SUMMARY
Soil organic matter (SOM) and specifically soil organic carbon (SOC) are known to play important roles in the maintenance as well as improvement of many soil properties. While agriculture is the area most concerned with key functions and critical levels of SOC, forestry and grazing as well as groundwater contamination and C sequestration are areas where knowledge about the functions of SOC is vital. This literature review aims to provide a comprehensive assessment of the current state of knowledge of the functions of SOC and its effect on the physical, chemical and biological properties of soil. Although these properties are discussed separately, it is important to emphasize the dynamic and interactive nature of the soil system and that changes in one property will likely affect other soil properties as well. Thus, functions of SOC almost always affect several different properties and engage in multiple reactions. We also highlighted that total SOC is often not a good indicator for assessing soil properties. Frequently, such properties are affected by specific pools with particular properties. Only by studying these pools separately and in conjunction with a specific function is it possible to understand what the key impacts of a SOC pool are.
Soil productivity is, therefore, determined by a combination of organic matter's influence on physical, chemical, and biological soil properties. It has been shown that the incorporation of crop residues into soil is beneficial to soils, improving one or more essential soil attributes.
CHAPTER ONE
INTRODUCTION
The soil consist of organic matter which is the organic matter component of soil, consisting of plant and animal residues at various stages of decomposition, cells and tissues of soil organisms, and substances synthesized by soil organisms (Baldock, JA and Nelson, PN. 2000). Soil organic matter (SOM) is the foundation for productive soil. It promotes healthy crops, supplies resources for microbes and other soil organisms, and regulates the supply of water, air and nutrients to plants (Wang, X.L., Sato, T., Xing, B.S., 2006). SOM can deliver over half of the nitrogen and a quarter of the phosphorous crops required, thus strongly influencing fertilizer requirements.
Forms and classification of soil organic matter have been described by Tate (1987) and Theng (1987). For practical purposes, organic matter may be divided into aboveground and belowground fractions. Aboveground organic matter comprises plant residues and animal residues; belowground organic matter consists of living soil fauna and micro-flora, partially decomposed plant and animal residues, and humic substances.
It is now widely recognized that SOC plays an important role in soil biological (provision of substrate and nutrients for microbes), chemical (buffering and pH changes) and physical (stabilization of soil structure) properties. In fact, these properties, along with SOC, N and P, are considered critical indicators for the health and quality of the soil. Since Lal’s (1993) initial definition of soil quality as the capacity of soil to produce economic goods and services and to regulate the environment, the term “soil quality”has been refined and expanded by scientists and policy makers to include its importance as an environmental buffer, in protecting watersheds and groundwater from agricultural chemicals and municipal wastes and sequestering carbon that would otherwise contribute to a rise in greenhouse gases and global climate change (Reeves, 1997). Doran and Parkin (1994) and Doran and Safley (1997) initially distinguished between“soil quality’’ and“soil health”before inclusively using the term“soil health”and defining it as “the continued capacity of soil to function as a vital living system, within ecosystem and land-use boundaries, to sustain biological productivity, promote the quality of air and water environments, and maintain plant, animal and human health”. However, the general perception of a healthy or high-quality soil is one that adequately performs functions, which are important to humans, such as providing a medium for plant growth and biological activity, regulating and partitioning water flow and storage in the environment and serving as an environmental buffer in the formation and destruction of environmentally hazardous compounds. In particular, the suitability of soil for sustaining plant growth and biological activity is a function of physical (porosity, water holding capacity, structure and tilth) and chemical properties (nutrient supply capability, pH, salt content), many of which are a function of SOM content (Doran and Safley, 1997). In general, increases in SOM are seen as desirable by many farmers as higher levels are viewed as being directly related to better plant nutrition, ease of cultivation, penetration and seedbed preparation, greater aggregate stability, reduced bulk density, improved water holding capacity, enhanced porosity and earlier warming in spring (Carter and Stewart, 1996; Lal, 2002).
FIGURE 1
Thus the objective of this work is to show the soil organic matter contributes to soil health by its effects on soil properties [physical, chemical and biological].
