Soil Organic Matter (SOM)
Soils represent a major pool (172 x 1010 t) in the cycling of C from the atmosphere to the biosphere and are the habitat for terrestrial photosynthetic organisms, which fix 11 x 10 10 t C per year, about half of which eventually finds its way into soils. Organic matter in soils is represented by plant debris or litter in various stages of decomposition through to humus and includes the living organisms in the soil. Above ground plants (phytomass) are generally excluded from discussions of soil organic matter, but living roots are generally included.
The following definitions will be followed:
The carbon cycle describes how carbon is circulated through the atmosphere, biosphere, pedosphere, and hydrosphere. The dead organic matter of the soil is colonized by (micro)organisms, which derive energy for growth from the oxidative decomposition of complex organic molecules. Decomposition is the biochemical breakdown of mineral and organic materials. During decomposition, anorganic elements are converted from organic compounds, a process called mineralization. For example, organic-N and -P is mineralized to NH 4+ and H 2PO4 -, and C is converted to CO2. The remainder of the substrate C used by the microorganisms is incorporated into their cell substance (biomass), which is called immobilization. The incroporated minerals are immobilized and released after the organisms die or decay. Humification is the formation of humus (complex organic polymers) from raw organic materials, such as fulvic acids, humic acids, or humin.
Global Climate Change
SOM models (e.g. DAISY, RothC, CANDY, DNDC, CENTURY, and NCSOIL) embody our best understanding of soil carbon dynamics and may be used to predict how global environmental change will influence soil carbon, and to evaluate the likely effectiveness of different mitigation options.
The content of organic matter content is a function of the soil forming factors. Jenny (1930) found that for loamy soils in the United States the effect of soil forming factor to OM were in the order:
Generally, cold and arid climate tends to slow down the microbial processes within soil, in particular decomposition and mineralization. Therefore, those soils contain large portions of organic matter as plant debris (macroorganic matter) than as humus. The same effect is observed in acid to very acid soils. The warmer the climate the higher the rates of microbial processes, i.e., the lower the organic matter content in those soils.
The soil moisture content also has a remarkable effect of soil organic matter decomposition and accumulation. Waterlogged soils tend to accumulate organic matter because the microbial processes, in particular decomposition and mineralization, are slowed down. In aquic moisture regimes the drainage and soil aeration is poor (anaerobic conditions). Anaerobic oxidation of organic residues is less efficient than aerobic oxidation. If organic matter is accumulated the soil development is towards organic soils (Histosols). Histosols generally form in wet, poorly aerated sites, such as shallow lakes and ponds, depression areas, swamps, and bogs and are the end product of natural eutrophication.
The rates of decomposition, even for simple substrates such as glucose, vary widely due to differnces in water content, temperature, pH and the availability of nutrients such as P and N to support microbial activity. However, the simpler monomers from carbohydrates, proteins, fats, and many polyphenolic materials are decomposed within weeks in soil environments. Polymers (complex compounds) such as hemi-cellulose or cellulose are decomposed more slowly and their resistance to decomposition increase with complexity. It is essential to emphasize that many of the organic compounds found in soils result from in-situ synthesis mediated by microbial processes. Some natural polymers may persist in soils for years:
Decomposition reactions are catalyzed by enzymes. Generally, when the C : N ratio is > 25, net immobilization occurs, whereas at ratios < 25 net mineralization is likely. Classically, organic matter has been characterized via various extraction/fractionation procedures into non-humic (lipids, carbohydrates and other 'simple' organic compounds) and the more complex humic susbstances (humic acids, fulvic acids and humin). These divisions do not align well with current understanding of the biological and biochemical processes operating during decomposition and stabilization of organic material in soil.
Generally, litter from coniferous trees, such as pine, are undergoing a slow decomposition, whereas the litter from deciduous trees, such as elm, ash, oak, and birch, are easy to decompose. Lignin (complex phenolic polymer) is a significant proportion of straw and coniferous litter, which takes a long time to decompose. Coniferous litter tend to be acidic and low in bases, which promotes greater amounts of soil weathering. Annual species, such as grasses, tend to add organic residues not only to the surface, but due to death and decay of the roots. Also, the residue from annual species tends to have higher base contents than are found with perennials. Therefore, a thicker, darker A horizon is formed under grass than under deciduous or coniferous forest.
A sequence for decomposition of litter would look like this, whereas it starts with low decomposition and ends with high rates of decomposition:
In Figure 4. the differences in organic matter content in a grassland and a forest soil profile are shown. Grassland soils contain more SOM than forest soils under similar environmental conditions. The distribution of SOM is more uniformly distributed through the grassland profile than in a forest soil.
On agricultural land the application of mineral fertilizers, manure or the practice of green manuring influence the organic matter content in soils. The application of manure tends to increase soil organic matter because of the supply of nutrients and organic material to the soil .
