Soil Organic Carbon – Role in Rainfed Farming Systems With Particular Reference to Australian Conditions

Abstract

As soil organic carbon is central to the functioning of all soils, we require a more fundamental understanding of the climatic
and management factors which influence its storage and persistence. The interest in carbon storage and sequestration has focused
attention on changes in soil organic carbon across different regions, climates and management systems. The major components
of soil organic carbon have different physical and chemical properties. A greater understanding of the quantities and composition
of these different components is required to gain an insight into the relative contributions soil organic carbon can make
to soil productivity. Whilst the texture and structure of the soil has an overriding influence on the capacity to store soil
carbon, management options more often influence the actual soil organic carbon content. This chapter addresses the function
of soil organic carbon in farming systems, including the role of specific fractions in key soil processes.

KeywordsSoil function-Organic carbon fractions-Carbon balance-Carbon sequestration-Carbon management

Full text

339
P. Tow et al. (eds.), Rainfed Farming Systems, DOI 10.1007/978-1-4020-9132-2_14,
© Springer Science+Business Media B.V. 2011
Abstract As soil organic carbon is central to the functioning of all soils, we
require a more fundamental understanding of the climatic and management fac-
tors which influence its storage and persistence. The interest in carbon storage
and sequestration has focused attention on changes in soil organic carbon across
different regions, climates and management systems. The major components of
soil organic carbon have different physical and chemical properties. A greater
understanding of the quantities and composition of these different components is
required to gain an insight into the relative contributions soil organic carbon can
make to soil productivity. Whilst the texture and structure of the soil has an overrid-
ing influence on the capacity to store soil carbon, management options more often
influence the actual soil organic carbon content. This chapter addresses the function
of soil organic carbon in farming systems, including the role of specific fractions
in key soil processes.
Keywords Soil function • Organic carbon fractions • Carbon balance • Carbon
sequestration • Carbon management
F.C. Hoyle (*)
Department of Agriculture & Food Western Australia, 3 Baron-Hay Court,
South Perth, WA 6151, Australia
e-mail: frances.hoyle@agric.wa.gov.au
J.A. Baldock
CSIRO Land and Water, Adelaide Laboratory, PMB 2, Glen Osmond,
SA 5064, Australia
e-mail: jeff.baldock@csiro.au
D.V. Murphy
Soil Biology Group, School of Earth and Environment,
The University of Western Australia, 35 Stirling Highway,
Crawley, WA 6009, Australia
e-mail: daniel.murphy@uwa.edu.au
Chapter 14
Soil Organic Carbon – Role in Rainfed
Farming Systems
With Particular Reference to Australian Conditions
Francis C. Hoyle, Jeff A. Baldock, and Daniel V. Murphy

340 F.C. Hoyle et al.
14.1 Introduction
Soil organic carbon (SOC) is central to the functioning of many physical, chemical
and biological processes in the soil ecosystem. Soil organic carbon is a term used
to define the total amount of organic carbon present in those fractions of soil under
2 mm diameter. Soil organic carbon is not the same as soil organic matter (SOM),
which includes roots and other organic particles, and contains other elements such
as hydrogen, oxygen, phosphorus, sulfur and nitrogen that are associated with car-
bon in organic molecules. On average, values of SOM are 1.72 times greater than
SOC. While SOC contributes little to the total soil mass (typically less than 10%),
it does contribute positively to a range of soil processes (Fig. 14.1).
These soil processes interact and are often positively correlated. For example,
SOC provides the dominant energy source for microorganisms (Chap. 6) allowing
them to perform the following functions that are considered important in defining
soil biological health:
1. Decomposition of plant and animal residues to form new SOC, which improves
pH buffering capacity and cation exchange capacity (CEC).
2. Transformation of nutrients from organic to inorganic molecules (e.g. from
organic N to NH4
+ and NO3
−), thus increasing nutrient availability to plants.
3. Formation and stabilisation of soil structure through bacterial polysaccharides
that ‘stick’ soil particles together and through fungal hyphae enmeshing soil
particles.
4. Degradation of pollutants and pesticides which otherwise would persist.
5. Production of gases (CO2, N2O, NH3, N2, CH4), some of which contribute to the
greenhouse effect and global warming.
Buffers pH Improves soil structural stability
Improves soil resilience
Biological
Physical
Chemical
Energy for biological processes
Large store of nutrients - N P S
K, Ca, Mg, Cu, Zn etc
Influences water retention
Mulching reduces water/soil loss
Buffers soil temperature
Complexes cations
Immobilises pollutants
Binds heavy metals
Fig. 14.1 The central role of soil organic matter in contributing to key soil functions and overall
soil fertility

