EESB05 Soil Lecture9.docx

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Environmental Science
Marney Isaac

Lecture 9 and Chapter 10 & 11 ● organisms in the soil are highly abundant and diverse ● Fauna is used to distinguish animals from flora (refers to the true plant and non-animal microorganisms) ○ macrofauna (>2mm) - all heterotrophs, largely herbivores and detritivores ○ mesofauna (0.1-2mm) - all heterotrophs, largely detritivores, and predators ○ microfauna (<0.1mm) - detritivores, predators, fungivores, and bacterivores. ● controlled by supply of decomposable organic matter ● mostly heterotrophic, use organic matter and carbon as energy ● decomposition of organic matter (heterotrophic bacteria) ● enzyme-mediated (inorganic) transformations ○ oxidation or reduction processes ● nitrogen fixation ● plant protection ● breakdown of toxic compounds ● decomposition of plant residues is an oxidative process ● readily decomposable substances breakdown first, leaving more resistant humus ● soil organic matter is a complex and varied mixture of organic substances; all organic matter contains carbon ○ rapid litter breakdown leads to rapid nutrient cycling ○ organic matter decay contribute to net CO2 atmospheric release ○ soils play a central role in the global carbon cycle, both as a Carbon repository, sink, and source ● global carbon cycle ○ plants take in carbon dioxide from the atmosphere ○ through photosynthesis, the energy of sunlight is trapped in the carbon-carbon bonds of organic molecules ○ some of these organic molecules are used as a source of energy by the plant via respiration; w/ the carbon being returned to the atmosphere as CO2 ○ remaining organic materials are stored temporarily as constituent of the standing vegetation, which will be added to the soil as plant litter or root deposition ○ CO2 also reacts w/ the soil to produce carbonic acid, and carbonates and bicarbonates of calcium, potassium, magnesium, and sodium; they are readily soluble and may be removed in drainage ■ carbonates such as calcite are less soluble and accumulate in soils under alkaline conditions ● the major types of organic compounds present in: ○ green plant tissue: cellulose, hemicellulose, lignins, protein, fat and waxes, and sugar and starches ○ elemental components are (dry matter): carbon, hydrogen, ash, and oxygen ○ plant residues are the principal material undergoing decomposition in soils, and are the primary source of soil organic matter ■ green plant tissue contains 60-90% water; when dried the dry matter consists of carbon,oxygen, and hydrogen (90-95%), rest when oxidized will turn into ash and smoke. ● the relative decomposition rates (from highest to lowest decomposition rates): ○ sugars, starches, and simple proteins ○ crude proteins ○ hemicellulose ○ cellulose ○ fats, waxes, and so forth ○ lignins and phenolic compounds ● decomposition: the breakdown of large organic molecules into smaller, simpler components; There are three phases of decomposition: ○ leaching of soluble cells ○ combination of fragmentation and leaching a decay products (the portion measured in litter bags) ○ involves the chemical alteration of organic matter ● when organic tissue is added to an aerobic soil, three general microbiological reactions can take place: ○ enzymatic oxidation of carbon compounds to produce carbon dioxide, water, energy, and decomposer biomass ○ release and/or immobilization of the essential nutrient elements, such as nitrogen, phosphorus, and sulfur, by a series of specific reactions that are relatively unique for each element ○ formation of compounds very resistant to microbial action, either through modification of compounds in the original tissue or by microbial synthesis ● an oxidation process - in a well-aerated soil, all of the organic compounds found in plant residues are subject to oxidation. ● organic compounds in plant tissue are grouped into broad classes; carbohydrates range in complexity, Lignins and Polyphenols are resistant to decomposition; proteins contain about 16% of nitrogen and decompose easily ● decomposition can either undergo active fraction, slow fraction, or passive fraction ○
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