Most of the soil organisms are concentrated in the top 15 - 25 cm of soil because C substrates are more plentiful there. Estimates of microbial biomass C range from 500 to 2,000 kg/ha to 15-cm depth. The macro- and mesofaunal biomass ranges from 2 to 5 t/ha, with earthworms making the largest single contribution. Microorganisms use litter and other organic compounds for respiration, where organic material is mineralized and CO2 and inorganic elements are released. The prokaryotes include the bacteria and actinomycetes, the eukaryotes include the fungi, algae and protozoa. They can be classified in heterotrophs, which require C in the form of organic molecules for growth, and the autotrophs, which can synthesize their cell substance from the C of CO2, harnessing the energy of sunlight (in the case of photosynthetic bacteria and algae) or chemical energy from the oxidation of inorganic compounds (the chemoautotrophs). Another way of subdividing the microorganisms is on their requirement of O 2: (i) the aerobes, those requiring O 2 as the terminal acceptor of electrons in respiration (ii) the facultative anaerobes, those normally requiring O 2 but able in anaerobic conditions to use NO 3 - and other inorganic compounds as electron acceptor in respiration (iii) the obligate anaerobes, those which grow only in the absence of O 2.
Table 1. Annual rate of litter return to the soil (White, 1987).
Franzmeier D.P., Lemme G.D., and Miles R.J., 1985. Organic Carbon in Soils of North Central United States. Soil Sci. Soc. Am. J., 49: 702 - 708.
Jenny H., 1930. A Study on the Influence of Climate upon Nitrogen and Organic Matter Content of Soil, Missouri Agr. Exp. Sta. Res. Bull. No 52, University of Missouri, Columbia, Mo.
Schreiner O., and Brown B.E., 1938. Soil Nitrogen. In: Soils and Men. USDA Yearbook, Washington D.C.: 361 - 376.
White R.E., 1987. Introduction to the Principles and Practice of Soil Science. Blackwell Sci. Publ., Oxford, London, Boston.
Cation Exchange Capacity
Interaction of SOM with Clay-Size Material
Field and laboratory experiments using additions of 14C-labelled organic compounds have been conducted to evaluate the fate of organic additions to soils of contrasting textures. The finer textured soils typically show a larger initial flush of microbial activity that is followed by greater incorporation and stabilization of organic matter in the soil than found in coarser textured soils.
Porosity exerts a strong influence on the fate of residues added to the soil because it define the domains in which microorganisms can function and those smaller domains into which organic molecules can migrate and become physically isolated from microbial attack. According to Kilbertus (1980), bacteria function only in pores that are at least 3 times their own diameter. Thus, bacteria are excluded from much of the pore space in soils, an effect that becomes more pronounced with increasing clay content. Thus, in clay-rich soils the physical separation between microorganisms and organic molecules can be extensive and account in part for their tendency to have larger accumulations than coarser-textured soils formed under otherwise comparable conditions.
It has been suggested that stabilization of organic molecules may occur between quasi-crystals (a packet of several layers) and within interlayers of 2:1 swelling clays such as montmorillonite. This mechanism has been inferred from examination of high resolution transmission electron micrographs that show presence of organic molecules within ~1.0 µm diameter pores between clay crystals. It is assumed that these domains provide considerable protection against microbial attack. Humic substances coat, partially or totally, mineral particles such as clay, often protecting the coated particles from weathering.
Cation Bridges and Retention of Organic Matter
In neutral and alkaline soils, Ca2+ and Mg2+ are the major cations responsible for bridging and the hydroxypolyvalent cations, Fe3+ and Al3+, serve a similar role in acid soils and those with a large amount of hydrous oxides. There are empirical observations that calcareous soils tend to have larger accumulations of organic matter than their non-calcareous neighbors. Liming experiments provide some insight into the role of Ca2+ in conversion of plant residue into stable organic matter. Addition of CaSO 4 or CaCO3 to soil containing 14C-labelled wheat straw produces an initial 'priming effect' on microbial biomass activity resulting in accelerated release of CO 2 that is followed by a greater retention and stabilization of organic matter than found in control treatments (i.e., no Ca 2+ addition). Thus, the 'priming effect' of Ca 2+ addition to the soil appears to be transient and the long term effect is one of stabilization of organic matter. The proposed mechanism of stabilization is the formation of Ca2+ cation bridges.
The mechanisms that control Fe3+ and Al3+ linkages with organic molecules are poorly understood. Fe3+ is only sparingly soluble in most soils and occurs mainly in hydrous oxide forms, some of which may be positively charged at low pH because of protonation or addition of hydrogen ions to surface exposed hydroxyl groups. Such positive charged surfaces may attract negatively charged organic molecules. A similar generalized mechanism probably operates with hydrous oxides of Al 3+ . However, at low pH soils may exhibit Al 3+ toxicity to vegetation, which would tend to limit C inputs into the soil.
The chelation process results in the formation of chelates, which are stable complexes containing organic compounds and metallic cations, which are trapped within the ring structures. The complexes can hardly be dissolved. Chelates formed with certain di- and polyvalent cations are the most stable, the stability falling in the order Cu > Fe = Al > Mn = Co > Zn.