34114 Soil Organic Carbon – Role in Rainfed Farming Systems
In addition to benefits related to soil biological health, the presence of SOC and
its associated nutrients also contribute positively to soil resilience – defined as the
ability of a soil to recover to its initial state after a deterioration event (see also
Chap. 1). For example, under conditions of prolonged drought accompanied by
crop failure and little return of crop residues to the soil, a soil with a high initial
content of organic carbon will return to its former state of soil health1 more rapidly
when the drought breaks than a soil with a lower SOC content.
The optimal level of SOC required for these functions in any particular soil is
difficult to quantify because different amounts and types of SOC may be required.
Irrespective of soil type, if SOC content is below 1%, water-limited yield potential
may not be achieved (Kay and Angers 1999) as the soil’s capacity to perform key
functions (Fig. 14.1) is constrained. Soil organic C contents of Australian soils
have been shown to vary from greater than 81 g C/kg soil (8.1% carbon) for alpine
humus soils (Organosols2) to less than 3 g C/kg soil (0.3% carbon) for desert loams
(Chromosols) (Spain et al. 1983). Australian soils under rainfed farming typically
have SOC contents in the range 0.7–4%. Enhancing the grain yield and harvest index
of agricultural crops often results in increasing proportions of the carbon fixed by
photosynthesis being exported from the site as grain, rather than returned to the soil
as residues. If the rate of carbon return to the soil is less than that being lost through
export, microbial decomposition and erosion, the SOC content will decline.
14.2 The Carbon Balance in Agricultural Soils
Carbon balance is used to refer to the net result obtained when all processes of carbon
addition and loss from a soil are summed. The soil carbon balance can be thought of
as a tipping scale. When the amount of carbon input into soil matches the loss of
carbon from the soil, the scale is balanced, and there is no net change in SOC content.
It is only when inputs outweigh losses (i.e. positive carbon balance) or losses are
greater than inputs (i.e. negative carbon balance) that SOC contents will change.
CO2 removed from the atmosphere and stored as either organic or inorganic
carbon for long periods of time (greater than an annual time scale) is considered
‘sequestered’. Places where carbon is stored are called carbon ‘sinks’ whilst places
where carbon is emitted or lost are termed carbon ‘sources’. As soil can store car-
bon, it represents a large potential sink but additionally it is also a potential source
of CO2. If the rate of input from plants and animals exceeds the rate of loss, SOC
accumulates (i.e. positive carbon balance), and the soil acts as a CO2 sink. If the rate
of loss is greater than the rate of input, SOC declines (i.e. negative carbon balance),
and the soil becomes a CO2 source. Thus, at any given time, the amount of carbon
retained in a soil is a reflection of the net difference in the carbon balance between
1 See Glossary.
2 This chapter uses the Australian Soil Classification see http://www.clw.csiro.au/aclep/asc_re_
on_line/soilhome.htm.

342 F.C. Hoyle et al.
historical inputs of organic carbon (e.g. photosynthesis/net primary productivity,
animal manure, compost) and the sum of organic carbon losses associated with
microbial decomposition (i.e. CO2 evolution), leaching of dissolved and particulate
carbon and wind or water erosion. Even a small loss of surface soil by erosion can
have a large impact on SOC sinks as this surface layer holds much higher SOC
concentrations than the deeper layers, and the smaller and lighter carbon-rich
particles are preferentially lost.
Erosion is greatly influenced by the percentage ground cover. The risk of wind
erosion, for example, can be decreased by covering a minimum of 50% of total
ground area with stubble or other residues. In Australian agriculture, typical soil
losses in a single year can be 60–80 t/ha from bare fallow, 8 t/ha under a crop and
0.24 t/ha under pasture; single, high-intensity storms can erode 70–300 t/ha through
both wind and water erosion. Since 1 mm depth of soil weighs approximately 10 t/
ha, erosion events in cropped soils represent a significant loss of topsoil with its
associated smaller and lighter carbon- and nutrient-rich fractions.
14.2.1 Potential SOC Content
The potential to store SOC is rarely achieved as sub-optimal climatic conditions
and soil management often restrict plant growth and return of plant residues
(Fig. 14.2).
Fig. 14.2 The influence of soil type, climate and management factors on the level of soil organic
carbon (SOC) that can be attained in a given soil (after Ingram and Fernandes 2001)

34314 Soil Organic Carbon – Role in Rainfed Farming Systems
The potential ability of a soil to retain organic carbon is based on its capacity to
protect (i.e. stabilise) SOC. Organic carbon is thought to be protected against
microbial decomposition by adsorption of organic compounds onto the surfaces of
mineral particles, by being within pores of less than 0.2 mm in diameter and by the
burial of organic materials within aggregations of mineral particles. In heavier
textured soils, aggregated clay particles physically protect organic particles from
microbial decomposition. Mechanisms of protection of SOC operate at soil aggre-
gate size scales ranging from micrometres (mm; i.e. one thousandth of a mm) to
centimetres (cm) and depend on the chemical and physical properties of the mineral
constituents and the 3-dimensional arrangement of mineral particles. Well-
aggregated soils are also less prone to erosion.
In contrast, a more rapid turnover of SOC occurs in soils with little or no clay
content; hence it is more difficult to increase the SOC content of coarse-textured,
sandy soil from crop residues alone. An example of the influence of clay content
on SOC is demonstrated in Fig. 14.3. This shows the range of SOC values mea-
sured in a 10-ha area under a cereal–legume rotation, where the clay content varied
from 3% to 52%. Soil organic carbon values increased with clay content, over a
fivefold range in values (min. 0.7%, max. 3.4%), reflecting differences in the
amount of plant growth (and thus residue returns) as well as physical protection as
clay content increased.
y = 0.64Ln(x) + 1.17
R2 = 0.92
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Clay content (%)
SOC (%)
3-5
(25)
30-35
(15)
10-15
(27)
20-25
(34)
40-52
(7)
35-40
(11)
25-30
(23)
15-20
(13)
5-10
(65)
Fig. 14.3 Influence of clay content on the range of soil organic carbon (SOC) values in a 10 ha
area of a paddock under cereal–legume rotation in the central agricultural region of Western
Australia. Solid circles represent the average SOC value for each clay content whilst open circles
represent the upper and lower SOC values for each clay content. The numbers of samples within
each clay content are shown in brackets (n = 220 in total). The soil contained no gravel

