Re: [compost_tea] N Bacteria and Tea

From: <soilfoodweb_at_aol.com>
Date: Sat, 10 Apr 2004 23:25:37 -0400
Hi Anthony -

You asked -

> Rhizobacter fixes N thru a mutual relationship with plants. Infections are initiated and nodules are formed then N can be supplied to the plant. They however they cannot fix N until a suitable infection is achieved and is all to do with the protein nitrogenase. - correct me if I am wrong.

The bacteria obtain sugars from the plant, and use that sugar to form the nodule, which has to be large enough so that the active bacteria in the nodule use up the oxygen before it reaches the center of the nodule.  Once the center becomes reduced in oxygen enough, then the nitrogenase enzyme (all enzymes are proteins) can to the job of converting N2 gas into NH2 (amide), attached to the carbon on a sugar molecule.  Which means a protein was produced. 

So, yes to your question - the bacteria have to make nitrogenase, a protein, in order to fix N.  They do this anaerobically.

 
> Azotobacter on the other hand can fix N directly from the air. So in the soil when does this N become available to the plant? Is it simply when they die from natural causes or does it only happen when a predator such as a protozoa or a nematodes feasts on it?

Azotobacter does the same process as Rhizobium, EXCEPT it does not form a nodule on the plant.  These bacteria still use plant carbon - but that carbon is released as sugars, proteins and carbohydrates from the root system as exudates.   Azotobacter has to grow a large enough colony so that the outer layers of bacteria use up the oxygen before reaching the center of the colony, so that the inner part of the colony lacks oxygen.  That means the nitrogenase enzyme can now function and fix N2 gas in the same fashion as Rhizobium does in the nodule. 


> My reason for asking is in relation to these two critters and compost tea. Rhizobacter is not present in compost piles as it needs a host plant to survive but I'm not sure about Azotobacter.

Azotobacter can be found in compost piles.  And worm compost too, apparently. 

> In a situation where a plant is in need of some N in a hurry, I'm considering adding some inocula of azotobacter as a 'bomb' just before applying some tea to speed up the nutrient cycling process. With the comment below (cut from an article on AZ) is seems heaps of O2 is needed for this guy to be happy, so can we presume he will be in a well managed compost or is he only in the soil?

> "Azotobacter nitrogenase is oxygen-sensitive, but it is believed that the extremely high respiration rate of Azotobacter (possibly the highest of any living organism) soaks up free
> oxygen within the cells and protects the nitrogenase."

To have N-fixation occur, you CAN'T have oxygen, but the bacteria take care of that situation for themselves. 

So, both Rhizobium and Azotobacter (as well as Azospirillum) grow aerobically, but the nitrogenase gene requires reduced oxygen levels.  Growing bacteria use up the oxygen, and so the center of the nodule, or the colony, is where nitrogen fixation occurs.

Hope this helps!


Elaine Ingham
President, Soil Foodweb Inc.
SFI Corvallis, OR
SFI Port Jefferson, NY
SFI Lismore, NSW, Australia
SFI Hilversum, The Netherlands
SFI Cambridge, New Zealand
SFI Culiacan, Mexico
SFI Jerome, Idaho
http://www.soilfoodweb.com

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Received on Sun Apr 11 2004 - 23:15:10 EDT

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