Soil Organic Matter
Soil Organic Matter Fractions
Decomposition or organic matter is a biological process that includes the physical breakdown and biochemical transformation of complex organic molecules of dead material into simpler organic and inorganic molecules. Its speed is determined by three major factors: soil organisms, the physical environment, and the quality of the organic matter (i.e., C:N ratio). The decomposition of plant and animal residues results in the production of a wide variety of materials of varying resistance to decomposition referred to as the labile and stable fractions of organic matter. The liable and stable organic matter fractions correspond, in general, to the organic residues and humus fractions, respectively. The relative contribution of each organic matter fraction to the total organic matter pool varies widely depending on soil type and farming system.
Labile Soil Organic Matter
The labile fraction of soil organic matter consists of any readily degradable materials from the plant and animal residues, and readily degradable microbial products. The liable fraction part of soil organic matter is the active, or easily decomposed, fraction. The active fraction is influenced strongly by weather conditions, moisture status of the soil, growth stage of the vegetation, addition of organic residues, and management practices, such as tillage. Labile organic matter is readily decomposed by microbes and is the principal energy source (e.g., sugars, oils, cellulose, and proteins) that fuels the soil food web. Upon decomposition, the nutrients are released in a plant-available form. The active organic matter fraction accounts for only a small fraction (10–20%) of the total organic matter in the soils but is most responsible for soil aggregation and nutrient recycling.
Stable Soil Organic Matter
The stable soil organic matter fraction consists of resistant compounds, which accounts for 80 to 90 percent of the total pool of soil organic matter, consists of organic compounds that are relatively resistant to decomposition because of either their chemical structure, their adsorption to clay particles, or their protection within microaggregates. The stable fraction is generally considered to be humus, highly resilient to weathering and closely bound up with the mineral portion of the soil, protecting it from further decomposition. The stable fraction is the result of successive steps in the decomposition of organic matter. The chemical composition of humus varies because it results from different types of plant and animal residues and depends on the action of living soil organisms. Humus-making material is made up of carbohydrates (simple sugars and starches), pectins, cellulose, and proteins.
Decomposition Rates of Organic Matter
Decomposition rate of organic matter refers to the time taken for organic matter to move into and through the various organic matter pools. This movement is a continual process and is vital to the functioning of all ecosystems. As new organic matter enters the soil it supports biological processes, releases nutrients through decomposition and contributes to soil resilience. The difference between organic matter inputs and outputs and the rate at which they are transformed determines the size and stability of the organic matter pools.
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