344 F.C. Hoyle et al.
Although soil bulk density and depth are not directly important to the stabilisation
of SOC (protection against biological decomposition), they can define the amount
of mineral material and/or surfaces that may interact with SOC, as well as the aera-
tion status of the soil which directly influences the rate of microbial decomposition.
The potential SOC content that a soil can achieve is a function of these defining
factors (Fig. 14.2). Thus for a soil to reach its potential SOC, inputs of carbon from
plant production must be large enough to fill the protective capacity of a soil as well
as to offset losses due to microbial decomposition.
14.2.2 Attainable SOC Content
The SOC content that can be achieved depends not only on the potential of the soil
to protect organic carbon but also on the productivity of the crop or pasture (net
primary productivity). As productivity depends on water supply, temperature and
solar radiation, attainable SOC may be less than potential SOC. These limiting
factors are largely outside the control of rainfed farmers. Where plant residue
returns are equal to or greater than those required to achieve the potential SOC, the
attainable SOC content equals the potential SOC. However, under most rainfed
agricultural conditions the availability of water will define an upper limit of plant
productivity below that required to attain the potential SOC, resulting in a lower
‘attainable’ SOC (Fig. 14.2).
Capture of CO2 from the atmosphere by photosynthesis contributes to carbon
sequestration through the return of plant shoot residues, root exudates and root
biomass to the soil. Some of this carbon will return to the atmosphere as CO2
through biological decomposition, but a component is likely to be sequestered
within the more stable fractions of SOC that are resistant to biological decomposi-
tion. Therefore carbon losses as CO2 to the atmosphere can be reduced by increased
movement of organic carbon into stable SOC pools or the microbial population.
This could be achieved by increasing plant growth (and the amount of photo-
synthetically-fixed CO2) or through improved agronomic and soil management
options that reduce losses through decomposition and erosion.
The positive relationship between annual rainfall and net primary productivity
(total dry matter production as defined using the equation of Lieth 1975), which
occurs where water is the major constraint to plant growth, is shown in Fig. 14.4
by the sloping solid line. Since productivity directly influences the potential
return of carbon to the soil in the form of roots and shoot residues, a positive
relationship between SOC and rainfall might be expected. However, none is evident
for a range of soil types used for grain production in Western Australia (Fig. 14.4).
Instead SOC data (collated from between 40 and 220 individual fields) reflect the
trend in actual productivity as determined by annual wheat yields estimated as
shire averages from 1960 to 2002 (Fig. 14.4). Altered cropping and management
strategies that improve crop productivity are therefore required to increase
SOC contents.

34514 Soil Organic Carbon – Role in Rainfed Farming Systems
14.2.3 Actual SOC Content
Reaching the attainable SOC is generally the best possible outcome for many rainfed
farming systems. But to achieve this, there must be no constraints to productivity
and associated carbon inputs such as low nutrient availability, weed growth, disease
or soil physical constraints. Such a situation virtually never exists, and the lower
plant productivity is reflected in the actual SOC.
Even where rainfall is deficient, many of the factors that restrict build up of
SOC are under the farmers’ control (Fig. 14.2). For example, rotational sequence
may alter the actual SOC as plant species vary in their efficiency at converting
water to plant biomass, in the amount and distribution of roots below ground
(Fig. 14.5) and in their tissue composition (i.e. lignin content). Soil constraints to
plant growth may be considered as factors limiting the actual SOC. Some soil
constraints (e.g. compaction) can be ameliorated through management. While
other constraints may be due to soil factors less readily managed – for example
saline conditions at depth.
Farmers may be able to regulate agricultural management to maximise organic
inputs and retain them. However, if the attainable SOC is much lower than the
potential SOC, only the addition of an external source of organic matter to the soil
will improve the situation (Fig. 14.2).
0
20
40
60
80
100
150 200 250300
Growing Season Rainfall (mm)
SOC (t C/ha; 0-10 cm)
0
1
2
3
4
5
6
Production (t C/ha /yr)
Fig. 14.4 Soil organic carbon (t C/ha; open circles with vertical lines indicating upper and lower
values; primary y-axis). Calculated net primary productivity (DM t C/ha/year; solid line; second
y-axis). Actual above-ground plant biomass (t C/ha/year; squares with dashed line indicating
linear trend; second y-axis). Data are plotted across a range of growing season rainfall (mm) in
Western Australia

346 F.C. Hoyle et al.
Variations in land use and agricultural management with respect to crop type, rotation
sequence, soil management, stubble management, and green manure incorporation can
influence the actual SOC. In general, soils under pasture have a higher SOC content
than those under cropping (Blair et al. 2006), while minimum tillage and stubble reten-
tion may improve SOC content in cropped soils (Chan and Heenan 2005). The addition
to and retention of organic matter in soil represents one of the primary input pathways,
whilst the adoption of no-tillage is primarily a protection pathway which contributes to
increased soil aggregate stability. Increased soil disturbance breaks down the physical
protection provided by soil aggregates, and exposes plant and animal residues and other
SOC to microbial decomposition, thus accelerating the rate of SOC decline. Extreme
climatic conditions such as drought or episodic events such as disease prevent farmers
from improving SOC status because there is less organic input and these provide real
challenges within a rainfed agricultural system (Table 14.1).
Fig. 14.5 Differences in below ground plant root architecture and biomass of (a) lupin and
(b) wheat plant simulated using ROOTMAP (produced by Vanessa Dunbabin, University of
Tasmania)

34714 Soil Organic Carbon – Role in Rainfed Farming Systems
Table 14.1 Management options for improving the potential for sequestration of organic carbon in soil
Option Source Example Benefits Factors addressed
Increase C
inputs
Increase active
plant growth,
longevity
and diversity;
increase ground
cover
Pastures Increases C inputs
through longer active
growth and reduced
fallow periods
Much of the historic C loss in Australian soils has been associated
with the clearing of native vegetation for agricultural production.
Land converted to an agricultural system often experiences
changes in the amount of SOC; typically, agricultural soils
experience organic carbon decline of between 25 and 75% with
continued cultivation
Perennials
Pasture cropping
Inter-cropping Provides range of residue
quality
Increased root biomass
Decreased erosion
Reduce fallow
periods
Add stable organic
amendments
Compost Increases C inputs Animal manure, composts and bio-solids added to the soil can
contribute both organic matter and nutrients where structural
integrity of the soil is maintained. Increasingly, by-products
from other industries are being considered (charcoal, canola
meal, compost etc). However, there may be insufficient evidence
to establish their impact on soil properties or crop production
on a broadacre basis. Management of humus provides some
potential for altering CEC, but almost all of the charges on
organic colloids are pH dependent (CEC increases as pH
increases). Thus increases in CEC are limited on acid soils
Processing wastes Contributes to stabilising
soil aggregates and
pore structure
Charcoal and char
Other
Increase plant
biomass
Manage soil for
constraints
Increases C inputs Appropriate soil and plant management to increase water and
nutrient use efficiency can result in higher plant biomass and
residue inputs to the soil where retained. The most limiting
factors must be assessed to determine the cost/benefit return and
end profitability of any specific management approach
Increase water use
efficiency
Increases profitability
Agronomy
management
(continued)

