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Chapter 11 Soil organic matter.

"We are part of the earth and it is part of us.... What befalls the earth befalls all the sons of the earth." Chief Seattle, 1852


In Chapter 10 you read about the various biogeochemical cycles of important nutrients in soil. In this chapter you will read about soil carbon, more specifically, soil carbon in the organic matter of soil. Soil organic matter, (SOM), is the debris of biological activity in soil. But unlike most debris it plays a critical role, influencing the chemical and physical properties of soil and the extent of biological activity that occurs therein. The amount and depth of the SOM in soils is one of the most important properties that influences soil productivity and its ability to sustain plant life and carry out other important ecological roles such as buffering and moderating hydrological cycles, physically supporting plants, retaining and delivering nutrients to plants, disposing of wastes and dead organic material, renewing soil fertility, and regulating major element cycles (Daily et al., 1997).


After reading this chapter, you should be able to:

* Describe the basic composition of soil organic matter.

* Identify five or six key roles that soil organic matter plays in soil systems.

* Understand the processes by which soil organic matter forms.

* Evaluate how soil-forming processes and human activity influence the amount of soil organic matter.


active and passive SOM


fulvic acid


humic acid




nonhumic compounds

soil organic matter (SOM)



In its broadest sense SOM is every organic compound that is in soil, which includes roots, animals, and microorganisms. Broadly classified, this means living organisms, fresh residues, and humus (the finely divided, nonrecognizable fraction of organic matter). In typical grassland soil the distribution of organic material would look something like that shown in Table 11-1. Humus, the nonrecognizable fraction of SOM, is by far the most abundant component.
SOM is usually partitioned
into humic and nonhumic

Excluding living organisms and plants for the moment, the SOM can be divided into two categories: nonhumic compounds and humic substances. The relative contribution of each category to soil is shown in Figure 11-1. Nonhumic substances have identifiable chemical composition and are readily available for microbial decomposition, so they are still in the process of decay. This fraction is often called the active component of SOM. Humic substances can be chemically analyzed but don't have a consistent, identifiable chemical composition. They are a dark-colored collection of compounds that resist further decomposition. One of the characteristics of humic substances is that these resistant materials are polymerized into new compounds such as fulvic acid and humic acid. Soil organic matter is unique in the sense that it reflects both degradative and constructive processes.


1. What are the types of organic matter in soil?

2. What are examples of nonhumic substances?

3. How do humic and nonhumic substances differ?

4. What fraction of organic matter makes the greatest contribution to SOM?

5. What property of SOM makes it unique?

Fresh plant residues are
about 10 percent of SOM.

It is much easier to ask this question than to answer it because SOM has neither an identifiable structure nor a uniform composition. For that matter, it is simultaneously forming and degrading, so the amount and distribution of SOM also continuously fluctuates.
Lignin is a key compound
involved in SOM formation.

Fresh plant and microbial residues make up about 10 percent of SOM. The most abundant and persistent microbial contributions are the cell wall polymers such as peptidoglycan, cellulose, and chitin. These polymers break down to release simple molecules like aminosugars and glucose. Plants, likewise, have structural polymers like cellulose and hemicellulose that are their principal contribution to SOM. Other compounds in fresh plant residue are sugars and starches, proteins and amino acids, waxes and pigments, pectin, fats, oils, lipids, organic acids, hydrocarbons, and lignin (Figure 11-2).

Lignin is perhaps second to cellulose in terms of the plant biomass returned to soil in residues and it is one of the key ingredients involved in forming soil humus. Lignins provide structural rigidity in plants, helping to glue cellulose polymers together and provide resistance to compression and bending. Lignin is a three-dimensional and highly branched molecule that forms by random condensation and polymerization reactions of just a few basic subunits called phenyl propanoids. The structure of lignin and its building blocks are illustrated in Figure 11-3.

Humus is fractionated based
on its solubility in acid and
alkaline solutions.

