Practice exam #3 - QUESTION 4
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Biological N-fixation is the process by which organisms convert Organic, plant unavailable atmospheric Nitrogen gas (N2) into inorganic, plant available forms of Nitrogen! It has a significant difference to both NH4 and Phosphate fixation, as those imply a different kind of “fixation”. Rather than being made available to soil organisms/higher plants, NH4 and phosphate are “fixed” within the crystalline structures of certain minerals and are thus made UNAVAILABLE until these are potentially weathered.
basic idea ok Jacob,
Note that atmospheric N2 gas is inert (not considered to be organic)
consider adding more specifics on ammonium fixation and phosphate fixation
Biological N2 fixation is the biological process of converting N2 in the atmosphere into ammonia (NH4+). It is one of the processes that convert N into plant-available form.
NH4 fixation is not a process of making N available to plants. Instead, it is one of the ways that available N is lost from the soil solution. Ammonium ions (NH4+) can be captured by phyllosilicate clays in their inner layers. Strength of the inner layers holding together is strong enough to prevent the ammonium from leaving, making the ions unavailable to plants.
Phosphate fixation is similar to NH4 fixation in the sense that they both make available nutrients unavailable. But different from the former, this fixation is a process where available phosphate ions in the soil solution are absorbed/reacted with metal (Fe, Al, Mn) compounds, clay minerals, and/or calcium compounds into plant-unavailable forms.
Please let me know if something in my explanation is off.
Eakin, consider Jacob's point that NH4+ fixed by phyllosilicate clays and fixed phosphate can be released by weathering (albeit slowly).
4.) Biological N2 fixation is the conversion of atmospheric Nitrogen to NO3- and NH4+. Making nitrogen more available to plants.
Ammonium fixation is the adsorption of NH4+ into the interlayer of clay minerals making unavailable to plants.
Phosphate fixation is the reaction of phosphate ions with other minerals in the soil making it unavailable to plants.
good Madison, but what fixes phosphate and how is pH involved - see notes from your classmates
Biological N2 fixation occurs when soil microbes transform atmospheric N2 gas into NH3 using the enzyme nitrogenase which is carried out by bacteria, cyanobacteria, and actinobacteria. This process adds nitrogen to the soil. NH4+ fixation occurs when ammonium is fixed within the interlayer space of certain kinds of phyllosilicate clay minerals that have an interlayer space the same size as the radius of NH4+. This results in NH4+ becoming unavailable to plants until the phyllosilicate clay mineral weathers. This is a transformation process of nitrogen within the soil. Phosphate fixation is when Fe, Al, and Mn compounds complex with certain forms of phosphate and crash out of solution. These phosphate will not be available until another chemical process breaks down the metal complex containing it. This is another transformation process within the soil, this time with phosphorus.
Blaise, with respect to P fixation consider both low and high pH.
phosphate fixation: indicate that phosphorous is tied up in a form unavailable for uptake by plants.
nitrogen fixation: organisms convert the inter nitrogen gas of the atmosphere to N-containing organic compounds that become available to all form of life through N cycle.
ammonium fixation: fixed in the interlayer spacing of clay mineral.