CHAPTER TWO
LITERATURE REVIEW
2.1 ORGANIC MATTER
Soil organic matter is any material produced originally by living organisms (plant or animal) that is returned to the soil and goes through the decomposition process (FIG 2). At any given time, it consists of a range of materials from the intact original tissues of plants and animals to the substantially decomposed mixture of materials known as humus (FIG 3). Most soil organic matter originates from plant tissue. Plant residues contain 60–90 percent moisture. The remaining dry matter consists of carbon (C), oxygen, hydrogen (H) and small amounts of sulphur (S), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg). Although present in small amounts, these nutrients are very important from the viewpoint of soil fertility management. Soil organic matter consists of a variety of components. These include, in varying proportions and many intermediate stages, an active organic fraction including microorganisms (10–40 percent), and resistant or stable organic matter (40–60 percent), also referred to as humus.
[Franzluebbers, A. J. 2010.] The amount and type of organic matter varies from soil to soil, but generally SOM can be divided up into three pools with different turnover times. SOM management should focus on strategies that build all three pools. This is the key to simultaneously building SOM and deriving benefits from its decomposition, including nutrient turnover, aggregate formation and water storage [Abrishamkesh S, Gorji M, and Asadi H, 2011].
SOIL ORGANIC MATTER (SOM) POOLS
Active SOM: The active SOM pool has a turnover time of months to years, and it includes constituents such as soil microorganisms that are involved in even faster turnover times. It is primarily composed of recent plant residues that are in the early stages of decomposition and of soil organisms. This active SOM pool is very important for nutrient release, and helps develop a soil’s slow SOM pool [FIG 4]. Slow SOM: The slow SOM pool has a turnover time that varies from years to decades. A soil’s physical condition and nutrient buffering capacity are both strongly influenced by this slow turnover pool of organic matter. Different from the stable SOM pool, the slow pool is also a source of nutrients including nitrogen and phosphorous. It includes decomposed materials, residues and microbial products that are protected through physical (for example, interior of soil aggregates) and biochemical processes [FIG 4].
Stable SOM: The stable SOM pool consists of material that is hundreds to thousands of years old. Sometimes this pool is called the passive pool, or humus. It is a highly recalcitrant pool (resistant to decomposition) that influences the CEC of the soil, and is important in soil physical processes such as aggregation. The amount of stable SOM is not strongly influenced by recent management practices and tends to increase with increasing clay concentration in the soil. However, management practices such as residue removal, burning, tillage and cover-cropping can have long-term effects on the relative and absolute amounts of stable SOM [FIG4]
2.2 EFFECTS OF ORGANIC MATTER ON SOIL PHYSICAL PROPERTIES
1] SOIL STRUCTURE AND AGGREGATE STABILITY
Soil structural stability refers to the resistance of soil to structural rearrangement of pores and particles when exposed to different stresses (e.g. cultivation, trampling/compaction, and irrigation). The interrelationship between SOC and soil structure and other physical properties has been extensively studied, and excellent reviews can be found in Tisdall and Oades (1982), Oades (1984), and Carter and Stewart (1996). It is well established that addition of SOM can not only reduce bulk density (Db) and increase water holding capacity, but also effectively increase soil aggregate stability. Angers and Carter (1996) noted that the amount of water-stable aggregates (WSA) was often associated with SOC content, and that particularly labile carbon was often positively related to macro-aggregate stability. Kay and Angers (1999) reported that a minimum of 2% SOC was necessary to maintain structural stability and observed that if SOC content was between 1.2-1.5%, stability declined rapidly. Boix-Fayos et al. (2003) showed that a threshold of 3-3.5% SOC had to be attained to achieve increases in aggregate stability; no effects on aggregate stability were observed in soils below this threshold. Haynes (2000) found that the mean weight diameter (MWD) of aggregates exhibited a curvilinear increase with carbon content, suggesting an upper limit of influence of SOC (FIG]
Carter (1992) found that maximum levels for an agronomically designed aggregation index (AI) were obtained at SOC contents of > 2.5% and at microbial biomass carbon contents of 250µg C/g soil, whereas maximum soil structural stability (determined by MWD) was found at SOC levels of 4.5%. Carter (1992) suggested that 2.5% could serve as a critical limit to define minimum concentrations of SOC required to provide optimum structural stability in fine sandy loams.
In fact, most studies report a linear increase of aggregate stability and aggregate size with increasing levels of SOM or SOC. While many studies agree on a positive relationship between aggregate stability and SOM, there is no agreement whether a defined threshold value exists for organic carbon levels, and Loveland and Webb (2003) concluded, after a review of several studies, that no universal threshold levels of SOC contents could be established.