During plant residue decomposition, a series of chemical, biochemical, and biological transformations takes place that returns C[O.sub.2] to the atmosphere, converts identifiable plant material into unidentifiable humic materials, and promotes the ecological succession of various microbial communities (Figure 11-4). Humus is generally fractionated into three components following extraction in NaOH: humin, fulvic acid, and humic acid. Humin is the non-NaOH-dispersible fraction. Humic acid is a NaOH soluble fraction that is insoluble at pH 2. The molecular weight of humic acid varies from 10,000 to 100,000 and it is composed of aromatic rings, cyclic nitrogen compounds, and peptide chains. The important functional groups of humic acids, those groups that give it pH-dependent charge and help bind soil elements and other organic compounds, are carboxyls, phenolic OH, alcoholic OH, and ketones. Fulvic acids are soluble in NaOH and at pH 2. They are smaller than humic acids, with molecular weights ranging from 1,000 to 30,000. Fulvic acids have the same functional groups as humic acids, but they tend to be more oxidized and more mobile. On the basis of chemical analysis, the composition of humic acid and fulvic acid will look something like Figure 11-5. However, the actual chemical composition of humic materials in soils will vary depending on the type of soil and climate.

Glomalin is a fungal product
contributing to SOM.

Another constituent of SOM, and one which has only recently been recognized, is a glycoprotein secreted by mycorrhizal fungi called glomalin. This compound is not extracted from soil during routine humus fractionation, but it can be recovered by vigorously autoclaving the soil in buffer. When this procedure is carried out, it reveals that glomalin constitutes considerable quantities of the organic material in soil.



1. What are the major plant components entering soil?

2. What is the basic chemical structure of lignins?

3. What happens to plant material as it gets incorporated into SOM?

4. What is the difference between fulvic and humic acid?

5. Why is glomalin important in soil?


Although you've just read about some of the details of SOM composition, what's really important is not what SOM is made of, but what it does and how it contributes to the formation, function, and fertility of soil. That will be the topic of this section.

What are the functions or roles of SOM?

* Soil organic matter is the major source of nitrogen and an important source of phosphorus, sulfur, and trace nutrients.

* Soil organic matter contributes to the cation exchange capacity (CEC) and anion exchange capacity (AEC) of soil, which makes it a major storehouse of exchangeable nutrients.

* Soil organic matter binds organic chemicals and pesticides, which reduces their movement and activity, and lessens their effect on the environment.

* Soil organic matter is a type of glue that binds clay and silt particles into larger structural units and contributes to desirable soil structure with improved percolation and tilth. The binding makes these aggregates water stable and gives them improved resistance to destructive forces such as water erosion.

* Soil organic matter helps move tightly bound and normally insoluble metals by forming water-soluble metal-organic matter complexes. It helps to chelate micronutrients and helps create spodic horizons in some soils, so it contributes to forming diagnostic soil horizons in soil taxonomy.

* Soil organic matter contributes to the water-holding capacity of soil-directly because it holds five times more water by weight than clay minerals and indirectly by its contribution to developing soil structure and porosity.

* Soil organic matter affects soil temperature directly and indirectly. Soil organic matter, mulch, and litter at the soil surface buffer the soil from extreme temperature changes, keeping soils cooler in summer and warmer in winter. The dark color of organic matter in soil adsorbs more solar radiation than lighter-colored soils with lower SOM contents; this leads to warmer soils. Indirectly, because SOM has a high water-holding capacity, it buffers the soil from temperature changes because the water has a very high heat capacity.

* Soil organic matter provides carbon and energy for the growth of heterotrophic microorganisms in soil and larger organisms such as earthworms.

* Organic acids in SOM contribute to mineral weathering.


1. How does SOM contribute to plant and microbial nutrition?

2. What is a dark-colored soil likely to mean?

3. How does SOM contribute to improved soil structure?

4. What are direct and indirect interactions of SOM and water?

5. Why are pesticide spills less damaging in soils with high SOM?


There are many steps in SOM formation. As noted before, SOM develops during simultaneous decomposition of nonhumic organic carbon entering soil, and formation of the decomposition products into humic materials such as humic acid and fulvic acid. The overall process is illustrated in Figure 11-6.


Peptides, amino acids, and aromatic compounds from lignin decomposition are the key ingredients that form the humic materials. Ultimately this results in the structures illustrated in Figure 11-7. It would be misleading to imply that humic material is small, linear and two-dimensional as Figure 11-7 suggests. Humic materials are large, shapeless, three-dimensional structures coating soil minerals. You can get a sense of the true nature of SOM from examining the complexity of the compound illustrated in Figure 11-8.