348 F.C. Hoyle et al.
Option Source Example Benefits Factors addressed
Improve soil
structure
Retain stubble and
maintain ground
cover
Zero tillage Decreases loss of organic
residues from soil
Generally the potential for increasing water-holding capacity of
soils is less than 5 mm. Increasing ground cover and intact roots
will help protect the soil surface from raindrop impact, minimise
the risk of both wind and water erosion, reduce evaporation and
buffer the soil against extremes of temperature
Pastures
Cover crops Protects soil from erosion
Do not burn Promotes water
conservation
Manage livestock Controlled grazing Limits soil compaction Near-surface soil compaction can occur with grazing livestock.
However, when the soil is moist and most prone to damage,
stock can be moved to parts of the farm where there is less risk
of soil compaction or where it is most easily rectified at a later
date. Removing or reducing stock (particularly cattle) when the
soil is wet and shortening the duration of high stocking rate will
help reduce the risk of compaction
Reduce stocking
rate
Protects soil from erosion
Rotational grazing
Reduce tillage Zero tillage Decreases erosion risk Increased soil disturbance breaks down the physical protection
provided by soil aggregates and exposes plant and animal
residues and other SOC to microbial decomposition,
accelerating the rate of SOC decline. Reduced or zero tillage
options will promote soil aggregation and provide greater
physical protection of SOC (tillage is likely to result in
continuing decline in SOC)
Reduce number of
workings
Protects stable SOC pools
Promotes water
conservation
Control traffic Guidance systems Reduces area of
land affected by
compaction
Using controlled traffic is an essential partner to soil loosening
for minimising or repairing compaction. Compaction can be
minimised by confining all or most tillage and traffic operations
to designated fixed vehicle pathways
Match wheel tracks
Supply
optimum
nutrients
Inorganic fertiliser Urea Enables rapid supply and
timing of nutrient
availability to increase
plant biomass
Soil nutrients need to meet crop demand and maximise profitability
without causing off-site pollution. Nutrient replacement should
equal nutrient removal. An understanding of which nutrients
are important, nutrient form and availability, and soil testing are
required
Superphosphate
NPK
Table 14.1 (continued)

34914 Soil Organic Carbon – Role in Rainfed Farming Systems
Option Source Example Benefits Factors addressed
Add organic
amendments and
wastes high in
nutrients (low
C/N)
Animal manures Enables longer term
supply of nutrients
and contributes to
biological turnover
Soil organic matter is a major storehouse of nutrients. A decline
in soil organic matter will reduce the ability of the soil to
provide adequate nutrients for crops over the long-term.
Efficient recycling of nutrients within the soil can offset the
cost of inorganic fertiliser, reduce energy costs, cut waste,
reduce potential environmental impacts and improve soil health.
Nutrient content varies greatly depending on the fertiliser
source, making it difficult to predict their effectiveness or cost
per unit of nutrient
Oilseed wastes
Legume green
manure
Use appropriate
rotations
Crop and pasture
legumes
N fixation contributes to
N release in soil
Legume crops fix N and generally result in increasing levels of soil
N which benefit subsequent non-legume phases. These residues
decompose more rapidly than those of cereals and release N in
subsequent seasons, but contribute less to building SOM than
plants with more stable residues such as cereals and grasses

350 F.C. Hoyle et al.
14.3 Soil Organic Carbon Fractions and Their Function
Soil organic carbon exists in a range of different types of materials that vary in size,
chemical composition, stage of decomposition and age. Newly incorporated
organic material is approximately seven times more decomposable than inherent
SOC (that present prior to addition of the new material). However, inherent SOC is
usually a much larger pool (15–225 t C/ha for the 0–30 cm soil layer). Though the
decomposition rate of inherent SOC may be low, it can result in significant miner-
alisation of both carbon and nutrients (Shen et al. 1989).
SOC should be considered as a continuum of different forms with turnover times
ranging from minutes for soluble root exudates (Jones et al. 2004, 2005) to hun-
dreds or thousands of years for highly resistant material (Anderson and Paul 1984).
Soil properties and processes are therefore influenced by the size and quality of the
SOC fractions.
A number of SOC fractionation schemes are in use (see Sollins et al. 1984; Sohi
et al. 2001), and these differ in the means by which separate components of SOC
are recovered. One such fractionation scheme (Fig. 14.6) involves the separation of
SOC based on different size classes and chemical composition:
Surface Plant Residues (SPR) and Buried Plant Residues (BPR) comprise leaf •
litter, plant stems or stubble and below-ground root matter. They represent
the main source of plant C inputs into broadacre rainfed farming systems where the
importing of composts and animal manures is uneconomical unless a source is
located nearby.
Particulate Organic Carbon (POC) represents the initial stages of organic matter •
decomposition; it still contains considerable energy ‘locked up’ within the bonds
of organic molecules.
Humus (HUM) is decomposed organic matter that is more biologically stable •
and turns over more slowly than POC. Changes in soil management or farm
inputs typically alter this fraction over a period of decades.
Resistant Organic Carbon (ROC) is the most biologically stable form of organic •
carbon and, in Australian soils, is dominated by char-type material derived
from plant residues after incomplete combustion. The size of the ROC fractions
is thus dependent on historical fire frequency. In Australian soils, Skjemstad
et al. (1999, 2001) determined that between less than 1% and 57% of SOC is
composed of fine char carbon, most of which is located in the less than 53 mm
fraction (Skjemstad et al. 1996, 1999).
The composition and size of different SOC fractions will influence the contribu-
tion that each type of SOC makes to the various functions typically ascribed to SOC
(Fig. 14.1). A conceptual representation of the contribution that SOC and its com-
ponent fractions make to a series of soil properties and processes is given in
Fig. 14.7. In Fig. 14.7, the width of the shape reflects the relative importance of
SOC to the function or process identified. For example, SOC will be most important
for defining CEC in soils with low clay content. As clay content increases the

