3 times the atmospheric pool of 760 Pg and 4.0 times the biotic pool of 620 Pg.Thus, the objective of this chapter is to illustrate the importance of world soils

in the global carbon (C) cycle The focus of the discussions is on the interactionbetween the soil and the atmospheric C pools under the changing climatic conditions,with specific reference on C dynamics in agricultural soils because of the need fortheir intense management to meet the needs for food, feed, fiber, and fuel production

6.2 SOIL AS MODERATOR OF EARTH’S CLIMATE

Soil affects and is affected by the climate through numerous interactive processes(Figure 6.1) Soil affects climate by influencing outgoing (albedo and the long waveradiation) and incoming (insolation) radiation through its effects on air quality andthe concentration of dust and other particulate materials, and changing amount anddistribution of precipitation through its effect on relative humidity and temperatureunder local conditions In turn, climate strongly affects soil properties1–3 through itsinfluences on rate and depth of weathering, intensity and severity of the cycles oferosion and deposition, quantity and quality of soil C pool and its stratification, andsoil reaction (pH) and the attendant changes in elemental composition and cycling

by flora and fauna (Figure 6.1) Strong interactive processes between soil and climatehave long been recognized by the Dokuchaev School in Russia.4 The interactionsbetween soil and the atmosphere (climate) are closely linked to those between soiland biosphere, soil and lithosphere, and soil and hydrosphere (Figure 6.2) Soil’seffects on climate are through its influence on the global C cycle Indeed, theinteractive processes between soil and the environment (e.g., biosphere, hydrosphere,lithosphere, and the atmosphere) are moderated through changes in soil processesincluding the global C cycle Physical, chemical, and biological processes andproperties are influenced by the climate (Table 6.1) Important soil processes that

FIGURE 6.1 Effects of (A) soil on the atmosphere, (B) atmosphere on soil, and of the interactive processes on the climate.

Soil Reaction Elemental Concentrations and Cycling by Vegetation

P Soil Erosion and Sedimentation

Atmosphere

Quantity and Quality of Soil C Pools and Stratification

Depth of Weathering and Horizonation

Relative Humidity

© 2006 by Taylor & Francis Group, LLC

are influenced by climate and have strong impact on the soil C cycles are soilaggregation and erosion, oxidation/mineralization of soil organic matter (SOM), andmethanogenesis In addition, climate also affects the N cycle through its impact onnitrification/denitrification processes and emission of N2O into the atmosphere Thus,natural or anthropogenic change in climate can drastically affect soil properties,5,6and the magnitude of change may depend on the antecedent conditions and thedegree of climate change.

6.2.1 S OILS AND THE G LOBAL C ARBON C YCLE

Soils provide numerous ecosystem services of value to humans and functioning ofthe biosphere.7–9 In this regard, the importance of soil in moderating the global Ccycle cannot be overemphasized Historically, soils have been the source of atmo-spheric enrichment of CO2 ever since the dawn of settled agriculture about 10,000

FIGURE 6.2 Interactive processes in soil with its environment with strong influence on the

global C cycle.

Atmosphere

• Concentration of:

(i) Trace gases (ii) Particulate Matter and Soot

Pedosphere

Lithosphere

• Depth distribution of soil C

Erosion Sedimentation

Biomass

Elemental cycling

years ago.10 Most agricultural soils have lost 25 to 75% of their antecedent SOCpool due to historic land use.11 The historic loss of SOC pool is estimated at 66 to

90 Pg C, of which the loss due to accelerated erosion by water and wind is 19 to

32 Pg C.12 Therefore, the SOC pool in most agricultural soils is drastically belowtheir potential maximum determined by the pedologic and climatic factors Thisdeficit in SOC pool, which can be filled through conversion to a restorative land useand adoption of recommended land use and management practice, is also called thesoil C sink capacity The C sink capacity of agricultural soils is estimated to beabout 35 to 40 Pg over a 50- to 100-year period.13,14 Sequestration of 1 Pg ofatmospheric C in soil is equivalent to reduction of atmospheric CO2 by 0.47 ppm.Total SOC pool to 1-m depth is 1500 Pg compared to 760 Pg in the atmosphere.11

