Diffusion through membranes 7 Sedimentation

Ammonification is another process that can result in N release. The simplest definition of the process is the release ofNH4 from organic matter (e.g., Herbert, 1999). It can occur by a number of different processes including remineralization by bacteria in the water column and sediments. Photochemical ammonification occurs abiotically when NH4+ is released from organic matter as a result of exposure to UV radiation (reviewed in Bronk, 2002 and Chapter 10 by Gryz-bowski and Tranvik, this volume). Ammonium efflux from cells has also been observed following urea uptake in a number of culture experiments (e.g., Price and Harrison, 1988 Rees and Bekheet, 1982 Uchida, 1976). The release may be due to passive diffusion through the cell membrane and is likely unavoidable because NH3 is lipid soluble. 

Filters collect liquid and solid particles by mechanisms including diffusion, impaction, interception, electrostatic attraction, and sedimentation onto the filter while allowing the gas to pass through. The types commonly used in atmospheric particulate collection are membranes, fibrous mats, or porous sheets. Different filter materials are used depending on the particular type of measurement being carried out, including Teflon, quartz fiber, nylon, silver, cellulose filters, glass fibers, and polycarbonate. The characteristics of each are summarized by Chow (1995). 

The in situ pore water sampler also called pore water equilibrator or dialyzer is based on the principle that given enough time, a contained quantity of water in the sampler and adjacent pore water will diffuse and equilibrate through a dialysis membrane, or other materials such as porous Teflon, with the surrounding water and its dissolved solutes. The in situ equilibrator can either be removed from the soil to collect pore water or the pore water can be collected through an attached tubing while the device remains in the sediment. 

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