35114 Soil Organic Carbon – Role in Rainfed Farming Systems
contribution of SOC to the total CEC of the soil will diminish. At high clay contents
(greater than 60%), SOC is not required to attain acceptable levels of CEC and thus
the CEC shape in Fig. 14.7 does not extend past 60% clay. Within the shapes, the
width of each shaded area reflects the perceived relative contribution that the dif-
ferent fractions of SOC will make. For the structural stability shape, at low clay
contents, the POC fraction would be expected to be the most critical form of SOC;
Surface plant
residues (SPR)
Resistant organic
carbon (ROC)
Non-living 85%
Plant roots
5-15%
Organisms
85-95%
Living 15%
Fauna
5-10%
Mite 500-2000 µm
Protozoa 10-80 µm
Bacteria 0.5-1.5 µm
Fungi 1-10 µm
Microorganisms
75-90%
Buried plant
residues (BPR)
> 2000 µm
Humus (HUM)
< 50 µm
> 2000 µm
Plant roots
> 2000 µm
0 − 70% vol.
Gravel
< 2000 µm
Soil
Particulate organic
carbon (POC)
50 − 2000 µm
Pores
and
voids 0.1 − 10%
Soil organic matter
(SOM)
Soil Minerals >90%
<2 µm
Sand
Silt
Clay
2-20 µm
20-2000 µm
Fig. 14.6 Schematic of soil organic matter fractionation scheme that represents different stages
of soil organic matter decomposition. Measurements refer to particle sizes

352 F.C. Hoyle et al.
while, at high clay contents, the humus form would be expected to be most critical.
It should be noted that the shapes presented in Fig. 14.7 are conceptual in nature,
and further research is required to derive accurate quantitative relationships.
14.3.1 Cation Exchange Capacity (CEC)
Like clay particles, humus (HUM) can hold nutrients through adsorption reactions
because of its large negative charge density at pH values greater than 5. This allows
humus to make a significant contribution to the CEC of a soil. The soil fractions
differ widely in their CEC, with HUM and ROC (see below) having the highest
CEC of all SOC fractions. The CEC of freshly-made charcoal does not appear to
acquire the same level of CEC as old chars extracted from soil. Thus, the addition
of HUM-type materials from composts or well-decomposed organic waste provides
some potential for altering CEC.
The impact of the HUM fraction on CEC is greater where the HUM fraction is
dominant in providing the CEC (as on light-textured sandy soils), compared to clay
soils which have a CEC associated with the clay complex. For example, in a soil
with 25% clay and 1.5% SOC, approximately one third of the CEC (of topsoil) can
be attributed to the SOC whereas in a sandy soil, with little or no clay content, SOC
accounts for almost 100% of the CEC of topsoil. The ability of SOC to hold and
release nutrients (that would otherwise leach deeper into the profile) in the upper
layers of sandy soils – and make these nutrients available for plant uptake – is a key
benefit of SOC.
CEC
Soluble
Particulate
Humus
Inert
Sands
Clays 100
80
60
40
20
0
Clay content (%)
Soil
Structure
Energy for
biological
processes
Provision
of
nutrients
Soil
thermal
properties
Fig. 14.7 Conceptual role of different soil organic carbon fractions (soluble, particulate, humus,
inert) on a range of soil functions

35314 Soil Organic Carbon – Role in Rainfed Farming Systems
However, almost all of the charges on organic colloids are pH dependent, limiting
the benefits in acid soils since CEC decreases as pH decreases. It is also important
to note that SOC-induced change is largely associated with the surface soil, and the
mineral fraction will continue to provide most of the CEC within the rooting depth
of a plant.
14.3.2 Soil Structure and Water Relations
Soil organic carbon stabilises soil aggregates and improves water infiltration into
soil by contributing to the development of a more porous soil structure. In addition,
surface plant residues, if present in sufficient quantities, can reduce evaporation and
buffer soil temperature.
Different fractions of SOC can hold up to several times their own weight in
water due to their porous nature. As SOC is likely to be concentrated in the
upper layers of the soil profile, plant systems with a proliferation of roots in
the upper soil horizon are more likely to access this moisture. Due to the energy
required to extract moisture from small pores, not all of the water is plant avail-
able. The relative contribution of SOC fractions to soil water-holding capacity
will decrease with increasing clay content. The additional water-holding capac-
ity (WHC) provided by SOC, above that due to soil texture alone, will be of most
benefit where rainfall distribution patterns result in low or variable soil water
conditions. Dependent on the season, this additional water storage capacity may
be of significant value.
Soil texture, soil structure, soil constraints and plant rooting depths are the crucial
factors determining the amount of water available for plants to access (plant-available
water or PAW). Although WHC tends to increase as clay content increases due to
changes in the soil pore structure (higher number of small pores), a portion of the
water remains unavailable to plants. This means that the influence of an increase in
SOC on plant-available WHC is not a constant but is dependent on soil clay con-
tent. For example, in Fig. 14.8 it can be seen that the influence of an increase in
SOC has a declining effect on plant-available WHC as clay content is increased.
In this example the amount of extra plant-available water holding capacity
increased from 2 mm in soils with 30% clay, to approximately 5 mm in those with
less than 10% clay. Although this may not be important in any one event, storage
of this amount of extra water over ten rainfall events would amount to 30 mm – the
equivalent of 600 kg/ha grain if a water use efficiency of 20 kg grain/mm water is
reached. It is important to note that it will be more difficult to obtain an increase in
SOC content of 1% on a sandy soil compared to a soil with higher clay content.
SOC can also influence plant-available water by stabilising soil structure and, in
particular, the pore size distribution within a soil. Table 14.2 shows measured
changes in the infiltration of water and its flow through the soil profile. In particu-
lar, it indicates the greater amount of water available under farming systems that
incorporated green manure crops compared to a winter fallow.

