Francis,

I am also interested in natural abundance 15N of soil ammonium and 
nitrate.
There is definitely no straightforward method as for bulk soil.
I have used the diffusion technique (see Stark & Hart, Soil Sci. Soc. 
Am. J. 60:1846-1855) for KCl extracts. I'd recommend it for ammonium. 
Nitrate is more difficult, because potential contamination from 
residual ammonium and there is contamination from reagents. If your 
soil N concentrations are high, the latter should not be as big a 
problem. If you have adequate funding, you could use the denitrifyer 
method for nitrate. There are a few service labs available.
You'd have to transport frozen soil extracts or fresh soil back to 
your lab- I don't know if this is logistically possible for you.  

Good luck,
Kirsten

Kirsten Stephan
PhD Candidate
Department of Forest Resources
University of Idaho
P.O. Box 441133
Moscow, ID 83844-1133

----- Original Message -----
From: Francis Brearley <[log in to unmask]>
Date: Saturday, May 20, 2006 8:21 am
Subject: [ISOGEOCHEM] 15N of ammonium and nitrate
To: [log in to unmask]

> Dear All,
> 
> I will be working in Jamaica this summer to look at 15N in plant-
> soil 
> systems.  I will be measuring 15N of bulk soil which will be 
> straightforward but I am also hoping to measure 15N of ammonium 
> and 
> nitrate (which are the forms of nitrogen that will be immediately 
> available to plants and are therefore of more interest).  Does 
> anyone know 
> of a method which can be used to measure 15N of these two 
> compounds.  
> 
> The main drawback I have is that I'll be in Jamaica while our IRMS 
> is in 
> Dublin.  This means that I need to be able to process and/or store 
> the 
> samples for a few weeks before being able to analyse them.  Also, 
> I won't 
> really have access to a lab so the easier and simpler the method 
> the 
> better.
> 
> Many thanks
> 
> Francis
>