Hello Dahmani,

Regarding your first question: you can keep the mineralized samples for any time before analysis; HOWEVER, make sure they are well closed as atmospheric ammonia readily dissolves in acids and can cause significant contamination.

I do not know about the second question. I and recommend automated flow injection analysis (FIA) in an analytical lab as this method is easy, fast and inexpensive.

Yes, as suggested by Dr. Tomas, you can keep and use later your samples to analysis.

There are several available methods, 

The cumulative nitrogen intake (rather than uptake) and then infer a rate. For this, you need, at 2 sampling times, the knowledge of the whole plant (or stand) dry biomass and mean tissue. In the open field, this is a classic method. However, if your experimental facility has a restricted area, it may be unworkable to sample whole plants along a year (furthermore, border effects are extremely high in terms of plant biomass and , and the harvest of a plant influences greatly the neighbours). You could envisage the subsampling of plant organs (leaves, stems & roots), but you have to know that [N] varies greatly with the organ's age, and exposure to

https://www.researchgate.net/post/Can_anyone_suggest_a_procedure_for_determining_nitrogen_uptake_in_the_plant_roots?_tpcectx=profile_questions

Yes.  As all say, the digest can be kept provided it is protected from absorbing ammonia.  If you are in doubt keep the digests closed and store them in a closed container over a non-volatile acid, e.g. H2SO4.

I don't know if you can use Nessler reagent directly on the digest, but you can use salicylateand the indophenol reaction.  The indophenol reaction is environmentally preferable to the Nessler reagent which contains a lot of Hg.  That is you produce a lot of toxic waste!!

Unless you have hundreds/thousands of samples to analyse, or the amounts of ammonia are too small to titrate accurately, you will be better off to distil and titrate rather than go to the trouble of validating a colorimetric method.   

Check the distillation recovery using  solution of an ammonium salt such as NH4Cl.  Once that is consistently 98-100% check the digestion recovery using pure TRIS base.  Often the digestion conditions need to be modified to obtain an acceptable N recovery from TRIS base in a short time. Under the appropriate conditions, using Pyrex tubes in a temperature controlled, Al-block at 400 degrees C, digestion should be complete in about 1 h. The monograph on the Kjeldahl method by Bradstreet is old but authoritative.

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