354 F.C. Hoyle et al.
14.3.3 Energy for Biological Processes
When soil organisms break down organic matter they use carbon as an energy
source and cellular building block, and use mineral nutrients for their growth and
metabolism. In using organic matter (including other organisms) as food, they
release CO2. Depending on climatic conditions, between 50% and 75% of the
carbon in fresh organic residues may be released as CO2 during the first year of
decomposition. Labile fractions of SOC (POC and dissolved organic carbon) are
the predominant sources of energy for soil micro-organisms. In cropping soils of
Western Australia, a strong positive relationship has been found between POC
and the total mass of micro-organisms measured as microbial biomass carbon
(MBC; Fig. 14.9).
y = −0.1229x + 5.5029
R
2
= 0.8212
0
1
2
3
4
5
6
010203040
Clay content (% of soil mass)
Change in plant available
water holding capacity
(mm water)
Fig. 14.8 The effect of increasing soil organic carbon by 1% in the 0–10 cm soil layer on plant
available water holding capacity (mm) at a range of different clay contents (% of whole soil mass)
on Chromosols of the mid-north region of South Australia
Table 14.2 Soil bulk density and water infiltration rates measured on a red sandy loam
(Chromosol) at sowing time in 1999, after eld pea green manure and fallow treatments were
imposed in 1998 at Mullewa, Western Australia
Measurement Fallow
Brown manure (green
manure crop desiccated
at flowering)
Green mulch (green
manure crop slashed
at flowering) LSD (P = 0.05)
Bulk density (mg/m3) 1.5 1.3 1.4 0.3
Sorptivity (mm/h) 13.6 16.7 17.4 3.4
Flow rate (mm/h) 47.0 27.3 30.1 8.8
Volumetric water (%) 7.0 11.3 12.1 6.3
Available water (mm/m) 69.6 113.2 121.3

35514 Soil Organic Carbon – Role in Rainfed Farming Systems
Land use and agricultural management practices alter the relative proportion of
labile SOC fractions, which in turn regulate microbial community composition and
function (Grayston et al. 2004; Cookson et al. 2005). In arable systems, these labile
fractions are often associated with rapid cycling of nutrients which can be corre-
lated directly with increased yield (Stine and Weil 2002). In contrast, ROC is con-
siderably less biologically available than other components of SOC (Baldock and
Smernik 2002). With mean residence times in the order of several thousands of
years, much of the ROC is more reflective of the historical conditions under which
the soil developed rather than more recent agricultural management practices; it can
be considered as an inherent fraction of the SOC that does not provide energy to
microbial processes.
14.3.3.1 Provision of Nutrients
As organic materials decompose, nutrients can be released (mineralised) or taken
up (immobilised) by soil organisms. Net nutrient mineralisation (the balance
between mineralisation and immobilisation) provides a measure of the influence
that decomposition processes will have on the supply of plant-available nutrients.
A primary control over net nutrient mineralisation is the carbon to nutrient ratio of
the organic materials being decomposed. As the carbon to nutrient ratio decreases,
the potential for a net release of nutrients into the plant-available pool increases.
0
100
200
300
400
500
600
700
0 500 1000 1500 2000 2500 3000
POC (kg C/ha; 0-10 cm)
MBC (kg/C ha; 0-10 cm)
Arable
Pasture
Forest
Linear (Arable)
R
2
= 0.86
Fig. 14.9 The relationship between particulate organic carbon (POC) in organic matter and
microbial biomass carbon (MBC) under a range of land uses in Western Australia. The line shows
the strong linear relationship for the arable sites

356 F.C. Hoyle et al.
As plant residues progress through the POC fraction to the more biologically
stable humus fraction, the extent of decomposition increases, and the carbon to
nutrient ratio decreases in magnitude and variability. Across 29 soils from south-
eastern Australia with SOC contents ranging from 0.8% to 5.7% in the top 10 cm
layer, C/N ratios of SOC fractions and their variance decreased from surface plant
residues (SPR) (more than 100) through to humus (less than 10) (Fig. 14.10). The
variations in C/N ratios measured for the SPR and BPR fractions suggest that nitro-
gen release dynamics will vary with the type of crops grown. High-N residues with
narrow C/N ratio, such as those obtained from pulses or pasture legumes, will result
in greater N release to the soil compared to low N residues (i.e. wide C/N ratio)
such as cereal crops.
The influence of C/N ratio (determined from the combined POC and BPR frac-
tions) on mineral N release (Fig. 14.11) illustrates that organic material with a C/N
ratio below 22 released more mineral N. In this example, as the C/N ratio of these
pools narrowed further, the availability of N increased rapidly. The N availability
after three different green manure treatments (lupin, field pea and oats) reflects
recent organic inputs, due to the chemical similarity between plant residues and
POC (Fig. 14.11). In contrast, organic material with a C/N ratio above 22 did not
supply additional mineral N to the soil–plant system. Residues with high C/N values
(e.g. wheat stubble with a C/N of approximately 80:1) can immobilise or ‘tie up’
soil N in the short term.
0
20
40
60
80
100
120
SPR BPR POC Humus
Type of organic matter
C/N ratio
(weight basis)
Maximum values
Minimum values
Fig. 14.10 The upper and lower boundaries for the C/N ratio of different SOM fractions mea-
sured across 29 south eastern Australian soils with total organic carbon contents ranging from
0.8% to 5.7%