TABLE 6.1

Soil Processes and Properties That Are Affected by Climate and That Strongly Affect the Soil Carbon Pool

I Physical (a) Structure/aggregation (i) Clay content and mineralogy

(ii) Cementing agents (sesquoxides, carbonates, organic polymers) (b) Erosion (i) Soil erodibility

(ii) Rate of new soil formation (iii) Transportability and sedimentation (c) Water retention and

transmission

(i) Plant available water capacity (ii) Least limiting water range (iii) Infiltration rate

(iv) Deep percolation (d) Crusting, compaction, and hard

setting

(i) Bulk density (ii) Porosity and pore size distribution (iii) Soil strength

II Chemical (a) Ion exchange (i) Elemental concentration

(ii) Ionic species (b) Leaching (i) Soil reaction (pH)

(ii) Charge density (c) Diffusion (i) Concentration gradient

(ii) Tortuosity III Biological (a) Oxidation/mineralization (i) Decomposition constant

(ii) C:N ratio and lignin/suberin contents (b) Soil respiration (i) Soil microbial biodiversity

(ii) Biomass C (iii) Soil enzymes (c) Methanogenesis (i) Methanogenic bacteria

(ii) Substrate composition (d) Nitrification/denitrification (i) Bacterial population

(ii) NO3 concentration

6.2.2 S OIL C ARBON D YNAMICS

The magnitude and rate of depletion of SOC pool depend on land use and soil/plantmanagement practices (Table 6.2) Practices that lead to severe depletion of SOCpool include deforestation, conversion of natural to agricultural ecosystems, biomassburning and residue removal, soil tillage, and extractive or fertility mining practices.These practices set in motion those processes that exacerbate mineralization of SOMand increase the rate and cumulative amount of CO2-C emission Attendant changes

in soil properties, with a positive feedback on emission of CO2 and other greenhousegases (GHGs), are reduction in the amount and stability of aggregates, increase inbulk density with a decrease in available water-holding capacity, and reduction inhydraulic conductivity and infiltration rate (Table 6.2)

The depletion in SOC pool due to conversion of natural to agricultural ecosystemsand soil cultivation occurs due to (1) reduction in the amount of biomass returned tothe soil, (2) increase in the rate of mineralization usually associated with the change

in soil temperature and moisture regimes, and (3) increase in losses of SOC pool due

to erosion and leaching In sloping and plowed soils prone to erosion by water and/ortillage,15 severe depletion occurs as a consequence of all three processes

The SOC dynamics in agricultural soils can be described by static and dynamicmodels The static model has been developed and used for five decades.16–19 It statesthat SOC equals gains minus losses of SOC (Equation 6.1)

(6.1)

where C is the SOC pool, K is the decomposition constant, and A is the amount of

C added to the soil through root biomass, crop residue, and other biosolids applied

as amendments, and t is time At steady state, when the addition of SOC by fication equals the loss by decomposition (and other processes), dC/dt = 0 and

humi-Equation 6.1 can be rewritten as

The decomposition constant K is influenced by practices, processes, and properties

outlined in Table 6.2 Sometimes Equation 6.2 is written in the form

where h is the humification efficiency, which is to 10 to 12% of the annual biomass

addition in temperate climates.20 In contrast to the static model, the dynamic nential model is an improvement over the static model.21–29 The two-componentdynamic model is shown in Equation 6.4

expo-C t = K1 A/K2 (l – e –k2t ) + COe –k2t (6.4)

dC

where C t is the SOC pool at time t, CO is the antecedent SOC pool at time t = O, K1

is the annual rate at which biomass is humified and added to the soil and is good for

SOC sequestration, and K2 is the annual rate of SOC loss by mineralization and erosion,

and K2 is bad for SOC sequestration A is the accretion or annual addition of C to the