The complexity of SOM
gives it some of its beneficial

It is the complexity of humic materials, the bulk of SOM, that give them some of their most beneficial properties. As you can see from Figure 11-8, the random polymerization of peptides and sugars into humic acids traps important plant nutrients like N and S in soil, which is why SOM is considered a great storehouse of plant nutrients. You can also see the humic materials binding silicate clay in Figure 11-8. This binding is the first step in developing soil aggregation and also in creating stable aggregates. Finally, because the material is complex, it resists decomposition. So the beneficial effects of SOM can be long lasting. However, SOM will decompose over time, and that is the subject of the next section.


1. Why does SOM formation require simultaneous decomposition and constructive processes?

2. What are the key building blocks of humic materials in soil?

3. What are some of the reactive groups in humic materials?

4. What are unique characteristics of humic materials that contribute to their beneficial functions?

5. How does SOM contribute to soil structure?

SOM forms active and
passive pools that differ
in terms of how readily
they decompose.

Soil organic matter decomposition is perhaps the major contributor to nutrient availability in soil, so it is vitally important to have a sense of how rapidly it decomposes. Soil scientists typically think of SOM in terms of active and passive SOM pools. The active pool is the fraction of soil SOM that is readily available (or labile) for microbial growth and activity. Fresh residues and soluble materials fall into this category, and their decomposition is quite rapid, occurring within days or weeks. This pool is characterized by materials that have simple structures and low C:N ratios. Low C:N ratios are important because they mean there is adequate N in the material to support growth of the microbial community on the available C; decomposition will readily occur and decomposition will not tie up or immobilize inorganic N in soil, preventing it from use by plants.

The passive pool of SOM resists decomposition because it is either physically or chemically protected. Physical protection means that during soil aggregate formation the SOM (either labile or nonlabile) is trapped in the developing aggregate and shielded from microbial attack, either because it is armored in clay minerals or, more likely, because it is located in pores too small to permit microbial access. Chemically protected SOM, however, is material that resists decomposition because of its chemical composition and high C:N ratio. Humin and humic acids fall in this category. They are large, complex, insoluble materials that are both physically and chemically difficult to degrade. The relative turnover times of the various SOM fractions are illustrated in Figure 11-9.
About 2 to 5 percent of
SOM decomposes yearly.

All told, about 2 to 5 percent of the SOM decomposes on a yearly basis. This varies from environment to environment, with warmer and drier soils having greater SOM decomposition rates than cooler and wetter soils (Table 11-2).



1. What are the active, protected, and passive components of SOM?

2. What mechanisms protect SOM from decomposition in soil?

3. Why do the different fractions of SOM decompose at different rates?

4. What are approximate turnover times for different pools of SOM?


If SOM is so important, then one of the central questions you have to ask yourself is where it is most likely to be found. Furthermore, if SOM accumulation is beneficial to soil function, then an equally important question is how quickly it forms, and whether there is anything that can accelerate that accumulation.

Amounts of Soil Organic Matter

The surface horizons of most well-drained mineral soils contain anywhere from 1 to 6 percent SOM depending on the climate and their management history. The O horizon of forest soils and poorly drained soils, such as Histosols, will frequently have higher SOM contents, ranging up to 20 to 30 percent C.

Distribution by Depth
Most SOM is in the upper
soil surface.

Most of the SOM is in the surface horizons of soil and progressively and sometimes rapidly decreases with soil depth. as Figure 11-10 illustrates. Spodosols, soils that form beneath cool, wet, coniferous forests, have a unique feature called a spodic horizon in which SOM increases. This horizon is formed by leaching of soluble humic compounds from the soil surface.

Some soils have buried A horizons, caused when wind- or water-borne soil is deposited over an existing soil. In this case, there is also an increase in SOM content at lower depths.

Effects of Soil-Forming Factors

You are already familiar with the five soil-forming factors. How does each of them, in turn, affect SOM accumulation?


Organisms (Vegetation)

The type of vegetation, or lack thereof, has a tremendous influence on SOM content. The extremes run from swamps and marshes, which contain soils that are almost entirely organic matter, to warm and cold deserts that have little if any organic matter. Table 11-3 illustrates how SOM content varies depending on the different plant communities present in the ecosystem.

SOM accumulates in cool
and moist environments.