The concept of aggregation as a process involving different organic binding agents at different scales was pioneered by Tisdall and Oades (1982) and based on their work, Oades and Waters (1991) introduced the concept of aggregate hierarchy. Large aggregates (>2000µm) were hypothesised to be held together by a fine network of roots and hyphae in soils with high SOC content (>2%), while 20-250µm aggregates consist of 2-20µm particles, bonded together by various organic and inorganic cements. Water stable aggregates of 2-20µm size, in turn, consist of <2µm particles, which are an association of living and dead bacterial cells and clay particles. The concept of aggregate hierarchy suggests that organic matter controls aggregate stability, and degradation of large (relatively unstable) aggregates creates smaller, more stable aggregates. Stabilisation of macro-aggregates occurs mainly via binding by fungal hyphae and roots. Particulate organic matter, on the other hand, serves as a substrate for microbial activity, resulting in the production of microbial bonding materials for micro-aggregates and for the encrustation of plant fragments by mineral particles. In this model, three principal organic binding agents are involved in the aggregate formation and stabilisation: transient, temporary and persistent organic matter. Transient organic binding agents are rapidly decomposed by micro-organisms and are thought to be mostly composed of glucose-like components (mono and polysaccharides), effectively lasting only for a period of a few weeks, after which their effect diminishes. Temporary organic binding agents are thought to consist of roots and hyphae and may persist for months and years. Persistent organic binding agents are composed of degraded humic materials mixed with amorphous forms of Fe and Al and Al-silicates. Tisdall and Oades (1982) proposed that the‘fresh’or ‘active’ part of SOM (consisting of mono- and polysaccharides, exudates from roots and fungal hyphae) was largely responsible for stabilisation of aggregates. They attributed the key aspect of aggregate formation by polysaccharides to the presence of functional groups, which upon deprotonation, become negatively charged and interact with positively charged oxides, producing stable organic-inorganic microstructures (Oades et al., 1989). However, they found that due to the variability of organic matter, the strength and time for formation of aggregates varied. For example, glucose-like components acted strongly in aggregate formation for the first 2-3 weeks of the experiment after which the effect declined. By comparison, cellulose showed the maximum effect after 6-9 months but was never as effective as glucose. Ryegrass residues were most effective after 3 months and maintained the effect for another 4-6 months, after which the effect declined (Oades et al., 1989). Based on these data, it is apparent that a specific group or groups of organic matter are key agents for aggregate formation and maintenance of structural stability in soils and Puget et al. (1995) stated that the type of organic matter was more critical to structural stability than the net amount of organic matter. This was further substantiated by observations from Haynes and Swift (1990), Haynes et al. (1991) and Angers and Carter (1996) who observed that after conversion from arable crops to pasture, stability of aggregates changed more rapidly than overall soil organic matter content (Fig. 13). However, there is no general agreement as to the type of organic matter essential for aggregation. This is most likely due to the fact that different types of organic matter perform different functions at different times during the aggregate formation and conservation process. In fact, Kay and Angers (1999) suggested that most or all SOC fractions were involved to different degrees in aggregate formation and stabilization.
2] WATER HOLDING CAPACITY
An important indicator of soil physical fertility is the capacity of soil to store and supply water and air for plant growth. The ability of soil to retain water is termed water holding capacity (WHC). In particular, the amount of plant-available water in relation to air-filled porosity at field capacity is often used to assess soil physical fertility (Peverill et al, 1999). Total plant available water (PAW) is the amount of water held between the wettest drained condition (field capacity FC, at matric suction of -10 kPa) and the water content at which plants are unable to extract water (permanent wilting point PWP, at matric suction of -1500 kPa). However, some studies use -10 kPa for coarse textured soils only and use -33 kPa for fine-textured soils (Bauer and Black, 1992). WHC of soils is controlled primarily by the number of pores and pore-size distribution of soils, and by the specific surface area of soils. In turn, this means that with an increase in SOC content, there is increased aggregation and decreased Db, which tend to increase the total pore space as well as the number of small pore sizes (e.g. Khaleel et al., 1981; Haynes and Naidu, 1998). These relationships highlight the interconnectivity between soil structure, Db and WHC. The effect of organic carbon on the WHC of soil is generally assumed to be positive but the types of carbon responsible for this effect and synergistic behaviour with other soil properties is not well understood. For example, de Jong (1983) and Haynes and Naidu (1998) found an increase in water content with increasing SOC content and Wolf and Snyder (2003) stated that an increase of 1% SOM can add 1.5% additional moisture by volume at FC. Emerson and McGary (2003) showed that per gram of additional carbon at -10 kPa suction, a 50% increase in water content was achieved (FIG). They suggest that the organic carbon from exudates [‘gel’) from ectotrophic mycrorhiza would bond soil particles, which would result in a change in the size of the pores and a change in water retention at -10 kPa.