35714 Soil Organic Carbon – Role in Rainfed Farming Systems
14.4 Monitoring Soil Organic Matter
Globally, the soil is a large sink containing approximately 1,550 Gt SOC, with an
additional 750 Gt of inorganic carbon (0–100 cm depth; Krull et al. 2004). SOC
accounts for more carbon than the combined total amount of carbon in the atmo-
sphere (780 Gt) and vegetation (550 Gt). In Australia, soils and vegetation are
estimated to contain 48 and 18 Gt carbon, respectively. Thus the carbon contained
in soils globally and in Australia is approximately 2.7 times greater than that
stored in vegetation.
Most Australian soils would be expected to contain more than 15 t C/ha in their
0–30 cm surface layer, which equates to a soil with a carbon content of 5 g SOC /
kg soil and a bulk density of 1 t/m3. Soil containing 50 g SOC /kg soil and a bulk
density of 1.5 t/m3 would have 225 t/ha C in the 0–30 cm layer. Using an average
wheat yield of 2 t/ha grain, a harvest index of 0.37, a carbon content of 450 g C/kg
residue, allowing for root dry matter and 50% decomposition, loss of crop residue,
an annual addition rate of carbon to the soil of about 0.8 t/ha is achieved. This is
approximately an 18th of the minimum SOC value of 15 t/ha C ha and a 280th of
the 225 t/ha C value. Consequently it is often difficult to measure management
induced changes in SOC on an annual basis given the small amounts of the C inputs
relative to the amount of inherent carbon present in a soil. Long measurement times
(more than 10 years) are usually required to detect significant management-induced
changes in total SOC content unless considerable external inputs of carbon are also
provided. Because of the continued decomposition of SOC, substantial amounts of
0
20
40
60
80
100
120
140
0102030405060
C:N ratio of combined BPR + POM
N release
(kg N/ha; 0-10 cm)
Oat
Field Pea
Lupin
Medic
Lucerne
Wheat
Fig. 14.11 The influence of the C/N ratio of the buried plant residue (BPR) plus particulate
organic carbon (POC) fractions on N release (assessed as potentially mineralisable N) in soil

358 F.C. Hoyle et al.
additional organic material are required to have a measurable effect on SOC over
the long-term (simulation modelling suggests an additional 2 t/ha of plant residues
retained each year for 20 years may increase total SOC by only 0.5%). Where rapid
changes in total SOC content have been reported in typical Australian farming
systems these have often been associated with changes in gravel and/or soil bulk
density not being taken into account. For example, as gravel content is increased,
the mass of soil (i.e. less than 2 mm) in a given volume is less, resulting in SOC
being concentrated in a smaller volume, within the <2 mm soil fraction (Fig. 14.12).
Where SOC comparisons are being made between sites with different bulk density
and/or gravel content (or at the same site in different years), a percent carbon value
must be adjusted to t/ha to allow for changes in soil density and gravel content.
In Australian agricultural soils, SOC content is highest in the 0–10 cm soil layer,
due to leaf drop, stubble return and the predominance of roots in the surface soil
layer. This is generally between 30% and 50% of the total soil C within a soil pro-
file. Because of its more biologically labile nature, the response time of this soil
layer to changes in soil management or inputs is likely to be more rapid than that
of SOC in deeper soil layers. SOC contents of the 10–30 cm and 30–100 cm layers
tend to be lower and demonstrate smaller management-induced change, with the
possible exception of soil under deep-rooted perennials. However, a study by
Macdonald et al. (2007) in the northern wheatbelt of WA showed that total organic
carbon (0–65 cm) did not differ significantly between adjacent native woodland and
a mixed grass/lucerne pasture, whilst there was clear evidence of N enrichment
0 % gravel
30 % gravel
60 % gravel
% SOC not adjusted for gravel
% SOC adjusted for gravel
0
0
1
1
2
2
3
3
4
4
5
5
6
6
Fig. 14.12 Calculated change in soil organic carbon content (%) for soils not adjusted (x-axis) or
adjusted (y-axis) for gravel content. The dashed lines illustrate that a soil test result of 4% SOC
once adjusted for 30% or 60% gravel would equate to 2.8% SOC and 1.6% SOC respectively

35914 Soil Organic Carbon – Role in Rainfed Farming Systems
under the grass/lucerne pasture system. In both cases, the major portion of the soil
carbon (about 80%) was present in the surface 15 cm.
Total SOC is unlikely to be a good predictor of the availability of C to microbial
communities and thus the level of biological activity that can be maintained in a
soil. Instead, the labile fractions of SOC that are sensitive to changes in land use
and management practice are considered more important indicators (McLauchlan
and Hobbie 2004; Haynes 2005; Hoyle and Murphy 2006). Quantifying manage-
ment-induced changes to the more dynamic SOC fractions can provide a more
rapid indication of the direction of SOC change. For instance, it can be seen from
Fig. 14.13 that although the directions of change for the POC and humus forms of
SOC were the same, the relative rates of response of the more labile POC fractions
were initially greater than that of the more stable humus fraction both in terms of car-
bon loss (when the wheat fallow system was implemented at year 0) and in terms of
carbon build-up (when the permanent pasture system was implemented at year 33).
14.5 Conclusions
Soil organic carbon plays a central role in the functioning of all soils including •
providing an energy source for biological processes, improving soil structure
and buffering chemical reactions.
Fig. 14.13 The influence of changing from a wheat–fallow rotation to permanent pasture on soil
organic carbon fractions simulated over a 75-year period