Climate naturally influences the type of vegetation that will be present in an environment. The climate, particularly temperature and annual precipitation, also has a significant effect on SOM contents through the dual and opposing processes of residue return to soil and SOM decomposition. At low temperatures and high precipitation SOM accumulates faster than it decomposes. Even though plant productivity is lower in colder environments, for much of the year the soils may be frozen, thus preserving the SOM content. This is one of the reasons why tundra and alpine regions (see Table 11-3) have such high SOM contents. However, if you compare temperate and tropical grasslands in terms of SOM content, tropical grasslands typically have much lower SOM contents, in part because they are hotter on a year-round basis, which promotes decomposition (Figure 11-11). Deforestation in tropical rainforests is particularly harmful because without the extremely high productivity of the overlying forest returning residues to soil, the high temperatures and rainfall in the tropics lead to extremely rapid loss of SOM from the underlying soil. As the temperature rises much above 35[degrees]C, SOM decomposition rates exceed organic residue deposition and SOM content decreases.


Topography and Aspect (Relief)
SOM accumulation
is influenced by
relief, and aspect.

Water, soil, and SOM are subject to gravity, so it is not surprising that topography has a significant effect on SOM deposition in the environment. Soils on steep slopes have greater erosion and more water runoff than soils on gentler slopes. Consequently, they tend to have lower SOM contents. Soils in slope positions that receive colluvial deposits tend to be wetter and have higher SOM contents. The distribution of SOM in a typical catena is illustrated in Figure 11-12. Convex positions on a slope tend to shed water and are drier than concave positions, which tend to accumulate water and consequently tend to have slightly higher SOM contents. A depression is a good example of a concave position in soil. Depressions modify the microenvironment of the soil by accumulating water, and so will tend to accumulate SOM.

Soil aspect is also important in determining the SOM content. Slopes that receive sunlight will tend to be several degrees warmer than opposing slopes that receive less sunlight. In the northern hemisphere, this means that south-facing slopes will be warmer and drier than north-facing slopes and consequently have less SOM. This is particularly applicable to forest soils. The cause is twofold. First, there is the increase in SOM decomposition due to higher temperature on south-facing slopes. Second, there is less water available in south-facing slopes because of the higher temperatures. This causes differences in the plant community, which in turn influence the amount and availability of the litter that is deposited.



Parent Material
Clay helps to preserve SOM
by forming crypts.

Parent materials that provide adequate nutrients for plant growth will tend to promote higher SOM contents. Parent materials will also influence the dominant texture of a soil. So, parent materials that lead to sandy soil formation will have relatively lower SOM contents than soils producing silty and clayey soils. Sandy soils are better drained and drier than other soils; they have fewer-plant available nutrients and typically support vegetation such as trees, which do not contribute as much available residue to SOM as grasses. For similar reasons parent material that produces finer-textured soils will promote SOM accumulation.

Clays play another important role in preserving SOM. The reactive groups of SOM, which are among the first groups subject to biological decomposition, are attracted to the charged groups on clay particles. This helps protect the SOM from decomposition. Furthermore, clay/OM interactions will lead to aggregation, which in turn physically protects SOM from decomposition. Consequently there is usually a positive relationship between the SOM content and the percentage of silt and clay in a soil (Figure 11-13).


How fast does SOM accumulate? It may take hundreds to thousands of years for the SOM to accumulate to relatively steady-state conditions in a mature soil. Chronological sequences in various soils suggest that the rates range from 1 to 12 g [m.sup.-2] per year (Schlesinger, 1997) depending on the environment. The SOM accumulation rates can be significantly higher, however, particularly in the first decades after the effects of human influence are removed. Table 11-4 illustrates the profound recovery of SOM content in abandoned soils with time.


Human Activities
Cultivation helps to
accelerate SOM

Cultivation has a dramatic effect on SOM (Figure 11-14). From a geological perspective, human activity can remove in an instant the SOM that had taken millennia to produce. With continuous cultivation more than half of the C in soil can be lost in a short period.

