I agree with Dave but would like to expand on one thing:

What exactly is the N in low N soils?  Are we talking an even 
distribution in humic material, etc?  Even if we are, the fact remains 
that a single rootlet or bug will throw the whole thing out.  Good luck 
with trying to grind and homogenise to that degree, even if combustion 
of the entire sample is thorough.  We always have real problems trying 
to get reproducibility with huge samples, and my feeling is that a lot 
of it genuinely represents aliquot variability.

I decline to do work like that anymore: too much labour (cleaning out 
columns) and perhaps not meaningful.

Hil



David A. Mucciarone wrote:
> Ruben-
>
> This may be one of those, is it worth it analysis. I will leave this 
> up to the individual because I'm sure this comment alone will get 
> folks going which is not my point. Consider this more a cautionary 
> note than a statement.
>
> You can combust this much material, but you will not be able to do 
> many of them before having to ash the top of the oxidation column 
> (inserts will help). You will need to use perhaps 9x10mm capsules 
> (double wrap them in 5x9mm will work) to get enough tin to help the 
> combustion. I would do some tests by mixing up some sample in the lab 
> of similar C:N ratio e.g., some organic matter with low N illite or 
> similar clay to check reproducibility at this sample size. Do your 
> best to homogenize your sample. You will also need a carousel that has 
> a _zero_ N blank as well as your capsules or this N will/can be a 
> significant contribution too.
>
> That said, if you have a lot of carbon associated with your sample 
> (i.e., high double digit or higher C:N ratios) this could cause 
> problems with the combustions efficiency.
>
> Now the cautionary note: Muller, 1977 on Geochim Cosmochim Acta v.41 
> p. 765-776 (there are other papers too), pointed out the issue of 
> interpreting C:N data with low N in marine sediments. The point 
> carries over to soils as well especially when clays are present. The 
> high CEC of clays will absorb inorganic N (e.g., such as NH4) and 
> other things that will be released along with the organic N when you 
> combust the sample. If you are after an predominate organic N signal 
> (obvious both inorganic and organic N are released during the the bulk 
> combustion), results in the less than 0.0?% range will be more 
> influenced by the presence of inorganic N skewing the isotopic and 
> concentration result.
>
> We do numerous C/N analyses on sediments and soils (among other 
> things) and samples with low N can be difficult to interpret. This of 
> course is data set dependent. In some cases the N present is all from 
> organic matter.
>
> So the answer to your question is, yes, you can combust your 200mg 
> sample with some playing around. However, the interpretation of a bulk 
> isotope values at this low an N may be another matter. I leave the 
> matter of convincing reviewers up to you.
>
> Good luck,
>
> Dave
>  
>

-- 
Hilary Stuart-Williams PhD
Research Officer - Stable Isotopes
Environmental Biology - The Research School of Biological Sciences
The Australian National University

Telephone (02) 6125 2099