3] SOIL COLOUR
Soil colour is often used as the highest categorical level in many soil classification systems, e.g. the concept of the Russian chernozem was centered around the thick dark soils of the Russian steppe and the Mollisol order of the US soil taxonomy is specifically defined to include most soils with relatively thick, dark surface horizons (Schulze et al., 1993). Generally good soil conditions are associated with dark brown colours near the soil surface, which is associated with relatively high organic matter levels, good soil aggregation and high nutrient levels (Peverill et al. 1999).
The effect of usually dark brown or black SOM on soil colour is important not only for soil classification purposes, but also for ensuring good thermal properties, which in turn contribute to soil warming and promote biological processes (Baldock and Nelson, 1999). Only about 10% of the solar energy reaching the earth’s surface is actually absorbed by the soil, which can be in turn used to warm the soil. Naturally, dark-coloured soils absorb more energy than light-coloured ones. However, this does not imply that dark-coloured soils are always warmer: since dark coloured soils usually have a higher amount of organic matter, which holds comparatively larger amounts of water, a greater amount of energy is required to warm darker soils than lighter coloured ones (Brady, 1990). Thus, the thermal property of soil is to a large degree influenced by water content, Db, soil texture (fine versus coarse) and soil colour. In addition, the surface cover of soil affects the heat transfer in and out of a soil, as bare soils warm up and cool off more quickly than those with a vegetation or mulch cover.
2.3 EFFECTS OF ORGANIC MATTER ON SOIL CHEMICAL PROPERTIES
Many important chemical properties of soil organic matter result from the weak acid nature of humus. The ability of organic matter to retain cations for plant use while protecting them from leaching, i.e. the cation exchange capacity (CEC) of the organic matter, is due to the negative charges created as hydrogen (H) is removed from weak acids during neutralization.
Many acid-forming reactions occur continually in soils. Some of these acids are produced as a result of organic matter decomposition by micro-organisms, secretion by roots, or oxidation of inorganic substances. Commonly used N fertilizers through microbial conversion of ammonium fertilizers, such as urea, and ammonium phosphates, such as mono-ammonium and di-ammonium phosphate, are converted rapidly into nitrate through a nitrification process, releasing acids in the process and thus increasing the acidity of the topsoil (FIG).
When acids or bases are added to the soil, organic matter reduces or buffers the change in pH. This is why it takes tonnes of limestone to increase the pH of a soil significantly compared with what would be needed to simply neutralize the free H present in the soil solution. All of the free hydrogen ions in the water in a very strongly acid soil (pH 4) could be neutralized with less than 6 kg of limestone per hectare. However, from 5 to more than 24 tonnes of limestone per hectare would be needed to neutralize enough acidity in that soil to enable acid-sensitive crops to grow. Almost all of the acid that must be neutralized to increase soil pH is in organic acids, or associated with aluminium (Al) where the pH is very low.
However, with large values of soil organic matter, the pH will decrease less rapidly and the field will have to be limed less frequently. A lime application of 1–2 tonnes/ha every 2–3 years might be sufficient to regulate the acidity.
Organic matter may provide nearly all of the CEC and pH buffering in soils low in clay or containing clays with low CEC. In comparing conventional and conservation tillage in Brazil, the highest values of soil CEC and exchangeable calcium (Ca) and magnesium (Mg) were found in legume-based rotation systems with the highest organic matter content (FIG).
In particular, systems with pigeon peas and lablab resulted in a 70-percent increase in CEC compared with a fallow/maize system.