In the long-term Morrow plots at the University of Illinois, over 125 years of continuous corn cultivation has reduced the SOM content by more than 60 percent. The SOM content decreases for several reasons. Cultivation aerates the soil briefly, which stimulates aerobic decomposition. It buries residue in the soil, which puts organic matter in closer proximity to decomposing microbes. Generally speaking, cultivated crops do not add as much organic residues to soil as do perennial grasses, and in many cases little residue is returned at all because it is harvested for forage or burned to facilitate planting. Tillage also serves to break up aggregates that have formed, and to some extent expose labile but physically protected soil C to microbial decomposition. Nevertheless, the SOM content in cultivated soils does not decline to zero regardless of the length of cultivation. Why? Because there remains the large pool of passive SOM, which is resistant to decomposition.


As a rough guide, approximately 5,000-8,000 Mg [ha.sup.-1] of residue are required to maintain the SOM of a soil once it has gone into cultivation (Larson et al., 1978). It is also possible to increase SOM content, though not usually to the levels previously found in undisturbed soil. Studies have shown an approximately linear increase in SOM content with increase in crop residues applied to soil (Figure 11-15).
Subsidence due to loss
of SOM is a major
problem in organic
soils that are drained
and aerated.

Drainage also has a dramatic effect on SOM content. In organic soils that contain 20 to 30 percent organic matter, SOM decomposition is retarded by wet conditions, and organic matter tends to accumulate. Once the soils are drained and aerated, decomposition of the accumulated SOM occurs at a much faster rate than its accumulation and a process called subsidence occurs. Subsidence is nothing more than a rapid decrease in the soil depth, sometimes as much as 2.5 to 5 cm (1 to 2 inches) per year. It is a major problem in organic soils in Florida, where subsidence has decreased the soil depth more than 3 m (10 ft) in the past 100 years in some locations.


1. How is most SOM distributed in soil?

2. How does topography influence the distribution of SOM?

3. How quickly does SOM accumulate in undisturbed soil?

4. Why does the SOM content of soil never go to zero?

5. What is subsidence and where does it occur?


Soil organic matter is one of the critical components contributing to a soil's ability to carry out necessary environmental functions such as supplying plant nutrients and facilitating soil aggregation. The SOM is composed of nonhumic and humic materials, basically representing plant and microbial residues that are in the process of decay, and organic materials that are unrecognizable and are in the process of polymerizing into compounds like humic and fulvic acid. One of the key ingredients in developing SOM is the polymerization of lignin and lignin decomposition products with peptides. The SOM can also be divided into active and passive fractions, which represents the SOM that is readily decomposed and that which is physically and chemically protected. Many soil-forming factors influence SOM distribution, and quantity in soil and human activity can both reduce and build up SOM content in soil.


1. What are major ecological roles of soil to which SOM contributes?

2. What percent of SOM is typically composed of humic materials?

3. Give some examples of typical plant materials that enter soil as residues.

4. What is lignin and what are the major components of lignin?

5. Diagram how lignin is involved in SOM formation.

6. Give two or three specific reasons why SOM is important to preserve.

7. Distinguish between humic and fulvic acids.

8. What is the relative difference in decomposition rate between the active and passive fractions of SOM?

9. What is the difference between chemical and physical protection of SOM?

10. How can human activity accelerate SOM decomposition?


The Plowman's Folly by Edward E. Faulkner (1977, Washington, DC: Island Press, gives an interesting alternative perspective on how conventional agricultural activities have contributed to the loss of SOM and consequently the loss of soil fertility and soil quality. For a more technical treatment of soil C and soil C cycles there are two important references that are invaluable: Cycles of Soil, 2nd ed., by F. J. Stevenson and M. A. Cole (1999, New York: John Wiley & Sons), which gives a thorough biochemical treatment of SOM, and Biogeochemistry: An Analysis of Global Change, 2nd ed., by W. H. Schlesinger (1997, San Diego, CA: Academic Press), which treats SOM in the context of soil as an ecosystem.


Daily, G., P. A. Matson, and P. M. Vitousek. 1997. Ecosystem services supplied by soil. In G. C. Daily (ed.), Nature's services: Societal dependence on natural ecosystems. Washington, DC: Island Press, pp. 113-132.

Larson, W. E., R. F. Holt, and C. W. Carlson. 1978. Residues for soil conservation. In W. R. Oschwald (ed.), Crop residue management systems. Spec. Pub. 31. Madison, WI: American Society of Agronomy.