TABLE 6.2

Land Use and Soil/Plant Management Practices That Exacerbate the

A Deforestation 1 Energy balance (i) Soil temperature

2 Water balance (ii) Soil moisture

3 Compaction (iii) Bulk density

4 Erosion (iv) Porosity

5 Shift in vegetation (v) SOC pool

6 Nutrient cycling (vi) Nutrient reserve

B Biomass burning 1 Energy balance (i) Soil temperature

2 Water balance (ii) Soil moisture

3 Nutrient balance (iii) Mineralization rate

4 Runoff/leaching (iv) Soil reaction (pH)

5 Net primary productivity (NPP) (v) Hydrophobicity

C Biomass removal 1 C/elemental cycling (i) SOC pool

2 Crusting/compaction (ii) Nutrient pool

3 Activity of soil fauna (iii) Bulk density

4 Runoff/erosion (iv) Infiltration rate

5 NPP (v) Soil temperature and moisture

regimes

D Soil tillage 1 Gaseous flux (i) Bulk density

2 Erosion/runoff (ii) Infiltration rate

3 Aggregation (iii) Stability and amount of

1 SOC depletion (i) Soil structure

2 Nutrient depletion (ii) SOC content

3 Elemental cycling (iii) Nutrient reserve

F Drainage 1 Anaerobiosis (i) Soil moisture and temperature

regimes

2 Methanogenesis (ii) Rate of mineralization

3 Nitrification (iii) Leaching

4 Denitrification (iv) Soil reaction

soil as crop residue or other biosolids Similar to the static model (Equation 6.1), the

first term [K1A/K2(1 – e –k2t)] is an estimate of the addition to the SOC pool through

crop residue, etc., and the second term (C0e –k2t) is an estimate of the decomposition

of C0 The difference between the two terms is the net amount of C t at any time Taking

the derivative of Equation 6.4 with respect to t leads to Equation 6.5:

(6.5)

which at equilibrium, when dC/dt = 0, gives Equation 6.6:

Similar to the K in Equation 6.3, K1 is strongly influenced by the quality of crop residue

(e.g., C:N ratio, lignin and suberin contents) In contrast, K2 is influenced by soilproperties, climatic factors, and management practices Good and bad farming/land-

use practices affecting the magnitude of constants K1 and K2 are outlined in Table 6.3.Models described in Equations 6.1 through 6.6 are based on several assumptions:30

1 The rate of mineralization depends on the amount of SOC at time t.

2 The rate of mineralization is not limited by lack of other elements (e.g., N)

3 The decomposition constants (K1 and K2) do not change over time

4 All components of the SOC pool are equally susceptible to mineralization

The objective of soil and crop management is to maximize C by moderating K1, K2,

and A through tillage methods, residue management, integrated nutrient

manage-ment, use of compost and biosolids, and cropping systems based on complex tions and use of cover crops

rota-6.3 SOIL CARBON SEQUESTRATION

Soil C sequestration implies transfer of a fraction of atmospheric CO2 into soil C poolthrough conversion of pant residue into humus, and retention of humus-C in soil for

a long time Enhancing the SOC pool of agricultural soils has numerous advantages

In comparison with engineering techniques (e.g., geologic sequestration, tion), SOC enhancement is a natural process, has no adverse ecological impacts, iscost-effective, and improves soil quality Restoration of soil quality through SOCenhancement improves biomass/agronomic productivity, improves water quality byreducing erosion and sedimentation and non-point-source pollution, improves air qual-ity by reducing wind erosion, and mitigates global warming by reducing the net rate

mineraliza-of enrichment mineraliza-of atmospheric CO2 In some cases, however, herbicide effectivenessmay be decreased in soils containing high SOC concentration.31,32

Several important mechanisms of protection of SOC sequestered in soil includephysical, chemical, and biological processes,33–38 some of which are describedbelow

dC

6.3.1 A GGREGATION

Physical protection of SOC is an important mechanism of increasing the residencetime of C in soil, and it involves its encapsulation within a stable aggregate Humiccompounds, comprising long-chain polymers, stabilize micro-aggregates againstdisruptive forces including chemical, mechanical, and biological processes Severalmodels have been proposed suggesting the role of SOC in stabilization of soilaggregates.39

