Packet 7.docx - Packet 7 Soil the Oxygen Cycle Slide 26...

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Packet 7: Soil & the Oxygen Cycle Slide 26 : Soil Oxidation – Reduction (Redox) Potential Another way to characterize the aeration status of a soil that can have environmental implications. Consider the Redox reaction: (2+) (3+) 2FeO + 2H 2 O  2FeOOH + 2H + + 2e - Fe(II) Fe(III) As the reaction proceeds to the right, Fe 2+ becomes oxidized and looses an electron to yield F 3+ . If the reaction proceeds to the left ferric iron, Fe 3+ is reduced by receiving an electron to form ferrous ion, Fe 2+ . The Redox Potential (E h) is a measure of the aeration status of a soil and dictates whether redox reactions are proceeding to the right or to the left Slide 27 : This graph depicts changes in dissolved oxygen & E h of a soil following water-saturation. As dissolved oxygen is consumed by soil respiration, the E h value (measured in volts) declines as well. The soil is said to go from an oxidizing to reducing condition. Substances such as iron (Fe) in soil minerals are reduced by becoming acceptors of electrons from soil metabolism. Slide 28 : The Eh status of a soil will dictate the redox form of certain elements found in soil. The practical importance of this is the potent greenhouse gasses (ex: CH 4 ) and toxic gases to plants (ex: C 2 H 4 ) are formed under reducing conditions found in waterlogged soils. Slide 29 : Excess soil wetness can inhibit oxygen entry into soils, resulting in oxygen depletion and anaerobic soil conditions. This can result in: direct plant injury, reduced organic matter of decomposition, an interruption of plant nutrient cycling leading to nutrient deficiencies in the plant, and a release of the greenhouse gasses methane & nitrous oxide. Thus in a so-called well aerated soil oxygen can readily enter into the soil and the soil is described as being aerobic; whereas in a waterlogged soil oxygen cannot enter into the soil and anaerobic soil conditions prevail. Soil plays a fundamental role in plant nutrient cycling. In a natural ecosystem, unaffected by human activity, most nutrients cycle between the plant and the soil in a closed loop. Nutrient losses due to, water runoff, leaching, soil erosion and volatilization are generally small so that the small additions from the atmosphere or from weathering of rocks keeps the cycle in balance. Simplified Agricultural Nutrient Cycle- In an agricultural ecosystems large quantities of nutrients are removed with crop yield. To maintain plant productivity, nutrients in the form of animal feed or fertilizers must be imported into the cycle. Packet 8: Soil and the Nitrogen Cycle Nitrogen : the most commonly deficient plant nutrient in natural & managed ecosystems; the most widely over applied nutrient in agricultural production & frequently responsible for water quality problems; 95 to 99% found in organic compounds within the soil-plant system. In forest areas, most N is in plant biomass
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& litter; in grassland systems, most N is in soil humus. These forms of N are organic nitrogen, which is unavailable to plants.
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