Raich, J. W., and W. H. Schlesinger. 1992. The global carbon dioxide in soil respiration and its relationship to vegetation and climate. Tellus 4413: 81-99.

Schlesinger, W. H. 1997. Biogeochemistry: An analysis of global change. San Diego, CA: Academic Press.

Troeh, F. R., and L. M. Thompson. 1993. Soils and soil fertility. New York: Oxford University Press.


Extracting fulvic and humic acids from soil is one of the easiest procedures a soil scientist can perform. It requires a strong base, a strong acid, and a little patience. The procedure is diagrammed below.

The humic acid can be collected by centrifugation after settling for twenty-four hours. The fulvic acids still in solution, along with other soluble compounds such as sugars and small polysaccharides, can be collected by freeze-drying. The procedure should be conducted in a nitrogen gas atmosphere to minimize the oxidation that takes place. Even so, it is impossible to extract humic and fulvic acids from soils without in some way changing the chemical and certainly the physical composition of the material.

TABLE 11-1 Average contents of organic materials in a
temperate grassland soil. (Adapted from Troeh and Thompson, 1993)

Organic Component            kg/ha       %

Plant roots                  16,800     8.4
Living macrofauna             1,826     1.0
Identifiable dead remains     4,480     2.2
Living microbial biomass     8,008      4.0
Humus                       168,000    84.4

TABLE 11-2 Turnover time of soil C in various ecosystems.
(Adapted from Raich and Schlesinger, 1992)

                  Soil C    Soil Respiration   Turnover Time
Vegetation Type   (Mg/ha)       (Mg/ha)           (years)

Swamps and
  marshes           723           2.0               520
Tundra              204           0.6               490
Boreal forest       206           3.2               91
  grassland         189           4.4               61
  forest            287           10.9              38
Desert scrub        58            2.2               37
  forest            134           6.6               29
Cultivated soil     79            5.4               21
  grassland         42            6.3               10

TABLE 11-3 Distribution of soil organic matter in various
ecosystems. (Adapted from Schlesinger, 1997)

                                 Mean Soil
                                   Matter     Total World
Ecosystem Type                    Content     Soil Organic
                                   (kg C/      C(Mg C x
                                 [m.sup.2])   [10.sup.9])
  Tropical forests                  10.4          255
  Temperate forests                 11.8          142
  Boreal forests                    14.9          179
  Woodlands and shrublands          6.9            59
  Tropical savannas                 3.7            56
  Temperate grasslands              19.2          173
Cultivated                          12.7          178
Tundra and alpine                   21.6          173
  Desert scrub                      5.6           101
  Extreme deserts, rocks, ice       0.1            3
Swamps and marshes                  68.6          137

TABLE 11-4 SOM accumulation in abandoned agricultural
and disturbed soils. (Adapted from Schlesinger, 1997)

                       Previous      Since
Ecosystem              Land Use      Abandonment

Subtropical forest     Agriculture            40
Temperate deciduous    Agriculture           100
  forest               Mining                 50
Temperate coniferous   Agriculture            50
  forest               Diked                 100
Temperate grassland    Agriculture             5
                       Agriculture            53
                       Mining             28-40

                       Rate of
                       (g C [m.sup.-2]
Ecosystem              [yr.sup.-1])

Subtropical forest          30-50
Temperate deciduous             45
  forest                        55
Temperate coniferous        21-26
  forest                        26
Temperate grassland            110

Figure 11-1 Contribution of different organic fractions to
SOM. (Adapted from Stevenson and Cole, 1999)

Nonhumic Substances

Lipids               (1-6%)
Carbohydrates        (5-25%)
Amino Acids          (9-16%)

Humic Substances

Up to 80%

Note: Table made from pie chart.

FIGURE 11-2 Typical composition of green plant residues.
(Adapted from Stevenson and Cole, 1999)

Sugars and Starches    (5%)
Protein                (8%)
Hemicellulose         (18%)
Lignins               (20%)
Cellulose             (45%)
Fats and Waxes         (2%)
Polyphenols            (2%)

Note: Table made from pie chart.
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Title Annotation:Section 4 Soil Biological and Biochemical Properties
Publication:Fundamental Soil Science
Date:Jan 1, 2006
Previous Article:Chapter 10 Soil biogeochemical cycles.
Next Article:Chapter 12 Physical management.

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