The classical model of Edwards and Bremmer34 illustrates the mechanism ofphysical protection of SOC through stabilization of micro-aggregates (Equation 6.7):

where Cl is clay particle, P is polyvalent cation (e.g., Fe3+, Al3+, Ca2+, Mg2+), OM is

organic molecule, and x and y are the number of these units bonded together by

cementing agents to form a secondary particle or a microaggregate The OM thus

TABLE 6.3

K1 Representing Humification Efficiency (good practices)

K2 Representing Loss by Erosion and Mineralization (bad practices)

1 Climate

(i) Rainfall High Low

(ii) Temperature Low High

(iii) Type Temperate, boreal, tundra, taiga Tropics, subtropics

2 Soil

(i) Clay High Low

(ii) Minerology 2:1, high-activity clays 1:1, low-activity clays

(iii) Water retention High Low

(iv) Type Heavy texture, poorly drained Light texture, excessively drained

3 Soil Management

(i) Tillage No-till, conservation tillage Plow tillage

(ii) Residue Surface mulch Incorporation, removal, burning (iii) Fertility Integrated nutrient management Nutrient deficit, fertility mining

4 Crop Management

(i) Rotations Complex Simple

(ii) Cover crops Winter cover crops Continuous cropping

(iii) Agroforestry With tree-based systems Without tree-based systems

(iv) Farming systems With animal and ley farming Without animal

5 Landscapes

(i) Slope gradient Gentle to none Undulating to steep

(ii) Position Foot slopes Summit and shoulder slopes (iii) Shape Concave/depositional Convex

(iv) Drainage density Low High

(v) Aspect North facing South facing

sequestered is physically protected against microbial processes and is not ized The strong bonding agents (e.g., polyvalent cations, long-chain organic poly-mers) stabilize the aggregate while weak bonds (e.g., Na+) disperse/slake the aggre-gate (Equation 6.8).

mineral-(6.8)Dispersion or breakdown of micro-aggregates (such as by raindrop impact or bythe shearing effect of flowing water) exposes the OM to microbial processesleading to emission of CO2 into the atmosphere Indeed, accelerated soil erosionenhances emission of CO2 from soil to the atmosphere.40 Predominant processesare water runoff, soil erosion, gaseous diffusion, crusting and compaction, anaer-obiosis, and depletion of SOC and nutrient pools Decline in soil quality, caused

by a range of degradative processes, exacerbates depletion of the SOC pool andemission of CO2

is also lower in the subsoil than in the surface soil

Illuviation of SOC occurs with bioturbation (e.g., earthworms) and movementwith percolating water from surface into the subsoil either as dissolved organiccarbon (DOC) or suspended colloid along with the clay particles Reprecipitation

of DOC in the subsoil following reaction with silica and other compounds anddeposition of clay-humus colloids in the deeper layers is another mechanism oftransfer of SOC from surface into the subsoil

6.3.3 S ECONDARY C ARBONATES

The soil C pool comprises two components: SOC and soil inorganic carbon (SIC)subpools Agricultural soils in arid and semi-arid regions also have the potential ofsequestering SIC The SIC subpool contains primary carbonates (e.g., calcite, dolo-mite, aragonite, and siderite) These primary carbonates are of lithogenic origin andoccur in soil due to weathering of the parent material In contrast, there are alsosecondary carbonates that occur in soil due to some pedogenic processes There aretwo mechanisms of SIC sequestration: (1) formation of secondary carbonates, and(2) leaching of bicarbonates into the ground water Visible accumulation of secondarycarbonates is a common occurrence in soils of arid and semi-arid climates.41 Sec-ondary carbonates occur as carbonate films, threads, concretions, and pendants.42They may also occur as laminar caps, caliche, and calrete.43 In gravelly soils,

[(C P OM) ]x y

Dispersion Aggregation

1− − y C P OM x 

Dispersion Aggregation

( 1− − )  xy CI P OM ( − − )