Topic%203%20Nutrient%20cycling%20notes - Trees and Forest...

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Trees and Forest Nutrient Cycling in Forest Ecosystems Nutrient requirements of an ecosystem change as a function of stand development. Nutrients are accumulated most rapidly during the early development of forests, and more slowly as the aboveground biomass reaches a steady state. Nutrients may be supplied from various sources: 1. Atmosphere (wet and dry deposition & gases) 2. Rock weathering, exchange & sorbed pools 3. Resorption 4. Organic matter turnover (including throughfall + stemflow) Nitrogen fixation 1. Free-living organisms (asymbiotic - some bacteria and blue-green algae) convert atmospheric N 2 to NH 3 using the enzyme nitrogenase . This reduction reaction (0 to -3 oxidation state) has a high metabolic cost and requires energy from the respiration of organic matter. Free- living heterotrophic bacteria that conduct N 2 fixation are usually found in organic-rich soils that provide a ready source of energy. Total inputs 1 to 5 kg N/ha/yr 2. Symbiotic nitrogen fixation by bacteria occurs in association with higher plants. Rhizobium and Frankia form associations with the roots of higher plants. Plants provide carbohydrates to the nitrogen fixers and in return receive nitrogen. Globally, plants spend about 2.5% of NPP on nitrogen fixation. Total inputs up to 100 kg N/ha/yr (especially after disturbance, such as fire or clearcutting, when light levels allow maximum photosynthesis). Rates of N 2 fixation appear to be inversely related to the N:P ratio in the soil. Atmospheric Deposition Wet Deposition ¾ Deposition of materials dissolved in rain or snow ¾ Fog & Cloud Waters: Fog deposition tends to have a much lower pH (1-1.5 units lower than the rain from the same air mass and much higher element concentrations (5-30 times greater). A serious problem at high elevations where fog commonly bathes the vegetation. Dry Deposition ¾ The result of gravitational sedimentation of particles during periods without rain. Includes aerosols, dust particulates and gases (gas absorption: some gases can be taken
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This note was uploaded on 04/22/2011 for the course PLS 144 taught by Professor Rice during the Fall '08 term at UC Davis.

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Topic%203%20Nutrient%20cycling%20notes - Trees and Forest...

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