Organic compounds (organic colloids < 2 micrometer) have the characteristic to increase field capacity because they tend to hydrolize. Generally, organic matter can hold up to 20 times its weight in water. This is important particularly for sandy soils to improve soil moisture conditions during summer seasons, when precipitation is limited and evapotranspiration rates are high. If organic matter becomes dry it is prone to wind erosion and can be transported over wide distances.
A hypothetical soil profile under deciduous species could be described as follows: There is a loose litter layer 2 - 5 cm deep under which the soil is well aggregated, porous, dark-brown in color, and has a granular structure. Below there is a deep A (approximately 30 - 50 cm) of a C : N ratio 10 - 15. The litter accumulation would be classified as mull. In contrast, a hypothetical soil profile under coniferous species could be described as follows: The surface litter is thick (5 - 20 cm) and ramified by plant roots and a fungi mycelium. There is a sharp transition between the organic and underlying mineral soil layers. The litter would be classified as mor.
O horizons are described in the field in terms of their relative degree of decomposition using the following subscript designations:
The symbol 'h' is used for the illuvial accumulation of organic matter but only in combination with B horizons. The 'h' indicates an accumulation of illuvial, amorphous, dispersible organic matter with or without sesquioxides. The influence of tillage or other cultivation disturbance that mix the surface layer is denoted by a 'p'. The symbol 'p' is only used with the master horizon A or O, even if the material mixed by cultivation is from an E, B, or C horizon.
Diagnostic Surface Horizons: Epipedons
A simple key to understanding the distinctions among the seven diagnostic epipedons centers on (i) distinguishing between mineral and organic surfaces, (ii) the thorough understanding of the definition of the mollic epipedon, and (iii) the awareness of the field settings and conditions where the less common epipedons (e.g., umbric, melanic, anthropic, plaggen) are likely to occur.
Figure 5. illustrates the criterion used to distinguish between mineral and organic soil materials. Note that as clay content increases the amount of SOM required to meet the organic soil material designation increases. The rationale behind this reflects (i) the intrinsic influence that particle-size has on SOM stabilization in soil, and (ii) functional behavior of SOM in relation to particle-size (i.e., SOM has a comparatively stronger influence on soil behavior with decreasing particle-size).
The following list describes the epipedons and their major characteristics.
Histic Epipedon: The histic epipedon has an aquic condition for some time in most years or has been artificially drained, and either,
Mollic Epipedon: The mollic epipedon has the following properties:
Umbric Epipedon: The requirements for the umbric epipedon are the same for the mollic, except that base saturation is <50%.
Anthropic Epipedon: The requirements for the anthropic epipedon are the same for the mollic, except that P2 O5 soluble in 1% citric acid is > 250 ppm.
Plaggen Epipedon: The plaggen epipedon is a cultural surface horizon produced by long continued manuring. Its color depends on the nature of the manure. Commonly it contains artifacts, such as bits of bricks and pottery through out its depth.
Melanic Epipedon: The melanic epipedon is a thick black horizon which contains high concentrations of organic matter, usually associated with short-range-order minerals or aluminium-humus complexes. The intense black color is attributed to the accumulation of organic matter from which "Type A" humic acids are extracted. This organic matter is thought to result from large amounts of gramineous vegetation, and can be distinguished from organic matter formed under forest vegetation by the melanic index. Additional information about the melanic index is outlined in Keys to Soil Taxonomy (KST).
Ochric Epipedon: The ochric epipedon does not meet the requirements of any of the epipedons listed above, but does show signs of surface soil formation (i.e., soil structure, darkening by organic matter).
The umbric epipedon can not be simply be distinguished from the mollic epipedon in the field. A determination of base saturation is required to distinguish the >50% base saturated mollic from the <50% base saturated umbric. The plaggen epipedon and anthropic epipedon are not commonly found and both owe their origin to local human manipulation of the soil. The histic epipedon has large amounts of organic material overlying mineral subsoils. The histic epipedon is not used in reference to the soils that are classified as Histosols. The mellanic epipedon has restricted occurrence and is associated with soils formed in volcanic materials. Table 2. Summary of epipedon names and important characteristics.
Diagnostic Organic Materials
Hemic Soil Material: In an unrubbed condition 1/3 to 2/3 of the total mass is composed of fibers (intermediate in decomposition between fibric and sapric).
Humilluvic Material: Illuvial humus that accumulates after prolonged cultivation of some acid organic soils.
Limnic Soil Material: These are organic or inorganic materials deposited in water by the action of aquatic organisms or derived from underwater and floating organisms. Marl, diatomaceous earth, and sedimentary peat (coprogeneous earth) are considered limnic materials.
Sapric Soil Material: In an unrubbed condition, less than 1/3 of the mass is composed of identifiable fibers and produced sodium pyrophosphate extracts with colors lower in value and higher in chroma than 10 YR 7/3.