360 F.C. Hoyle et al.
Clay content is a key determinant of the potential for soil to store organic •
carbon; however, in most circumstances the amount of organic carbon in a soil
is limited by climatic and soil induced constraints to plant growth and thus
organic matter returns.
The level of SOC is the result of the balance between inputs (e.g. plant residue •
and other organic inputs) and losses (e.g. erosion, decomposition).
Organic carbon stored in a soil exists in a range of different fractions that vary •
in their size, chemical composition, stage of decomposition and function.
Greater insight into soil function can be gained by monitoring SOC fractions •
rather than considering only the total amount of organic carbon present.
A primary challenge for farmers is to sustain a profitable farming system for the •
long term, which requires continued addition and maintenance of organic inputs.
Monitoring is essential to assess whether management induced changes are •
depleting or restoring the soil resource, and in understanding the impact of
changing land use and climate.
References
Anderson DW, Paul EA (1984) Organo-mineral complexes and their study by radiocarbon dating.
Soil Sci Soc Am J 48:298–301
Baldock JA, Smernik RJ (2002) Chemical composition and bioavailability of thermally altered
Pinus resinosa red pine, wood. Org Geochem 33:1093–1109
Blair N, Faulkner RD, Till AR, Crocker GJ (2006) Long-term management impacts on soil C, N
and physical fertility: part III: Tamworth crop rotation experiment. Soil Tillage Res
91:48–56
Chan KY, Heenan DP (2005) The effects of stubble burning and tillage on soil carbon sequestra-
tion and crop productivity in southeastern Australia. Soil Use Manage 21:427–431
Cookson WR, Abaye DA, Marschner P, Murphy DV, Stockdale EA, Keith WT (2005) The contri-
bution of soil organic matter fractions to carbon and nitrogen mineralization and microbial
community size and structure. Soil Biol Biochem 37:1726–1737
Grayston SJ, Campbell CD, Bardgett RD, Mawdsley JL, Clegg CD, Ritz K, Griffiths BS, Rodwell
JS, Edwards SJ, Davies WJ, Elstone DJ, Millarda P (2004) Assessing shifts in microbial com-
munity structure across a range of grasslands of differing management intensity using CLPP,
PLFA and community DNA techniques. Appl Soil Ecol 25:63–84
Haynes RJ (2005) Labile organic matter fractions as central components of the quality of agricul-
tural soils: an overview. Adv Agron 85:221–268
Hoyle FC, Murphy DV (2006) Seasonal changes in microbial function and diversity associated
with stubble retention versus burning. Aust J Soil Res 44:407–423
Ingram JSI, Fernandes ECM (2001) Managing carbon sequestration in soils: concepts and termi-
nology. Agric Ecosyst Environ 87:111–117
Jones DL, Hodge A, Kuzyakov Y (2004) Plant and mycorrhizal regulation of rhizodeposition.
New Phytol 163(3):459–480
Jones DL, Healey JR, Willet VB, Farrar JF, Hodge A (2005) Dissolved organic nitrogen uptake
by plants – an important N uptake pathway? Soil Biol Biochem 37(3):413–423
Kay B, Angers DA (1999) Soil structure. In: Sumner ME (ed) Handbook of soil science. CRC
Press, Boca Raton, pp A-229–A-276
Krull ES, Skjemstad JO, Baldock JA (2004) Functions of soil organic matter and the effect on soil
properties. GRDC report, Project CSO 00029

36114 Soil Organic Carbon – Role in Rainfed Farming Systems
Lieth H (1975) Modelling the primary productivity of the world. In: Lieth H, Whittaker RH (eds.)
Primary productivity of the biosphere. Springer, Berlin, pp 237–263
Macdonald AJ, Murphy DV, Mahieu N, Fillery IRP (2007) Labile soil organic matter pools under
a mixed grass/lucerne pasture and adjacent native bush in Western Australia. Aust J Soil Res
45:333–343
McLauchlan KK, Hobbie SE (2004) Comparison of labile soil organic matter fractionation
techniques. Soil Sci Soc Am J 68:1616–1625
Shen SM, Hart PBS, Powlson DS, Jenkinson DS (1989) The nitrogen cycle in the Broadbalk
wheat experiment: 15N-labelled fertilizer residues in the soil and in the soil microbial biomass.
Soil Biol Biochem 21:529–533
Skjemstad JO, Clarke P, Taylor JA, Oades JM, Mcclure SG (1996) The chemistry and nature of
protected carbon in soil. Aust J Soil Res 34:251–271
Skjemstad JO, Taylor JA, Janik LJ, Marvanek P (1999) Soil organic carbon dynamics under long-
term sugarcane monoculture. Aust J Soil Res 37:151–164
Skjemstad JO, Dalal RC, Janik LJ, McGowan JA (2001) Changes in chemical nature of soil
organic carbon in Vertisols under wheat in south-eastern Queensland. Aust J Soil Res
39:343–359
Sohi SP, Mahieu N, Arah JRM, Powlson DS, Madari B, Gaunt JL (2001) A procedure for isolating
soil organic matter fractions suitable for modelling. Soil Sci Soc Am J 65:1121–1128
Sollins P, Spycher G, Glassman CA (1984) Net nitrogen mineralization from light-fraction and
heavy fraction forest soil organic matter. Soil Biol Biochem 16:31–37
Spain AV, Isbell RF, Probert ME (1983) Soil organic matter. In: Soils – an Australian viewpoint,
CSIRO/Academic Press, Melbourne/London, pp 551–563
Stine MA, Weil RR (2002) The relationship between soil quality and crop productivity across
three tillage systems in south central Honduras. Am J Altern Agric 17:2–8