2.4 EFFECTS OF ORGANIC MATTER ON SOIL BIOLOGICAL PROPERTIES
The biological effects of SOM are primarily to provide a reservoir of metabolic energy that drives biological processes (Baldock and Nelson (1999) stressed that one of the most fundamental functions of SOM was the provision of metabolic energy which drives soil biological processes. In essence, it is the transformation of carbon by plant, micro- and macro-biological processes that provides energy and results in the establishment of a cycle, that connects above- and belowground energy transformations (FIG)] , To act as a supply of macro-and micro-nutrients and to ensure that both energy and nutrients are stored and released in a sustainable manner[Most of the nutrients in SOM are derived from the mineralization of SOM and become available for plant uptake during decomposition and for this reason, the particulate organic matter fraction is often considered the most important proportion of SOM in providing nutrients to plants (Wolf and Snyder, 2003)]. Importantly, biological processes in turn influence both soil chemical and soil structural properties as they greatly affect soil structure and soil redox reactions.
Soil micro-organisms are of great importance for plant nutrition as they interact directly in the bio-chemical cycles of the nutrients. Increased production of green manure or crop biomass aboveground and belowground increases the food source for the microbial population in the soil. Agricultural production systems in which residues are left on the soil surface and roots left in the soil, e.g. through direct seeding and the use of cover crops, therefore stimulate the development and activity of soil micro-organisms. The roots of most plants are infected with mycorrhizae, fungi that form a network of mycelia or threads on the roots and extend the surface area of the roots. In undisturbed soil ecosystems, e.g. in conservation agriculture, colonization with mycorrhizal fungi increases strongly with time compared with colonization under natural vegetation (Figure). Fine roots are the primary sites of mycorrhizal development as they are the most active site for nutrient uptake. This partly explains the increase in mycorrhizal colonization under undisturbed situations as rooting conditions are far better than under conventional tillage. Other factors that might stimulate mycorrhizal development are the increase in organic carbon (C) and the rotation of crops with cover crop/green manure species.
Another consequence of increased organic matter content on biological properties is an increase in the earthworm population. Earthworms rarely come to the soil surface because of their characteristics: photophobia, lack of pigmentation and tolerance to periods of submergence and anaerobic conditions during rainfall. Soil moisture is one of the most important factors that determine the presence of earthworms in the soil.Through cover crops and crop residues, evaporation is reduced and organic matter in the soil is increased, which in turn can hold more water. Residues on the soil surface induce earthworms to come to the surface in order to incorporate the residues in the soil. The burrowing activity of earthworms creates channels for air and water; this has an important effect on oxygen diffusion in the root-zone, and the drainage of water from it. Furthermore, nutrients and amendments can be distributed easily and the root system can develop.
Resilience has been defined by Baldock and Nelson (1999) as the capacity of an ecosystem to return to its initial state after disturbance. In that respect, resilience is a soil property and an indicator of how well a system is able to recover. Together with soil resistance (the inherent capacity to withstand disturbance) it ultimately defines the stability of soil. Nannipieri et al.(2003) found that soils with a greater microbial diversity were more resistant and resilient to perturbations than soils with less diverse communities. Griffiths et al. (2000) found that soils fumigated with chloroform were more resistant and resilient to conditions such as heating if they had a high microbial diversity. Degens et al. (2001) noted that an arable soil was less resistant to microbial cell stresses and other disturbances, compared with a pasture soil. Since the SOC content, CEC and microbial biomass were also greater in the pasture soil, these factors were suggested to have increased the resistance of soil microorganisms to stresses and disturbances.
These results indicate that the resilience of a soil is really a measure of the functionality of the whole ecosystem. Therefore it is governed by the adequate performance of physical, biological and chemical functions, which in turn is to a large extent determined by the SOM content and its chemical composition. In essence, resilience of a soil is a measure that provides a conclusive analysis as to how well the different functions of a soil are carried out and the ability of the ecosystem to maximise the utilisation of the SOM resource.
CHAPTER THREE
CONCLUSION
Soil organic matter encompasses the soil biota, and plant and animal tissues at varying stages of decomposition, which can be separated into different pools and these pools are specific in the way that they affect soil properties [physical, chemical and biological].
Soil organic matter has a lot of effects[which is mostly positive] on soil properties being that it betters the physical properties by impacting on the soil structure and texture which entails water holding capacity, aggregation and colour. It impacts on the chemical by acting upon the ph, buffer capacity and CEC. It impacts on the biological by acting as a nutrient supplier, source of energy for various reactions in the soil, improves the resilience of the soil.
All these effects on soil properties [physical, chemical and biological] together with other effects on the soil which include increase in proliferation of micro-organism, increase in available forms of nutrients to plants, etc leads to an increase in soil fertility.
This makes organic matter an important aspect of Agriculture.
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